discussion - inflibnetshodhganga.inflibnet.ac.in/bitstream/10603/19621/16/16...erdtman (1969)...
TRANSCRIPT
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5 DDIISSCCUUSSSSIIOONN
During the past few decades, the study of pollen morphology has
assumed great significance in the realm of morphological and comparative
botany due to the realization that the pollen grains reflect in them, to a fair
degree, the facts and facets of the biology of the mother plant they belong
to. The structural uniqueness of the pollen wall, the exine has no parallel in
any other morphosystem comprising the plant body. The developmental
morphology itself is unique as the sporopollenin material constituting the
exine has its origin in the gametophytic pollen protoplasm initially, and in
the sporophytic tapetal tissue in succession. The high degree of strength
and tenacity of the exine which is acknowledged as one of the most
resistant material in the organic world is necessitated by the fact that the
function of the exine is the protection of the most vital life material, the
male gametophyte, a critical partner in the biology of reproduction.
The advancements in microscopy, and particularly electron
microscopy, have its significance in our understanding of the structure and
sculpturing of the pollen wall, which has led to the effective use of new
morphological parameters to taxonomic and phylogenetic purposes. The
propounding of the triphyletic theory of angiosperms as the basis of the
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morphological evolution of apertural characters in pollen (Nair, 1979) is an
outstanding example of the effective application of pollen morphology in
plant phylogeny as well as in straightening the ambiguities and problems of
taxonomy. In making morphological analysis of exine characters relevant for
applying them in plant taxonomy and evolution, it is held that the characters
relating to the aperture are primary, exine ornamentation secondary and the
others tertiary in their degree of importance. Principles have been laid and
methods evolved for the evolutionary interpretation of each morphological
category of exine characters (Nair, 1970b) which have facilitated the
increasing use of pollen morphology in comparative botany. The present
study is expected to provide adequate pollen exine information for use in the
taxonomy and evolution of the myrtalean families.
Based on the palynological information gathered in a sizeable number
of taxa of the six families of the Myrtales from the South Indian region, a
brief discussion on the pollen exine morphosystem of these families is made.
In addition aspects such as pollen morphological evolution in the group, role
of palynology in the systematics of the order and their interrelationships,
evolution and affinities are also considered.
5.1 Pollen Morphological analysis
Pollen grains, the male gametophytes of flowering plants have two
main concentric layers in the wall, the inner intine and the outer exine. The
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morphological characteristics of the pollen grains are manifested in the
outer layer, the exine. A brief discussion of the different pollen
morphological features of the order Myrtales based on the data obtained
from the present investigation is attempted (Table 1). Morphological
analysis of pollen is made in relation to the degree of importance of the
various characters in plant taxonomy, phylogeny and evolution; the
germinal aperture being of primary importance, exine ornamentation
secondary, and the other characters such as exine strata, size and shape
tertiary (Tewari and Nair, 1979).
5.1.1 Pollen aperture
Apertures are specially delimited, generally thin-walled areas in the
outer pollen wall or exine, through which the pollen tube usually emerges
during germination. Although most apertures seem to represent thinner
areas in the exine or aperture membranes, in some pollen grains they are as
thick or thicker than the exine on nonapertural areas on the same grain
(Walker and Doyle, 1975). Apertures serve a second major function in that
they allow for volume-change accommodations (harmomegathy) in pollen
grains subjected to changes in humidity (Wodehouse, 1935; Payne, 1972).
Some apertures may serve both for pollen tube exitus and harmomegathic
changes, while some pollen grains have “pseudo-apertures” which appear to
function solely in a harmomegathic capacity. Harmomegathic mechanisms
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involve the reaction of the complete pollen wall to the turgor pressure of
the cytoplasm (Blackmore and Barnes, 1986). During harmomegathic
movements, different stresses are exerted on the pollen grain wall
depending on whether cytoplasmic volume changes are accomodated
mostly by apertural or mesocolpial areas of the exine (Crane, 1986). This
phenomenon could contribute to the diversity of exine substructures as has
been observed in Eperua. In contrast to the pollen of most other legume
taxa, where volume changes are accommodated by apertural exine, in
Eperua the mesocolpial exine tends to curve inwards when dehydrated.
This may be correlated with the relatively small aperture size (Banks and
Rico, 1999). As discussed by Blackmore and Barnes (1986), it is important
to consider the effects of harmomegathy on the wall structure in the
context of an overview of all functions, phylogenetic history and
ontogenetic constraints and further study including experimental
observations on living grains are required for a better understanding of
this.
Almost all palynological discussions of plant relationships and
phylogeny are based on the aperture form, their number and distribution,
and position (designated as the NPC; N=Number; P=position;
C=character). Erdtman (1969) proposed the NPC-system for the
classification of pollen grains based on the number, position and character
of the aperture.
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With respect to the number of apertures, pollen may be classified as
inaperturate (without aperture), mono-aperturate (with one aperture), di-
aperturate (with two apertures), tri-aperturate (with three apertures), or
poly-aperturate (with more than three apertures). Number of apertures and
their distribution in the pollen grain forms one among many trends in the
evolution of pollen grain morphology (Walker and Doyle, 1975; Van
Campo, 1976; Chanda et al., 1979). An increase in the number of apertures
from the first angiosperm pollen grains to more recent ones appear to be a
trend (Van Campo, 1976; Chanda et al., 1979). The more apertures a
pollen grain has, the more quickly it loses its ability to germinate, and
shorter its life expectancy (Dajos et al., 1992). The more apertures a grain
has, the more is it susceptible to external stresses. Consequently many
apertured pollen grains probably dry up much more quickly than few-
apertured ones, since the pollen wall is very much reduced in the aperture
region (Dajos et al., 1992). In a number of taxa it has been observed that
there is a correlation between the ploidy level and the size of the aperture.
As the ploidy increases the aperture size also increases correspondingly
(Nair and Ravikumar, 1984).
The external characters of pollen, such as size, number of pores and
sculpturing of the exine, are widely used for identification purposes, and
the number of pores on periporate pollen has been used as a diagnostic
character for taxonomic purposes in several families such as the
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Caryophyllaceae, Plantaginaceae and Chenopodiaceae (McAndrews and
Swanson, 1967). Davarynejad et al. (1995), in their study, used pollen
morphology as an indicator for the identification of male pistachio trees
for Pistacio vera varieties. However, it is recognized that these characters
are more or less variable, particularly as a result of the effects of climate
on pollen-forming plant (Kurtz and Liverman, 1958), and several studies
have attempted to demonstrate the relationship between pollen
characteristics and various climatic factors. Species with many pores tend
to have larger pollen grains than species with few pores. This increase in
size may be caused by an increased adaptation to different systems of
animal pollination as well as the level of polyploidy that may be
manifested by larger grains as the ploidy level increases. Mineral nutrition
as well as the genetic factors may also cause variation in pollen size. In
addition, a correlation with climatic factors has been postulated for the
genus Ilex, in which the pollen size increases both with latitude and
altitude (Lobreau-Callen, 1975). There is a trend towards a larger size of
pollen grains from the semi-arid sites to the more arid sites, and which is
also negatively correlated to the site altitude (Belhadj et al., 2007). This is
not in accordance with Kurtz and Liverman (1958) who reported the
occurrence of smaller pollen grains caused by aridity. Another perspective
is given by Do¨mnez and Pinar (2001), who reported a trend towards
greater pollen size from the less to the more specialized taxa in Iris
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species, supporting the general idea that pollen size increases with
evolution. The pollen of angiosperms for instance, has evolved towards an
increasing number of apertures that offers a potential selective advantage,
because it increases the likelihood of contact between the germination site
and the stigmatic surface (Dajoz et al., 1991). Pollen fertility, as well, may
provide additional information of taxonomic importance when correlated
with morphological traits and may help to determine whether a plant
species is successfully adapted to certain ecological conditions (Qureshi et
al., 2002). Crane et al. (1974) reported that pollen fertility could vary
between species, types and years. This variation may be due to different
ecological conditions between the different sampling regions as has been
suggested for Pistacio species (Ozeker et al., 2006).
Beaulieu et al. (2008) found a strong positive relationship between
genome size and cell size, suggesting that genome size may partly
determine, or be correlated with pollen size. The correlation between the
pollen size and genome size was studied in 464 species by (Knight et al.,
2010), but found only a weak relationship between the two in their
analysis.
The members of Myrtales generally are triaperturate. The aperture
number does not show any variation in Rhizophoraceae. The number of
apertures is three, except in Calycopteris floribunda (Combretaceae), and
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Clidemia hirta and Dissotis rotundifolia (Melastomaceae) where
occasional four-apertured grains occur. Myrtaceae showed variation in
aperture number in Psidium 3, 4 and 5 apertured; 3 and 4 apertured grains
in Pimenta, Melaleuca, species of Eugenia and Syzygium. In Lythraceae 3
and 4 apertured grains are met with in Lagerstroemia indica, Cuphea
ignea and Rotala malampuzhensis. In Rotala densiflora the pollen grains
are 4-zonoaperturate which agrees with the previous report by (Graham et
al., 1990). In Onagraceae 3 and 4 apertured pollen grains are present in
Ludwigia suffruticosa; in Fuchsia arborescens the pollen grains are
2-zonoaperturate and in F.fulgens 3, 4 and 5 apertured grains occur.
Apertures may be elongated (furrow-like) or circular (pore-like). The
elongated apertures are termed as colpi, and the circular ones as pori.
Pollen grains with colpus are colpate and with porus, the porate type.
Colpate pollen is essentially restricted to dicotyledonous angiosperms. The
apertures are situated either in the outer layer of the exine (sexine) or in the
inner layer (nexine) or both. An aperture which is a feature of the sexine is
termed as the ectoaperture (ectocolpus, ectoporus), and the one which is a
feature of the nexine is termed as the endoaperture (endocolpus,
endoporus). The outer and the inner faces of the apertures in the colpate
and porate grains may be congruent or incongruent. When the outer and
inner faces are incongruent the apertures are termed colporate and
pororate. The frequency of the three major apertural types (colpate,
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colporate, porate) observed in the present group show that colporate one is
the predominant type (101 species, 36 genera, 6 families) followed by
porate (4 species, 2 genera, 2 families) and the least being colpate type (3
species, 2genera, 2 families) as shown in Table 2 and Fig. 2.
Table 2. Distribution of aperture forms in South Indian Myrtales.
Colpate Colporate Porate
3-co
lpat
e
3-sy
n co
lpat
e
3,4-
Col
pora
te
3,4-
Syn
colp
orat
e
3,4-
hete
ro
colp
orat
e
2-Po
rate
3-Po
rate
3,4,
5-po
rate
Family
G S G S G S G S G S G S G S G S Rhizophoraceae - - - - 5 7 - - - - - - - - - -
Combretaceae - - - - - - - - 6 12 - - - - - -
Myrtaceae - - 1 2 4 5 8 27 - - - - - - - -
Melastomaceae - - - - - - - - 8 26 - - - - - -
Lythraceae - - - - 5 8 1 3 4 6 - - 1 2 - -
Onagraceae 1 1 - - 1 7 - - - - 1 1 - - 1 1
Although the prominent aperture type is colporate in Myrtales,
variations occur in aperture structure. Myrtaceae have syncolpate apertures
in Barringtonia racemosa and B.acutangula but colporate in others. A few
Species of Eugenia and Syzygium, Melaleuca leucodendron, Pimenta
pubescens, Callistemon citrinus, Eucalyptus, Careya arborea and
Barringtonia asiatica (Myrtaceae) and Cuphea species (Lythraceae) have
syncolporate apertures while, Eugenia uniflora, Syzygium jambos,
S.hemisphericum, S.zeylanicum, S.occidentalis, and S.aromaticum have
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parasyncolporate apertures. Exclusively porate apertures have been met
with, in Sonneratia (Lythraceae) and, species of Fuchsia (Onagraceae),
while species of Ludwigia have brevicolpate apertures and in all the
species of Onagraceae studied they are protruding or aspidote type. In
Trapa (Onagraceae) the aperture is of the colpate type.
Fig. 2. Distribution of aperture type in South Indian Myrtales
The apertures usually have aperture membranes. The membranes are
psilate (smooth), granulate (provided with granules), crustate (thickly beset
with coarse granules), etc. In certain apertures the membranes are
sometimes conspicuously thickened so as to assume almost the same
appearance as the interapertural parts of the pollen grains. The
subcategories of the basic apertural morphoform is formed on the basis of
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the nature and form of endoapertures (endocolpium, endoporium), the
apertural membranes, operculum etc. in addition to the number, position
and distribution of the apertures. In the colporate grain the endocolpium
(ora) is usually circular or it may be extended or elongated transversely
(lalongate) or elongated longitudinally (lolongate). The distribution of the
endoaperture type in the taxa of the order Myrtales studied presently is
shown in (Table 3 and Fig.3). Lalongate endoaperture had the highest
frequency with (35 species, 14 genera, 3 families); the endoaperture being
rectangular-lolongate in all the 7 species of Rhizophoraceae presently
studied, the endoapertures being fused in species of Rhizophora and
Bruguiera, a feature not met with in any other myrtalean families and faint
type was exhibited by (25 species, 13 genera, 4 families). Circular
endoaperture was observed in (26 species, 12 genera and 5 families) and
lolongate in (8 species, 5 genera and 2 families) with the lowest frequency.
Table 3. Distribution of endoaperture types in South Indian Myrtales
Faint Circular Lalongate Lolongate Rectangular lolongate Family
G S G S G S G S G S Rhizophoraceae - - - - - - - - 5 7 Combretaceae 2 2 1 2 5 8 - - - -
Myrtaceae 7 17 2 2 3 7 4 6 - -
Melastomaceae 3 5 1 1 6 20 - - - -
Lythraceae 1 1 7 14 - - 1 2 - -
Onagraceae - - 1 7 - - - - - -
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Fig. 3. Distribution of endoaperture type in South Indian Myrtales
In some taxa, the aperture is covered by a cap-like sporopollenin
termed as the operculum. Operculate grains have been observed in
Myrtaceae (Rhodomyrtus) Melastomaceae (Memecylon, Melastoma) and
Lythraceae (Rotala, Nesaea) in the present study. Operculate grains are
characteristic of Gramineae and supposed to be highly evolved.
According to Clarke (1977) the term heterocolpate covers pollen
grains with ectocolpi bearing endoapertures alternating with ectocolpi
which do not. “Subsidiary colpi or the pseudocolpi differ from a normal
furrow in that it is not an exit for the pollen tube” (Faegri and Iversen,
1964) while, according to Erdtman (1971) it is the “Colpoid streaks not
functioning as apertures”. Muller (1981) considers the term to be a
misnomer because pseudocolpi function in volume changes of the pollen
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grain during expansion and contraction in response to moisture content
(harmomegathy of Wodehouse, 1935) therefore he recommends the term
“subsidiary colpi”. Subsidiary colpi or pseudocolpi, generally have a
thinner ektexine than the surrounding mesocolpial areas but in contrast to
the colpi all exine layers are usually represented. The thinning of the ektexine
is gradual, and thus the subsidiary colpi often are not as clearly delimited as
the colpi, although the endexine is increased in thickness just as in the colpi.
The surface sculpture in the pseudocolpi is often different from that of the
colpi. They have been observed in Lythraceae, Acanthaceae, Boraginaceae,
Hydrophyllaceae, Leguminosae and Verbinaceae (Erdtman, 1971; Nowicke
and Skvarla, 1974; Faegri and Iversen, 1975; Ferguson and Skvarla, 1981,
1983; Raj, 1983), Crypteroniaceae (Muller, 1975) and Melastomaceae
(Vasanthy, 1976). Subsidiary colpi have also been observed in Lythraceae,
Combretaceae, Melastomaceae (Patel et al., 1984).
In the present investigation pseudocolpi have been observed in
families such as, Combretaceae (6 genera and 12 species),
Melastomaceae (8 genera and 26 species), and Lythraceae (3 genera and
4 species). Subsidiary colpi or pseudocolpi are either equal to the number
of colpi (isomerous), alternating with them as in Combretaceae and
Melastomaceae or there can be additional subsidiary colpi, as observed in
Rotala, Ammania and Nesaea (Lythraceae), where they are twice the
number of aperture. Distributon of pseudocolpi in the myrtalean families
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is given in Table 4 and Fig. 4. Subsidiary colpi have been reported in
members of Lythraceae by Erdtman (1952), Campos (1964), Graham
(1977), Patel et al., (1984) and Graham et al. (1990). They are reported
to be located on just one polar face of the grain in the Oliniaceae (Patel et
al., 1984), and hence considered as half subsidiary colpi, while in some
members of Alzateaceae, they are reported to be rudimentary or
incipient.
Table 4. Distribution of Pseudocolpi in Myrtalean Families of South India
Number of Pseudocolpi
Three Six
Family
G S G S
Rhizophoraceae - - - -
Combretaceae 6 12 - -
Myrtaceae - - - -
Melastomaceae 8 26 - -
Lythraceae - - 3 4
Onagraceae - - - -
Intercolpar concavities were originally reported by Wodehouse
(1928) in the tribe Mutisieae of Compositae. These are structurally and
functionally similar to subsidiary colpi, distinguished only in that they are
markedly larger. Intercolpar concavities have also been described in
Calyceraceae (Skvarla et al., 1977) and Hoplestigmataceae, Verbenaceae
and Olacaceae by Erdtman (1971) and Raj (1983). In the Myrtales
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intercolpar concavities have been observed in Myrtaceae (2 genera, 2
species) and in Melastomaceae (1 genus, 1 species).
Fig. 4. Distribution of pseudocolpi in South Indian myrtalean familien
Position of the apertures is determined quite early and may be
correlated with the position of the meiotic spindle poles (Blackmore and
Barnes, 1990). With respect to the position of the aperture, pollen grains
are described as polar when the apertures are located on the poles. Polar
apertures may be either proximal or distal. The proximal apertures are met
with in the pteridophytes. Some primitive angiosperms also have proximal
apertures (Nair, 1968). Zonal or equatorial apertures are located on the
equator and global apertures scattered over the surface of the pollen grain.
The zonal and global positions of the apertures are restricted to the
angiosperms alone. The position of the aperture in the order of
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evolutionary progress is proximal, distal, zonal and global. All the taxa
presently studied have zonal apertures.
Viscin threads are sporopollenin containing threads and form most
remarkable structural features of pollen grains in some angiosperms.
Viscin threads are themselves nonsticky, thin, long, flexible, nonelastic
fibers and basically located on the surface of the pollen tetrads or single
grains. One end is free and the other end is attached to the exine of the
pollen. Viscin threads occur in some unrelated families viz., Onagraceae,
Ericaceae, and Caesalpiniaceae (Skvarla et al., 1978; Graham et al., 1980;
Waha, 1984; Hesse, 1984; Takahashi and Skvarla, 1990). Morphological
and chemical variations of viscin threads are notable, smooth pattern
(Rhododendron) and beaded pattern (Oenothera) (Yamada, 1988). The
threads in Onagraceae, Ericaceae and Caesalpiniaceae chemically contain
sporopollenin (Skvarla et al, 1978; Graham et al., 1980; Waha, 1984;
Hesse, 1984), elastic fibers devoid of sporopollenin occur in some
Orchidaceae (Vijayaraghavan and Shukla, 1980) and probably different in
some Caesalpiniaceae (e.g. Delonix: (Cruden and Jensen, 1979; Hesse,
1984). Since the viscin threads hold large number of pollen grains together
from dehisced anther, it is presumed that these threads play an important
role in pollination. Viscin threads may tie the pollen grains like ropes to
insect hairs and bristles.
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Viscin threads have been observed in Onagraceae in the present
study in 2 genera and 4 species. The structure of these threads appear to be
different in these genera, being smooth and tufted (L.adscendens and
L.peruviana), long and twisted (L.suffruticosa) and long and mycelia-like
(F.fulgens).
Permanent tetrad formation represents an advanced character over
solitary grains. In some instances however monads may have evolved from
tetrads (Walker, 1971), and in such cases solitary grains represent an
advanced rather than a primitive character state. Compound pollen grains
in the form of dyad, triad, tetrad, polyad or pollinium occur in more than
56 families of angiosperms (Knox and McConchie, 1986). Among
dicotyledons pollen tetrads are found both, in ancient families possessing a
number of primitive features such as Winteraceae, Monimiaceae,
Annonaceae, and in advanced families like Onagraceae, Ericaceae,
Proseraceae and Mimosaceae. Recently they have also been reported in
Amaryllidaceae (Meerow et al., 1986) and Empetraceae (Kim et al.,
1988). Two common binding systems in the permanent tetrads are
recognized in about 20 families that have been examined (Knox and
McConchie, 1986): the cross wall cohesion mechanism where exine
bridges occur between the grains as in Acacia (Kenrick and Knox, 1979;
Knox and McConchie, 1986), Calluma (Dahl and Rowley, 1991), Drosera
(Takahashi and Sohma, 1982), Hedycarya (Sampson, 1977), Onagraceae
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(Skvarla et al., 1975) and Winteraceae (Sampson, 1981) etc. The simple
cohesion mechanism, refers to the binding of adjacent microspores simply
at the tectal/ bacular level, without the presence of the exine bridges, as in
Asimina (Waha, 1987; Gabarayeva, 1992, 1993), Elaeocharis (Dunbar,
1973), Leshenaulti (Knox and Frederich, 1974), Pyrola (Takahashi and
Sohma, 1980) and Typha (Skvarla and Larson, 1963; Takahashi and
Sohma, 1984). The mechanism of tetrad pollen formation in two species
of Annona by the rotation of microspores has been observed by Tsou
and Fu (2002).
Microspores are unique entities providing important morphological
criteria for taxonomic considerations of plants. Palynologically the various
taxa are either stenopalynous or eurypalynous. In the stenopalynous taxa,
the different sporomorphs are of the same basic type, while in the
eurypalynous ones, they are of more than one type.
Distribution of stenopalynous and eurypalynous conditions in the
myrtalean families presently studied is as follows:-
Eurypalynous families:
COMBRETACEAE 3-zonocolporate
4-zonocolporate
MYRTACEAE 3-zonosyncolpate
3-zonocolporate
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3,4-zonocolporate
3,4,5-zonocolporate
3-zonosyncolporate
3,4-zonosyncolporate
3-zonoparasyncolporate
3,4-zonoparasyncolporate
MELASTOMACEAE 3-zoncolporate
3,4-zonocolporate
LYTHRACEAE 3-zonocolporate
3,4-zonocolporate
4-zonocolporate
3-zonosyncolporate
3,4-zonosyncolporate
3-zonoporate
ONAGRACEAE 3-zonocolpate
3-zonocolporate
3,4-zonocolporate
2-zonoporate
3,4,5-zonoporate
In the eurypalynous families, further groupings at generic level may be
possible based on aperture character. The proposed groupings are as below.
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COMBRETACEAE
Grains 3-zonoheterocolporate Terminalia
Anogeissus
Lumnitzera
Combretum
Quisqualis
Grains 3,4-zonohetercolporate Calycopteris
MYRTACEAE
Grains 3-zonosyncolpate Barringtonia
Grains 3-zonocolporate Syzygium
Grains 3-zonocolporate (longicolpate) Rhodomyrtus
Grains 3-zonocolporate (brevicolpate) Eugenia
Grains 3,4-zonocolporate Syzygium
Grains 3,4,5-zonocolporate Psidium
Grains 3-zonosyncolporate Eucalyptus
Melaleuca
Syzygium
Careya
Barringtonia
Grains 3,4-zonosyncolporate Callistemon
Psidium
Syzygium
Pimenta
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Grains 3-zonoparasyncolporate Syzygium
Grains 3,4-zonoparasyncolporate Syzygium
MELASTOMACEAE
Grains 3-zonoheterocolporate Osbeckia
Melastoma
Sonerila
Medinilla
Memecylon
Tibouchina
Grains 3,4-zonoheterocolporate Clidemia
Dissotis
LYTHRACEAE
Grains 3-zonocolporate Rotala
Woodfordia
Lawsonia
Lagerstroemia
Punica
Grains 3-zonoheterocolporate Rotala
Ammania
Nesaea
Grains 4-zonocolporate Rotala
Grains 3,4-zonocolporate Rotala
Lagerstroemia
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Grains 3-zonosyncolporate Cuphea
Grains 3,4-zonosyncolporate Cuphea
Grains 3-zonoporate Sonneratia
ONAGRACEAE
Grains 3-colpate Trapa
Grains 3-zonocolporate Ludwigia
Grains 3,4-zonocolporate Ludwigia
Grains 2-zonoporate Fuchsia
Grains 3,4,5-zonoporate Fuchsia
Stenopalynous Family:
RHIZOPHORACEAE
Grains 3-zonocolporate Rhizophora
Kandelia
Bruguiera
Carallia
Blepharistemma
5.1.2 Exine sculpturing
Pollen wall is made up of two layers, the inner intine which surrounds
the cytoplasm and the outer exine. The exine is exceedingly hard and
resistant. Chemically seen the exine is composed of sporopollenin, a
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substance resembling lignin. It shows morphological features that are of
high diagnostic value which is beautifully revealed by means of scanning
electron microscopy (Nilsson, 1992). The exine generally consists of two
layers, inner homogeneous, nonsculptured (nexine) and an outer variously
sculptured (sexine) layer. The exine is highly resistant and durable. The
exine is also known to be associated with mode of pollination (Knox,
1984).
The pattern of pollen wall sculpturing is species specific and in
majority of cases is determined by the sporophyte (Quiros, 1975).
Blueprint for different sculpturing patterns is laid down at the time of
meiosis, while the tetrads are still encased in the callose wall. Pollen wall
morphology has often been shown to contain taxonomically interesting
information (e.g. Walker and Skvarla 1975, Skvarla et al.,1977).
The surface ornamentation of the exine is a significant morphological
character helping a great deal in the categorization of the various genera and
species within the family and genera (Nair and Sharma, 1965). The
morphoform categorization based on exine ornamentation is particularly
useful in the grains of the unipalynous taxa. The exine surface often presents
various types of ornamentation forms. The two broad categories of
ornamentation patterns are – the depression type and the excrescence
(projection) type. The basic forms are the spinate (spinulate), baculate,
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clavate, gemmate, verrucate and granulate among the excrescence types, the
reticulate (retipilate, foveolate, fossulate, scrobiculate), lophate, striate and
the rugulate among the depression types.
The excrescence type and depression type represent two parallel lines
of evolution with their possible genesis in morphoforms without any
ornamentation. The excrescence form and depression form of ornamentations
are the end products of the same developmental phenomenon and the result of
a secondary activity in exine organization. The primitive angiosperms have
pollen with more or less psilate pattern. Regarding the phylogeny of the
ornamentation pattern it has been argued that a character which would
serve to increase the protection of the protoplast of the spore or pollen
should be considered more primitive. The phylogenetic trend may be from
the psilate to the reticulate to the spiny to the echinate.
The taxa belonging to the order Myrtales presently investigated
possess various types of exine sculpturing. Although the taxa have both
depression type as well as excrescence type of exine ornamentation, the
depression type predominates in the majority of the taxa. The psilate or
smooth exine occurs in (7 species, 5 genera, 4 families), while the different
excrescence types, such as spinulate have been observed in one taxa only,
Myriophyllum tuberculatum, (Haloragaceae, which has been included in
Myrtales by Takhtajan, 1980); granulate in (27 species, 11 genera and 5
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families), while tuberculate in only one species of Myrtaceae. The taxa
belonging to the order exhibited a higher frequency of the depression type
sculpturing such as reticulate (5 species, 3 genera and 2 families); the
reticulate pattern is met with in only one member of Combretaceae
studied, (Lumnitzera racemosa) and four taxa of Myrtaceae (Barringtonia
species and Careya arborea, which were later on shifted to a separate
family Lecythidaceae); scrobiculate in one species of Rhizophoraceae;
punctate in 6 species and 4 genera of Rhizophoraceae; foveolate (9
species, 5 genera and 3 families); striate (14 species, 4 genera and 2
families); rugulate (33 species, 12 genera and 5 families); areolate (4
species and 3 genera and 2 families), the highest frequency of exine
sculpturing being the rugulate (depression type) followed by granulate
(excrescence type). The distribution of exine sculpturing patterns in the
South Indian taxa of Myrtales presently studied has been given in Table 5,
Fig. 5.
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131
132
5.1.3 Exine stratification
In certain mosses and ferns the sporoderm consists of perine, exine
and intine. The exine comprises an inner, usually homogeneous layer
(nexine), an outer layer (sexine) composed of more or less radial processes
and one or several layers (tegillum), lying more or less parallel to the
general surface of the nexine. The processes may be classified according to
their shape (verrucae, gemmae, baculae, pila, etc.). In certain cases
(bacula) they may have a root in the nexine, protrude as infrategillar
elements; penetrate (or in certain cases by amalgamation) form a tegillum,
continue (if there are two tegilla) as infrategillar elements penetrate the
upper tegillum, and come to an end with a distal suprategillar part. Certain
processes (spinae, spinulae) are generally borne on tegilla only. A tegillum
may be defined as a layer or (layers) formed whenever two or more
processes are united by the deposition of material upon and/or between
their distal parts. According to the areal extension of the tegilla (tegillum),
the sexine is generally striate, reticulate or tectate (areolate). When a
tegillum is present, the tegillum and everything connected with its outer
surface may be referred to as ectosexine, whereas the supporting bacula or
the layer that may be found in their place may be referred to as
endosexine. The nexine often seems to consist of an outer thicker and less
refracting part (ectonexine) and an inner, thinner, probably less resistant
more refracting part (endonexine). In some plants, e.g. Echinops, the outer
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layer (referred to as foot layer by Faegri) of the ectonexine differs from the
inner layer particularly with regard to the staining properties. The
endonexine can be distinctly seen as an individual stratum e.g. Epilobium
species.
Pollen wall enclosing the protoplasm of the grains consists of two
fundamentally different layers, the inner intine and the outer exine. The
intine is hyaline and completely covers the pollen protoplasm. The exine
covers the pollen surface excepting the germinal apertures where it is
absent or greatly reduced. The exine of pollen grains can be divided into
an inner homogeneous layer, endine (syn. nexine, Erdtman, 1952) and an
outer heterogeneous layer, the ectine (syn. sexine, Erdtman, 1952). The
ectine is composed of radial rods, the columellae, which are either free at
their tips or are united to form a layer called tegillum. It appears that the
lumina and spaces between the columellae provide storage space for exine-
bound substances, such as lipids and recognition proteins (Heslop-
Harrison, 1976), as in many pollen grains. Evolution of similar traits in
different lineages can be considered as evidence for convergent adaptive
change (Brooks and McLennan, 1991).
The diversity of pollen surface patterns has been interpreted as a
result of an adaptation for rapid germination, protection from desiccation
and harmomegathic mechanisms within the closely related taxa (Nowicke
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and Skvarla, 1979). In order to see if a correlation exists between pollen
sculpturing patterns and habitat within sect. Tithymalopsis, the tectal
variation and habitats of the species were mapped on the phylogenetic
tree proposed by the morphological data (Park, unpubl. data). The
resulting tree shows that the reticulate pollen grains have arisen twice
through independent evolution within section Tithymalopsis. In addition,
reticulate or microreticulate pollen grains are commonly associated with
dry, sandy habitats, because the foveolate patterns only appear in mesic
habitats.
The subject of exine stratification and its nomenclature has long been a
moot point in palynology. But, it has been very aptly dealt with by Faegri and
Iversen (1975), who distinguished between the massive and lamellate
endexine on the one hand, and the ectexine on the other. Ectexine most
often comprised a foot-layer and a tectum separated by columellae. As is
now well-known, it may also exhibit a granular structure below a more or
less obvious tectum. Endexine and ectexine, in Faegri's opinion, differed in
their ontogeny. In both gymno- and angiosperms the endexine is indeed
deposited on periclinal membranes that are apparently produced by the
plasmalemma, and share in tripartite structure (Rowley and Dunbar 1967,
Rowley and Southworth 1967, Dickinson and Heslop-Harrison 1968,
Willemse and Keznickova 1980). A review of most of the work on
endexine ontogeny has been presented by Nabli (1979) who himself
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studied it in several Labiatae. In angiosperms the membranes are short,
and in most cases sporopollenin accretion is such that they are no longer
apparent in the mature grain.
5.1.4 Pollen size and shape
Pollen and spore shapes and size do not apparently possess much
phylogenetic significance. Walker (1976); Le Thomas (1980, 1981), and
Walker and Walker (1984) argued that large boat-shaped, granular and
monosulcate pollen grains are the primitive type in angiosperms. Variation
in pollen size and shape are of less diagnostic value. However, this has
been shown to be of value in certaian instances, especially as an index of
aneuploidy (Nissen, 1950), and for correlating with chromosome number.
The value of pollen size in taxonomy has been emphasized in the
angiosperm family Rubiaceae (Mathew and Philip, 1983). The size of
pollen grains may be affected by the method of preparation and hence can
be rather an unstable character. Acetolysis is known to swell grains to
varying extents depending on the duration of the treatment (Reitsma,
1969). The angiosperm pollen exhibit a tremendous size range, from about
2 to 5µm in Myosotis (Boraginaceae) to over 300µm in Cymbopetalum
(Annonaceae). The pollen size classes are determined based on length of
the longest axis and are minute when less than 10µm, small (10-24µm),
(25-49µm) medium, (50-99µm) large, (100-199µ) very large and greater
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than 200 gigantic ones. Approximately a dozen angiosperm families have
species with pollen grains as large as or larger than 200µm.
Data of pollen size in the myrtalean families studied have shown wide
variation. In the order Myrtales pollen grains of the taxa of the constituent
families are generally medium sized with the exception of a few taxa with
large grains such as Lythraceae (Sonneratia, Punica, Lagerstroemia),
Myrtaceae (Barringtonia, Careya) and Onagraceae (Ludwigia, Fuchsia,
Trapa), while those in the herbaceous members of Lythraceae and members
of Myrtaceae are generally of smaller size. In some cases significant size
variation of pollen grain has been noticed in the same taxon. Pollen size
variation is exhibited by Lagerstroemia indica, L.speciosa, Rotala species
(Lythraceae), Osbeckia species (Melastomaceae) etc.
The shape of the pollen grain is unfixed and hence this character is
not generally considered as a reliable parameter in morphological
analysis of pollen (Nair, 1970). Pollen grain shape is correlated largely
to aperture type which in turn is correlated to polarity and symmetry.
The shape of the pollen grains varies from species to species.
Angiosperm pollen may be either nonfixiform (without definite shape)
or fixiform (with definite shape). Nonfixiform shape is rare in
angiosperms and occur in marine angiosperms such as Zostera, where
the pollen is long (2000µ) and thread-like. Normal fixiform angiosperm
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pollen is divided into two basic shape classes, boat-shaped and globose.
The boat-shaped pollen may be further divided into a number of
subtypes depending on the ratio of the longer equatorial axis to the
shorter.
The outline in polar view or amb, is circular, triangular, or in other
geometrical shapes while in the equatorial view the ratio between the
polar and the equatorial diameters multiplied by 100 gives an indication
of the shape. Based on the shape angiosperm pollen can be grouped into
nine classes such as peroblate grains, oblate, suboblate, oblate-
speroidal, spheroidal, prolate-spheroidal, subprolate, prolate and
perprolate grains (Walker and Doyle, 1975). Most of the different shape
categories were noticed among the myrtalean families except
perprolate. Generally the pollen is oblate or peroblate in Myrtaceae and
Cuphea species, while Careya, Barringtonia and Couroupita have
spheroidal to prolate shape; least frequency is observed in prolate and
spheroidal shape classes.
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Table 6. Distribution of pollen shapes in South Indian Myrtales
Pero
blat
e
Obl
ate
Subo
blat
e
Obl
ate-
sphe
roid
al
Sphe
roid
al
Prol
ate-
sp
hero
idal
Subp
rola
te
Prol
ate
Family
G S G S G S G S G S G S G S G S
Rhizophoraceae - - - - - - 1 1 - - 3 4 2 2 - -
Combretaceae - - - - - - 5 5 - - 3 6 1 1 - -
Myrtaceae 3 7 7 22 - - - - 1 1 2 2 1 1 1 1
Melastomaceae - - - - 1 4 2 3 1 1 6 10 3 8 - -
Lythraceae 1 2 1 1 - - - - - - 2 2 6 11 3 3
Onagraceae - - 2 3 1 2 1 1 1 2 1 1 - - - -
Data of pollen size and shape are made use of in constructing pollen
keys for taxa at various levels. The primitive shape of angiosperm pollen is
clearly boat-shaped, with globose pollen representing an advanced
character-state. The presently studied taxa of Myrtales exhibited much
variation with regard to pollen shape. All the six families of the order have
globose pollen grains with a range of shapes. With regard to the shape of
the pollen grains, Clarke (1980) has reported that in related species of
dicotyledons oblate grains are more advanced than the prolate ones. The
pollen possess peroblate, oblate (Myrtaceae and genus Cuphea of
Lythraceae), oblate-speroidal, prolate-spheroidal, spheroidal, subprolate
and prolate shapes in different families.The distribution of different shapes
139
of pollen in the South Indian Myrtales presently studied are given in Table
6 and Fig. 6.
Fig. 6. Distribution of pollen shapes in South Indian Myrtales
5.2 Pollen features in myrtalean families
A general scenario of the pollen morphology of the Myrtales has
been dealt in detail by Patel et al. (1984). The pollen grains in the core
families of the order seem to be basically 3-zonocolporate, and less
frequently two to more than four apertures have been reported. Further the
pollen grains are basically tectate and frequently characterised by pseudocolpi
(intercolpate furrows or “rugae” Erdtman, 1952). The pseudocolpi are not
actual apertures, but conspicuous colpus-like thin parts of exine. In their
early ontogeny, they differ from true apertures by not being subtended by a
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thick layer of intine. Pollen tetrads occur in some groups, especially in
Onagraceae, where another peculiarity is the occurrence of viscin threads.
Pollen data so far known in the group show that pseudocolpi are
present in families like Lythraceae, Oliniaceae, Combretaceae and
Melastomaceae, but are absent or very indistinct in many Lythraceae, (incl.
Punicoideae, Sonneratioideae and Duabungoideae), Trapaceae, Alzateaceae,
Psiloxylaceae, Heteropyxidaceae, Myrtaceae and Onagraceae. More or less
distinct intercolpate depressions in the pollen grains connect the distinctly
heterocolpate pollen grains with other types and make the feature
somewhat vaguely defined (Dahlgren and Thorne, 1984). However,
presence of pseudocolpi is so significant in the Myrtales, and so rare
outside the order, that it may be possible to attach great phylogenetic
significance to its distribution.
Distribution of pseudocolpi in the families of Myrtales presently
studied (Table 4 and Fig.4) show that the occurrence is most common in
Combretaceae followed by Melastomaceae and Lythraceae, while in a few
(Rhizophoraceae, Myrtaceae and Onagraceae) they are nonexistent.
The family Lythraceae shows great variation in the occurrence of
pseudocolpi (Patel et al., 1984). Lythraceae generally posses 3-colporate
pollen grains, several genera with pseudocolpi (as in Ammania, Rotala and
Nesaea in the present study). In Lythraceae, there are six pseudocolpi, two
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alternating with each aperture, as in Ammania, Rotala and Nesaea. A few
other genera lack pseudocolpi. Lawsonia (present study; Campos, 1964)
has incipient or rudimentary pseudocolpi. Those and the pollen grains of
Lafoensia, where the pseudocolpi are very faint, could be interpreted as
incipient or reduced. A study of the Lythraceae pollen morphology may
indicate that distinct or faint pseudocolpi seem to occur in derived genera,
as contented by Dahlgren and Thorne (1984). As far as variational
occurrence of pseudocolpi is concerned, Lythraceae are outstanding in the
order, as it may be argued whether absence of pseudocolpi represents an
original or advanced state.
Sonneratia and Duabunga have anguloaperturate pollen grains with
short colpi (Muller, 1969, 1978). In Sonneratia intercolpate depressions
resembling pseudocolpi occur. The pollen grains of Punica resemble those
of Lythraceae. They are 3 or rarely 4-colporate and lack pseudocolpi.
Whether the characteristic pollen grains of Trapa with their
meridional crests of folded exinous material meeting at the poles represent
a heterocolpate type or rather a type with pronounced intercolpate
depressions need to be ascertained. The heterocolpte pollen grain type
represents a distinct category in the order. They include very peculiar
shapes such as that in Oliniaceae where the pseudocolpi are restricted to
one hemisphere (Patel et al., 1984). It appears evident that the families or
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subfamilies with clear heterocolpate pollen grains form a coherent group
along with some others where pseudocolpi are absent or very indistinct
(Lythraceae, subfamily, Punicoideae, Duabungoideae and Sonneratioideae,
Combretaceae, subfamily Strephonematoideae and Alzateaceae) or where
pseudocolpi are “missing or doubled” in number (Lythraceae, subfamily
Lythroideae). None of the families outside Myrtales showing considerable
similarities to them possess pseudocopi.
A critical overview of the aperture scenario of the various myrtalean
families is highlighted as under:
Rhizophoraceae
The pollen morphological studies on the Rhizophoraceae, as a part of
floristic or general morphologic surveys has been done by (Erdtman, 1952;
Kubitzky, 1965; Huang, 1968; Bonnefille, 1971; Guers, 1974; Geh and Keng,
1974; Sowunmi,1973; Straka and Friedrich,1984; Thanikaimoni,1986, 1987).
Comparative palynology in Rhizophoraceae has focused on the mangrove
genus Rhizophora, primarily in connection with the recognition and study
of paleo-shorelines (Kuprianova, 1959; Langenheim et al., 1967;
Assemien, 1969; Rakosi, 1978; Sowunmi, 1981). Light Microscopic,
Scanning Electron Microscopic and Transmission Electron Microscopic
study on the pollen morphology of 39 species belonging to Anisophylleaceae
and Rhizophoraceae has been carried out by Vezey et al., (1988). SEM
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studies have been centred on the tribe Rhizophoreae (Tissot, 1979; Bertrand,
1983; Ludlow-Wiechers and Alvarado, 1983). Blepharistemma membranifolia
was examined by Murthy (1992); and the present study showed similar
results to that of the previous observations.
The Rhizophoraceae have tricolporate, radially symmetrical and
isopolar pollen in all the species presently studied. The shape of pollen
grains differ from subprolate (2 species), to prolate-spheroidal (4 species)
and oblate spheroidal (1 species). The colpi are long with smooth
membrane in (Rhizophora mucronata, Kandelia candel, Blepharistemma
serratum) and granular membrane in (R.apiculata, Bruguiera gymnorhiza,
B.cylindrica, Carallia bracheata). Endoaperture in the family has been
rectangular-lolongate in all the species studied, a feature not met with in
members of any other myrtalean families. Vezey et al., (1988) observed
fusion of the endoapertures in varying degrees which could be observed in
the species of Bruguiera and Rhizophora in the present investigation. The
exine sculpturing in the majority of the species has been punctate
(R.mucronata, Kandelia, Bruguiera, Blepharistemma), scrobiculate in
R.apiculata and faintly granular in Carallia. Thus the family appears to be
more or less homogeneous palynologically.
Light and ultrastructural data on Myrtales pollen (Patel et al., 1985)
are generally comparable to the data of Vezey et al., (1988). The majority
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of pollen morphological characteristics in Anisophylleaceae and
Rhizophoraceae occur in a broad range of families throughout the
angiosperms. It seems significant that none of these families could be
connected to Myrtales on palynological grounds.The colporoidate or fused
endoapertures possessed by Rhizophoraceae or Anisophylleaceae have no
counterpart in Myrtales. However, the pseudocolpi of Myrtales pollen do
not correspond to features of either Anisophylleaceae or Rhizophoraceae
pollen (Vezey et al., 1988).
Combretaceae
There are no modern detailed studies on the pollen morphology of
Combretaceae (Thanikaimoni, 1984). Earlier palynological work in the
family is scanty except that of Erdtman (1971) on Quisqualis indica as
having colpi alternating with “pseudocolpoid thin walled areas” and made
comparisons with, Cacoucia, Combretum and Terminalia. He also studied
Laguncularia racemosa, but did not mention about the presence or
absence of subsidiary colpi. Q.latialata was described as having three
pseudocolpi by Lobreau et al. (1969). Sowunmi (1973) described pollen of
Combretum glutinosum and four species of Terminalia in general as
resembling each other and possessing a characteristic shape, aperture
system and exine. All grains were noted to have “colpoid streaks” or
subsidiary colpi alternating with colpi. Guers et al. (1971) and Guers
(1974) also showed a fundamental similarity in the pollen of Combretum
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(6 species), Conocarpus erectus, Pteleopsis diptera and Terminalia (3
species). They reported colpi alternating with subsidiary colpi in all of
them, and the SEM of Combretum aculeatum was similar to that of
C.cacoucia.
From a comparative point of view, Erdtman (1971) held that pollen
grains more or less similar to Combretaceae occur in Melastomaceae, (also
Lythraceae and Penaeaceae). The grains in Haloragaceae, Hernandiaceae,
Myrtaceae, Punicaceae, Sonneratiaceae etc. are different. The SEM
observations of Patel et al. (1984) indicate several rather distinctive groups
possessing subsidiary colpi, or not, and other exine ornamentation
features. Their first group comprises taxa having colpi alternating with
subsidiary colpi possessing various surface patterns. The second group is
characterized by an echinate surface and subsidiary colpi. A third group
consists of a single genus with echinate surface, but lacks subsidiary colpi.
The fourth group also consisits of a single genus and have punctate surface
and lacks subsidiary colpi. The fifth group represented by a single genus
lacks subsidiary colpi and is different from the rest of the Combretaceae in
having a reticulate surface which has not been observed in any other
member of Myrtales core families (Patel et al., 1984). But such a
distinction could not be made out in the present study of 12 taxa. The
details of the pollen characters in the taxa studied here by and large
correspond to the first group of Patel et al. (1984).
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In the Combretaceae presently studied, pollen is tricolporate (rarely
tetracolporate as in Calycopteris floribunda), heterocolpate, radially
symmetrical and isopolar. The shape of the grains is prolate-spheroidal in
majority of the species (Terminalia catappa, T.chebula, T.elliptica,
T.paniculata, C.floribunda and Quiqualis indica) subprolate in (T.cuneata
and T.bellirica) and in a few species oblate-spheroidal (Anogeissus latifolia,
Lumnitzera racemosa, Combretum latifolium and Q.malabarica). The colpi
are long with acute ends and granular surface (T.catappa, Calycopteris) and
smooth surface (T.bellirica, Combretum and Quisqualis). Endoapertures are
lalongate in most of the species (10 species) and circular in two species
(T.catappa, T.elliptica). Mesocolpial extensions over the endoaperture have
been observed in T.catappa, T.elliptica and Calycopteris. The predominant
exine surface ornamentation met with in Combretaceae has been the
rugulate type (all species of Terminalia and Anogeissus), rugulate-
verrucate in Calycopteris, granulate in Combretum, reticulate in
Lumnitzera (a condition not met with in any other core Myrtales) and
psilate in Quisqualis.
Generalized comparison with the pollen of the Myrtales core families
agree with Erdtman’s (1971) suggestions. The pollen of the first group of
Patel et al. (1984) comprising six genera having colpi alternating with
subsidiary colpi showed resemblances with that of Penaeaceae and
Melastomaceae and some Crypteroniaceae, and the rest of the groups
147
showed resemblance to other Myrtales (Patel et al., 1984). The heterocolpate
pollen of Combretaceae show broad similarities to the heterocolpate members
of Melastomaceae observed in the present investigation of Myrtales such as
(Memecylon, Osbeckia, Tibouchina and Dissotis) which is in consensus
with the reports of Erdtman (1971).
Myrtaceae
In the extensive bibliography of myrtalean palynology, Thanikaimoni
(1984) lists only three references that allude to electron microscopy of
Myrtaceae pollen, making it obvious that SEM-TEM investigations are
greatly needed in the family. The most comprehensive taxonomic light
microscopic study of Myrtaceae pollen was that of Pike (1956) from the
Southwest Pacific area including 71 genera and 300 species, and McIntyre
(1963) examined pollen of 18 New Zealand taxa and found that most of
them could be recognized upto genera. Electron microscopic studies were
limited to that of Gadek and Martin (1981), who examined the pollen of 28
species in seven genera of subtribe Metrosiderinae with Light Microscope
and SEM. They found a greater range of pollen morphology within the
family than was heretofore recognized, and in some instances pollen could
be identified to the generic and specific levels. Transmission Electron
Microscopic studies were carried out by Lugarden and van Campo (1978)
and Gadek and Martin (1982). Some studies on pollen morphology and
wall organization have been done in a few species of Eucalyptus (Gadek
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and Martin, 1982; Patel et al., 1984; Heslop Harison and Heslop Harrison,
1985; Zhou and Heusser, 1996; Pickett and Newsome, 1997). The
ultrastructure and cytochemstry of the pollen of E.globulus has been
studied in detail (Eliseu and Dinis, 2008) and the pollen is 3-
parasyncolporate with rugulate mesocolpia, and psilate apocolpium and
margo. In E.globulus (Eliseu and Dinis, 2008) and in E.rhodantha (Heslop
Harrison and Heslop-Harrison, 1985) the intine is three-layered under the
apertures forming a complex oncus. The outer layer is pectic and compact,
and according to Heslop-Harison and Heslop-Harrison (1985, 1991) is
bordered by a cap or operculum that represents an extension of the foot
layer of the ektexine. The same was reported by Gadek and Martin (1982)
who considered the remnant of the foot layer to form a colpus membrane.
In general, Myrtaceae pollen is tricolporate (triporate in Tristania
nereifolia), radially symmetrical and isopolar or heteropolar, and in lateral view
the pollen is oblate-peroblate, elliptic with obtuse or truncate sides (Reitsma,
1970). In polar view it is triangular, goniotreme with straight or curved sides
and with acute or obtuse corners. Colpi vary in length. In the taxa studied
presently, the number of apertures varied from three to four as in Psidium,
Melaleuca, Callistemon, Pimenta, Rhodomyrtus, S.calophyllifolium,
S.salicifolium, S.cumini, S.samarangense, S.hemisphericum, S.malaccensis,
S.aromaticum and S.jambos and three, four or five in P.cattleianum. In polar
view it is triangular with straight or curved sides and with acute or obtuse
149
corners. Colpi vary in length, and are either syncolpate or parasyncolpate,
and generally have smooth surface. Endoapertures are lalongate.
Intercolpar concavities are present in species of Callistemon, Eucalyptus,
Syzygium jambos and S.occidentalis. Heteropolar grains are due either to
the nature of the colpi (e.g., long colpi on one pole and syncolpate on the
other pole) of the two polar faces as observed in Psidium guajava or to the
different shapes (concave, convex, straight) of the two polar faces. In
Eugenia capuli and in some grains of Luma chequen and Ugni molinae,
one polar face is concave (or straight, in Chamaelaucium) and the other
face convex. The pollen is free except in Myrtus communis and Psidium
littorale where tetrahedral tetrads are present along with monads (Patel et
al., 1984). Operculate grains have been observed in Rhodomyrtus (present
study).
Based on the nature of colpi, Pike (1956) recognized three pollen
types in the Myrtaceae: (1) longicolpate grains; (2) syn- or parasyncolpate
grains, and (3) brevi- or brevissimicolpate grains. All these types are
present in the taxa examined in the present study.
1) Longicolpate type: here the colpi are long; “colporate grains are
longicolpate, when the colpi are longer than the distance between
their apices and the poles” (Pike, 1956). Hypocalymna angustifolium,
Myrceugenella apiculata and Ugni molinae belong to this group. The
150
pollen shape is also similar in Myrceugenella and Ugni: triangular
grains with convex side and slightly protruding apertures in polar
view and an elliptic shape with acute ends in lateral view. The
surface in Myrceugenella is verrucate-granular with some larger
verrucate elements at the poles. Ugni shows unique surface pattern
with multiangular units that have acute corners. These units are large
and scattered at the poles, but become smaller and more tightly
packed toward the equator. The surface appeared granular around the
endoapertures (Patel et al., 1984). In the present study longicolpate
grains have been observed in Psidium, Rhodomyrtus and Pimenta.
The exine surface was rugulate in Psidium; granulate in Rhodomyrtus
and areolate in Pimenta. The shape of the grains was triangular with
acute corners; the colpus narrow with a smooth surface.
2) Syncolpate and parasyncolpate type: In syncolpate grains, the colpi
are either straight, meeting at the poles without becoming wider, or
curved where they are wider at the poles and form a triangular area.
In parasyncolpate grains, the colpi bifurcate at the poles and their
branches meet and outline a triangular apocolpium. Some grains are
syncolpate on one pole and parasyncolpate on the other pole while
others also combine with longicolpate grains and result in long/syn/
and long/para forms. These combinations were reported in Myrtus
communis, Psidium littorale, Eucalyptus ficifolia, E.robusta,
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Heteropyxis natalensis, Temu divaricatum and Eugenia elliptifolia
(Patel et al., 1984). In the present investigation, pollen was
longicolpate on one pole and syncolpate on the other pole has been
observed in Psidium guajava. Syncolpate grains have been observed
in Callistemon, Eucalyptus, Melaleuca, Eugenia uniflora, Syzygium
benthamianum, S.rubicundum, S.gardneri, S.calophyllifolium,
S.travancoricum, S.salicifolium, S.cumini, S.samarangense,
S.aqueum, S.malaccensis, S.laetum, S.macrosepala and S.occidentalis
and parasyncolpate in Pimenta, S.zeylanicum, S.hemisphericum,
S.jambos, and S.aromaticum.
The taxa included in the syncolpate and parasyncolpate type are
further grouped according to the presence or absence of intercolpar
concavities. Pollen with clearly defined intercolpar concavities have
been observed in Acmena smithii, Callistemon citrinus,
C.teretifolius, Calothamnus validus, Eucalyptus ficifolia, E.robusta,
Heteropyxis natalensis, Melaleuca hypericifolia, M.raphiophylla,
Tristania conferta and T.lactiflua while the “depressions” are not as
markedly depressed as in the above mentioned taxa in Eugenia
elliptifolia, Metrosideros nervulosa and M.polymorpha. Melaleuca
hypericifolia has a verrucate-granular surface in the intercolpar
concavities, and the surrounding areas are scabrate. In Callistemon
teretifolius, C.citrinus and Heteropyxis the surface of the mesocolpia
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is rugulate, and it is rugulate-verrucate in the intercolpar concavities
(Patel et al., 1984). Syncolpate pollen with intercolpar concavities
has been observed in the present study in Callistemon, E.tereticornis,
S.jambos and S.occidentalis; exine surface ornamentation was
granulate; and the exine surface ornamentation being different in the
intercolpar concavities and in the rest of the mesocolpium.
In pollen without intercolpar concavities the surface is more or less
uniform over the entire surface of the grain. The surface in Myrtus
communis, Psidium littorale, Eremaea pauciflora, Melaleuca
preissiana and M.decussata the surface is granular-verrucate-
rugulate. There are fine, branched, irregular channels separating the
surface elements (Patel et al., 1984). Pollen without intercolpar
concavities occur in the majority of the taxa presently studied. The
surface in Luma chequen, Pilidiostigma glabrum and Rhodamnia
argentia is verrucate-granular. In Luma, the colpi are curved and
form a large thin walled triangular area at the poles that has
irregularly scattered verrucae and granules on it. In Pilidiostigma, at
the parasyncolpate pole, scattered granules form a triangular
apocolpium. The surface is smooth near the endoapertures in both the
taxa. In Rhodamnia larger verrucate elements occur along the
margins of the mesocolpia, even near the endoapertures (Patel et al.,
1984). The exine surface in the taxa studied is variable; it is psilate
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(Psidium cattleianum, Syzygium rubicundum, S.gardneri,
S.caryophyllatum), granular (Callistemon, Eucalyptus, Rhodomyrtus,
Syzygium zeylanicum, S.salicifoliu, S.hemisphericum, S.jambos,
S.macrosepala and Couroupita) areolate (Pimenta, S.salicifolium,
S.aqueum), foveolate (Melaleuca), macroreticulate (Barringtonia,
Careya), rugulate-verrucate in Eugenia uniflora or rugulate (Psidium
guajava, Eugenia mooniana, Syzygium benthamianum,
S.calophyllifolium, S.samarangense, S.malaccensis, S.aromaticum,
S.mundagam).
3) Brevicolpate type: In the brevicolpate pollen, the length of the colpi
is equal to or less than the distance between their ends and the poles.
In the brevissimicolpate grains, the colpus length is less than that of
underlying endoaperture (Erdtman, 1971). Chamaelaucium
uncinatum has brevissimicolpate grains (Patel et al., 1984).The
brevicolpate grains occur in Eugenia mooniana (present study) and
the surface sculpturing is rugulate-verrucate.
In the present study, endoapertures are faint in most of the species
(17), lalongate in nine species and lolongate in four species. Exine
surface ornamentation in the Myrtaceae studied have been different;
psilate pattern has been observed in four species, granular in 12
154
species, foveolate in one, rugulate in eight species, areolate in three
species, macroreticulate in four species and rugulate-verrucate in two.
In Barringtonia, Careya and Couroupita of Myrtaceae in the present
study, the pollen is 3-zonoaperturate, and the pollen shape varies from
prolate, prolate-spheroidal to spheroidal. The pollen grains of
Barringtonia, B.acutangula and B.racemosa are 3-zonosyncolpate and
large in size; but B.asiatica and Careya has 3-zonosyncolporate grains
with lolongate endoaperture. In Couroupita guainensis 3-zonocolporate
grains have been observed which have comparatively smaller size. In
Careya the aperture membrane bears two rows of gemmate-verrucose
sculptural elements flanking the underlying endoaperture (apertural
verrucae). The apertural verrucae in B.asiatica are less prominent. In
B.asiatica and Careya the endoapertures are bordered by marginal ridges
which in turn are having marginal grooves to the mesocolpial side. In the
polar areas the exine is thickened forming the polar cushions. The exine
surface ornamentation in Barringtonia and Careya has been
macroreticulate, while in Couroupita it is minutely granulate.
The three genera (Barringtonia, Careya and Couroupita) have pollen
quite different from the rest of the Myrtaceae as well as other core
Myrtales. These three genera itself may be categorized into two groups
based on pollen morphology: the first group including Barringtonia and
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Careya with comparatively larger pollen, macroreticulate exine sculpturing
and syncolpate or syncolporatae pollen; the second group in the present
study includes only Couroupita with smaller grains, 3-zonocolporate
aperture and granular exine sculpturing. The first group including the
species of Barringtonia and Careya and the second group (Couroupita
only in the present study) is comparable to the Planconia type and
Lecythis type proposed by Erdtman (1952).
Although the pollen of Myrtaceae is essentially uniform, in some
taxa minor differences make it possible to recognize particular genera or
species. Within the family there is no particular feature that separates
pollen of the Myrtoideae from that of Leptospermoideae, but the pollen of
Chamaelaucieae differs markedly from that of all other tribes in the family
(Pike, 1956). Within a tribe the taxa are usually similar, but it is possible
for closely related genera to have quite distinct pollen. On the other hand
pollen of widely separated genera may show some similarities. Grains of
the different species of the same genera are usually indistinguishable or
they may be similar in general features but show a comparatively large
range in size.
The Myrtaceae pollen does not appear to have any close similarities
to taxa from the other core families of the Myrtales. Some superficial
similarity exists with Onagraceae pollen (through the shape of the grains
156
as well as the short colpi in some members of the two families) but the two
are clearly recognizable. Some similarity has been observed between the
pollen of Myrtaceae and Lythraceae as suggested by Erdtman (1971)
through Cuphea, but more studies are required to establish the
relationship. Pollen of Lecythidaceae and Sapindaceae has been examined
using SEM (Muller, 1972, 1973; Muller and Leenhouts, 1976; Mori et al.,
1980) and are not similar to Myrtaceae pollen. However, in Sapindaceae
there are several taxa with triangular, parasyncolpate grains (Muller and
Leenhouts, 1976).
Melastomaceae
The pollen morphological studies in the Melastomaceae have not
gone beyond the light microscopic level before Guers (1974) who
examined Osbeckia decandra, Dicellandra barteri and Calvoa orientalis
electron microscopically, which was part of a light microscope study of
eight genera and twenty species from tropical Africa. Of these Dicellandra
and Calvoa compare favourably with the second group of pollen in
possessing intercolpar concavities and O.decandra with the first pollen
group having heterocolpate grains with subsidiary colpi of Patel et al.,
(1984). Earlier palynological studies of Melastomataceae have been
undertaken by Erdtman (1952), Nair (1965b), Huang (1972) and Patel et
al. (1984). Detailed pollen morphological investigations in relation to
taxonomy were initiated by Erdtman (1952). He stated that Melastomataceae
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pollen grains are prolate-spheroidal, have three ori (orial) colpi and three
pseudocolpi, with the sexine as thick as the nexine or slightly thinner and an
obscure pattern. He also suggested that the pollen morphology did not
support the opinion that Memecylon and related genera should be referred
to a separate family and that the pollen grains were similar to the pollen
grains of Combretaceae and Lythraceae, but not to Myrtaceae.
A more detailed paper on this family was produced by Patel et al.
(1984) in which the pollen morphology of 19 genera was studied. The
pollen grains are generally monads although tetrads and polyads occur in
Miconia melanotricha and Tococa spadiciflora. The grains are
tricolporate, radially symmetrical, isopolar and spheroidal to subprolate in
equatorial view with a circular, hexagonal or triangular shape in polar
view. The exine sculpturing in the mesocolpia is variable, striate and
rugulate. They also suggested that the Melastomataceae pollen can be
divided into three pollen groups; heterocolpate with subsidiary colpi,
heterocolpate with intercolpar concavities and tricolporate.
In the heterocolpate group with subsidiary colpi, the three colpi (four
in Votomita) alternate with three (four in Votomita) elongate, narrow
subsidiary colpi. This type of pollen is present in (Trembleya flogiformis,
Tibouchina urvilleana, Tristemma littorale, Dissotis brazzae, Marumia
nervosa, Dissochaeta celebica, Osbeckia polycephala, Acanthella sprucie,
158
Memecylon normandii, Mouriri glazioviana, Votomita monadelpha,
Miconia hondurensis, M.alypifolia, M.caesia, Comolia stenodon and
Tococa broadwayi). The surface sculpture is variable; smooth, punctate,
rugulate-punctate, verrucate-rugulate, or striate (Patel et al., 1984).
Colpi are long and some grains are syncolpate in Dissotis, Acanthella
and Dissochaeta with acute ends and a smooth surface or the surface is
granular. Endoapertures are lalongate and elliptic. Extensions of the
mesocolpia over the endoapertures are present. A horizontal bar is often
persistent over the open endoapertures (Patel et al., 1984).
In the second group “heterocolpate” with intercolpar concavities,
three intercolpar concavities - large elliptic, thin-walled, depressed areas
are present on the mesocolpia. The remaining thick-walled portion of the
mesocolpia forms a more or less narrow band around the intercolpar
concavities (Adelobotrys tessmannii, Allomorphia caudata, Bredia hirsuta,
Astronia cumingiana, Oxyspora paniculata and Miconia melanotricha).
The grains are spheroidal in lateral view and circular to triangular or
hexagonal in polar view. Colpi are long and narrow with acute ends and a
more or less smooth surface. Extensions of the mesocolpia are present over
the lalongate-elliptic endoapertures. The surface of the meso and
endocolpia is variable; it is psilate, striate-rugulate, fine rugulate-verrucate,
159
smooth-punctate, coarse with many channels and pits, verrucate or
granular-verrucate (Patel et al., 1984).
The third group, tricolporate is represented by Tococa stephanotricha
and T.formicaria, which have a rugulate surface and very short colpi.
Another species of Tococa, T.spadiciflora could not be assigned to any of
the three groups described above. It consists of polyads composed of basic
units of tetrahedral tetrads. Internal bridges on the proximal faces of
individual pollen grains maintain tetrad unity, while the external bridges
on the distal faces of the tetrads, frequently along the aperture margins,
maintain polyad unity (Patel et al., 1984).
Pollen grains of Melastomaceae in the present investigation are
tricolporate in all the taxa studied except in Clidemia and Dissotis where it
is tri and/or tetracolporate, radially symmetrical and isopolar. Grains are
prolate-spheroidal (10 species), subprolate (9 species), oblate-spheroidal (3
species), suboblate (3) and spheroidal (1 species). All species of
Melastomaceae studied are heterocolpate with subsidiary colpi except one
(Medinilla) in which intercolpar concavities occur. On the basis of pollen
morphology the grains of the family could be classified into two groups
such as, heterocolpate with subsidiary colpi which comprise all the taxa
studied except Medinilla and heterocolpate with intercolpar concavities
consisting of Medinilla in the presently studied taxa.
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In the heterocolpate group with subsidiary colpi, three colpi alternate
with three elongate, narrow subsidiary colpi. Surface of the colpus is
granular in Sonerila species while it is smooth in Osbeckia, Dissotis,
Melastoma, Memecylon and Tibouchina. Pollen grains seem to be
operculate in M.malabathricum and Memecylon. The colpi are very long
and almost meet at the poles in Osbeckia and Melastoma. The endoaperture
is lalongate in all the species studied except Memecylon flavescens where it
is circular, and faint in M.grande. Mesocolpial extensions over the
endoapertures were observed; and often it forms a horizontal bar over the
endoaperture in (Osbeckia, Sonerila and Tibouchina).
The surface of the meso- and apocolpia is variable; psilate in the
mesocolpium and granulate in the intercolpar concavities in Medinilla;
foveolate in (O.aspera var.aspera, O.courtallensis, O.octandra,
O.leshenaultiana, O.cupularis, M.flavescens), rugulate-fossulate (O.aspera
var.travancorica, O.reticulata), rugulate in (O.wynadensis, Dissotis),
granulate-rugulate (O.parviflora), matted in Melastoma, striate in all
species of Sonerila studied and granulate in all species of Memecylon
studied (except M.flavescens), Clidemia and Tibouchina.
The ornamentation can be used for the identification of pollen grains
at the type or subtype levels. At the LM level the pollen morphology of
Melastomaceae is in agreement with the observation of Patel et al. (1984)
161
which indicates that pollen grains with subsidiary colpi or with intercolpar
concavities are not important for group identification. The range of
variation in Melastomataceae pollen is pronounced. It is, therefore, not
possible to separate the genera (of Thai Melastomataceae) using pollen
morphology except for one genus, Pternandra. However, all grains studied
can be divided into three groups by LM and five types using both LM and
SEM (Chantaranothai, 1997).
Generalized comparisons with pollen from the core families of the
Myrtales show that pollen of the Melastomaceae, Combretaceae and
Lythraceae have a number of features in common and indicate that these
three families are closely related. However, Melastomaceae pollen is more
like that of Combretaceae than of Lythraceae. On the basis of pollen
morphology there is no evident difference between the subfamily
Memecyloideae and the others and so this study follows Erdtman (1952) in
keeping this subfamily in the Melastomaceae. Palynological evidence does
not confirm the tribal and subfamily classification except in the case of the
genus Pternandra in the tribe Kibessieae and subfamily Astroniodeae,
according to Clarke (1879), Krasser (1893) and Hutchinson (1969).
Among the genera examined by Patel et al. (1984) Mouriri, Votomita
and Memecylon have been segregated with several other genera from
Melastomaceae sensu stricto as Memecylaceae or subfamily Memecyloideae
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(eg.Dahlgren and Thorne, 1984; Johnson and Briggs, 1984). All three of
these genera belong to the heterocolpate group, but are not delimited as a
group in anyway fom the other heterocolpate genera of Melastomaceae,
although Votomita is unusual in having four colpi and subsidiary colpi, and
a bead-like colpus surface. Further studies on this group are essential to a
proper understanding of the relationships between Melastomaceae sensu
strico and Memecyloideae. Memecylon belong to the group heterocolpate
with three subsidiary colpi and does not show enough pollen morphological
evidence in favour of its segregation from Melastomaceae sensu stricto.
Within the limited context of this study the heterocolpate pollen with
subsidiary colpi show some similarities to other core families. At the SEM
level some resemblance is evident with Combretum, Bucida, Conocarpus,
Pteleopsis, Terminalia, Ramatuella, Guiera, Poivera and Lumnitzera (Patel
et al., 1984). In the present investigation also resemblance is observed in the
pollen grains of Melastomaceae and Combretaceae in the grains possessing
heterocolpate pollen with subsidiary colpi. Also Calycopteris floribunda
(Combretaceae) and Clidemia hirta and Dissotis rotundifolia
(Melastomaceae) have 3, 4-zono heterocolpate pollen grains.
Lythraceae
The available pollen morphological data including the present
findings indicate that Lyhraceae have the most diverse pollen morphology
163
in the Myrtales. Much of the diversity concern the aperture system for e.g.
tricolporate grains is the dominant condition exhibited by species
belonging to several genera such as Physocalymma, (Cos Campos, 1964),
Rotala (Guers, 1970), Heimia (Graham, 1977), Adenaria, Pleurophora,
Galpinia (Erdtman, 1971) and Diplusodon (Muller, 1981) and also in
members of several genera presently studied such as Rotala, Lawsonia,
Lagerstroemia, Woodfordia and Punica, heterocolpate grains with
isomerous subsidiary colpi are reported in Lythrum (Cos Campos, 1964;
Guers, 1970; Huesser, 1971) and also heterocolpate with additional
subsidiary colpi in three genera (Rotala, Ammania, Nesaea). The same
condition was reported in a few genera by Erdtman (1971) and Muller
(1981). Grains with three meridional ridges that alternate with apertural
fields are reported in Lafoensia, Crenea and Lagerstroemia (Muller, 1981)
and also in Lagerstroemia indica and L.speciosa presently observed.
The family is a palynologically interesting group in the order
Myrtales as evidenced from the present study as well as earlier reports on
the pollen morphology of Lyhraceae by Cos Campos (1964), Graham et
al. (1968), Guers (1970), Erdtman (1971), Graham and Graham (1971),
Graham (1977), Muller (1981) and Graham et al., (1990). Much diversity is
exhibited in the aperture structure of the members. Most of the genera have
tricolporate grains (Rotala, Woodfordia, Lawsonia, Lagerstroemia
microcarpa and Punica), heterocolporate grains with six subsidiary colpi
164
have been observed in (Rotala, Ammania and Nesaea) while grains with three
meridional ridges alternating with apertural fields (Lawsonia, L.speciosa,
L.indica), syncolporate grains (Cuphea) and triporate grains in Sonneratia.
4-apertured grains have been noticed along with 3-apertured ones in L.indica,
C.ignea, R.malampuzhensis while R.densiflora have exclusively 4-apertured
grains, and this is in consensus with the report by (Graham, 1990). The
Lythraceous pollen morphology may indicate that pseudocolpi seem to occur
in derived genera. The family is outstanding in the order, and it may be
argued whether absence of pseudocolpi represents an original or, by
secondary loss an advanced state (Dahlgren and Thorne, 1984).
Pollen morphological diversity is also evident in the shape of the
pollen grains. Majority of the grains have prolate and subprolate shape
while in Cuphea the shape is oblate to peroblate. Size of the grains also
show variation as large grains occur in (Sonneratia, Lagerstroemia and
Punica) in contrast to the small to very small grains in Ammania, Rotala,
Nesaea etc. Exine ornamentation also showed much variation from psilate
(Cuphea), pebblate (Woodfordia), rugulate (Lagerstroemia, Lawsonia),
rugulate-verrucate (Sonneratia), striate (Cuphea, Nesaea, Rotala) and
granulate (Punica).
Pollen grains of Sonneratia are triporate with meridional ridges
alternating with apertural fields and polar caps as has been observed by
165
Muller (1969, 1978). The three well-developed meridional ridges alternate
with three apertural fields. The apertural field has a protruding pore in the
centre. The surface is verrucate in the apertural fields, verrucate-rugulate
on the ridges and the polar caps are psilate. In addition to the three large
ridges, six smaller ridges are present, two in each apertural field. The
meridional ridges and smaller ridges are prominent in S.alba and not
observed in S.caseolaris. In the present study the pollen of S.alba observed
resembles S.caseolaris of Patel et al. (1984) (i.e. S.alba of Muller, 1978)
in contrast to their reports that their “SEM results show that at least
superficially, S.caseolaris appears more similar to Muller’s SEM of S.alba
than to his SEM of S.caseolaris. However, it should be noted that the
similarity is observed in the well-developed meridional ridges and
apertural fields in S.alba of the present study and Muller’s S.alba”.
Pollen diversity at the infrageneric level has been documented in the
genus Cuphea in which wide variation has been reported with pollen
ranging from basic tricolporate-spheroidal to tricolporate-syncolporate-
oblate, triangular (Graham et al.1968; Guers, 1970). The species of the
genus presently studied exhibited tricolporate-syncolpate-oblate, triangular
grains. Starting with a basic oblate, oblate-tricolporate, striate, tectate grain
the genus Cuphea was shown to be remarkably eurypalynous with great
variation at sectional, subsectional, specific and varietal level. Moreover
166
Cos Campos (1964) have described multiple pollen types produced by
individual plants or anthers in the genus.
A comprehensive study by Lee (1979) including 26 genera of the
family has reported subsidiary colpi and three major pollen groups in the
family – (1) 3-pseudocolpate – Lythrum, Pemphis, Peplis, Rhynchocalyx;
(2) 6-pseudocolpate – Ammania, Capuronia, Crenea, Lagerstroemia,
Lawsonia and Rotala; of these, the same condition was noticed in
Ammania and Rotala; and 3) nonpseudocolpate in Adenaria, Alzatea,
Cuphea, Decodon, Didyplis, and Heimia.
In his study of the Lythraceae, Muller (1981) examined pollen grains
of Crenea, Diplusodon, Lafoensia and Lagerstroemia as well as
Sonneratia and compared the results on a harmomegathic functional
standpoint, and several structural pollen types were established, starting
with a tricolporate longiax prototype. He has postulated that the relation
between pollen form and function was indicative of adaptive radiation in a
few directions such as (1) there is a trend towards increasing the number of
colpi (i.e. subsidiary colpi), (2) harmomegathic functions are transferred
from individual colpi to flexible apertural fields alternating with
meridional ridges, (3) harmomegathic function is transferred to prominent
poles and (4) harmomegathic function is lost in the ecto- and endo-apertures.
In his concept of usefulness of pollen morphology, Muller (1981) has
167
emphasized the need for ecologic interpretations of function and for detailed
ultrastructural studies to uncover those characters which reflect ancient
phylogenetic links. He has held that this can best be illustrated by considering
the possible affinities between the genera Lafoensia, Lagerstroemia, and
Sonneratia assuming that the latter may be placed in the Lythraceae. If,
convergences in the recent pollen morphology is stressed Lafoensia will be
considered closely related but, the differences in ultrastructure and
heterocolpate nature in some pollen types of this genus would argue against
affinity with Sonneratia. He has further pointed out that if fossil evidence is
taken into consideration, the genus Lagerstroemia appears a much stronger
candidate, although its present-day type show less similarity with a living
Sonneratia type. In a more general sense, Lafoensia is considered to be
similar to Sonneratia of subfamily Sonneratioideae (Muller, 1981);
Diplusodon similar to Duabunga of subfamily Duabungoideae; while
Lagerstroemia bears resemblance to Punica of subfamily Punicoideae. These
similarities would strongly support the view of Dahlgren and Thorne (1984)
that Sonneratia, Duabunga and Punica should be regarded as separate
subfamilies independently related to Lythraceae.
Onagraceae
Onagraceae pollen has been the object of studies using light
(Mitroiu, 1961-1962; Ting, 1966; Brown, 1967) and electron microscopy
168
(Skvarla et al., 1975, 1976, 1978). These studies have shown that the
pollen of the 17 genera and approximately 650 species possess distinctive
features such as viscin threads, a spongy paracrystalline ektexine, apertural
protrusions and a large central body; tetrads and /or polyads in certain
groups; and an exine surface composed of circular, globular, and elongate
or rod-like elements.
The pollen of Onagraceae studied here have 3-zonoaperturate
condition as the predominant feature with variations in aperture number
such as 2 (Fuchsia arborescens), 3, 4 and 5-aperturate (F.fulgens). The
grains are 3-colpate in Trapa, brevicolpate in Ludwigia and porate in
Fuchsia. In all the taxa of Onagraceae presently studied the pollen have
certain distinctive features such as a large central body and apertural
protrusions; viscin threads have been observed in L.adscendens,
L.suffruticosa, L.peruviana and F.fulgens; Viscin threads have different
surface patterns: smooth in L.peruviana, L.adscendens, F.fulgens twisted
in L.suffruticosa; pollen tetrads occur in L.perennis. L.prostrata and
L.suffruticosa, and polyads in L.peruviana.The exine surface has rugulate
ornamentation in all the species of Ludwigia studied except L.adscendens
where it is areolate, and granulate in Fuchsia and Trapa.
Trapa is distinguished by the possession of 3-zonocolpate grains with
three prominent meridional ridges on the grain and apertures that are
169
protruding and swollen as elongated domes. The lens-shaped colpus is
covered by the meridional ridge which is formed by the folding of
ektexine, and they are united at the poles to form a greatly enlarged
triangular base. The ridges enclose a cavity also. The extremely thick
endexine, surface sculpturing and protruding apertures of Ludwigia pollen
resemble Trapa in a remote sense although there are no difficulties in
recognizing both. Trapa thus bears unique features that are quite different
from the rest of the Onagraceae presently studied, and its separation from
Onagraceae into a distinct family could be justified on palynological
grounds.
The Onagraceae pollen is distinctive in the order by considering the
extremely thick endexine, surface sculpturing, and the protruding apertures
and the pollen of Ludwigia resemble that of Trapa in a remote sense. The
short colpi of Hauya, Gongylocarpus and Boisduvalia (Patel et al., 1984) and
Lopezia (Skvarla et al., 1976) and the brevicolpate condition in Ludwigia in
the present study bears a distant similarity with that of Myrtaceae. Meridional
ridges observed in Ludwigia pollen, L.alternifolia, (Patel et al., 1984) and
L.adscendens and L.peruviana of the present study are unique and should not
be confused with other taxa in the Myrtales possessing meridional ridges.
Most genera of Onagraceae shed their pollen exclusively as monads
but some shed their pollen as tetrads or polyads. Pollen tetrads are
170
characteristic of Boisduvallia, most species of Epilobium and some species
of Camissonia and Ludwigia. Presence of viscin threads, as noticed in
Ludwigia, L.adscendens, L.suffruticosa and L.peruviana and Fuchsia
fulgens in the present study has been reported earlier in several taxa of the
family by Patel et al. (1984). These are extensions of the exine surface
which has different surface patterns like smooth or segmented, tightly
compound-twisted and incised-compound (present study and Patel et al.,
1984). According to Skvarla et al. (1978) some of the patterns are difficult
to categorise and may represent intermediate or transitional forms. For
example the obliquely inclined-segmented-beeded threads as observed in
Fuchsia by Patel et al. (1984) or as mycelial threads (present observation).
It is considered that the viscin threads are an integral part of the pollen
grain wall, namely the ektexine as documented by Skvarla et al. (1978)
and Nowicke et al. (1979). There are different interpretations regarding its
relationship with the exine such as “an attachment point” indicating a
specific area on the exine surface where the threads emerge, or it is not an
inherent part of the exine, but as a later addition. For avoiding these
confusions, a different term in the place of “attachment point” is used
namely, emergence area (Patel et al., 1984). Structures similar to viscin
threads in Onagraceae are known in other families like Ericaceae (Skvarla et
al., 1978), Leguminosae (Graham et al., 1980). Later study of Patel et al.
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(1985) interpreted these threads ends attached to different pollen grains
indicating that they are exinal connections rather than viscin threads.
Sculpturing of exine surface is variable in the family. Taxa studied
here had rugulate, heteromorphic-rugulate, areolate and granulate exine
ornamentation types.
Other surface features of significance in the family are long colpi and
prominent ridges in Ludwigia as reported by Patel et al. (1984) as against
short colpi in the present study of the genus. Prominent meridional ridges
are of two types such as those occurring on the polar faces and extend to
the equator in the area midway between the apertural protrusions, and
lateral ridges which occur between the apertural protrusions which are
joined with the meridional ridge. Those noticed in the present study is of
the former type.
Trapa is a small genus with three species accommodated in the
Onagraceae of sensu Bentham and Hooker (1865) of which one species,
T.natans has been studied here. It is distinguished by the possession of 3-
zonocolpate grains with three prominent meridional ridges on the grain
and apertures, that are protruding and swollen as elongated domes. The
other species of the genus, namely T.japonica, was studied by Patel et al.
(1984); almost similar pollen morphological features were observed except
the better developed domes.
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Structurally the meridional ridge of Trapa is different from that
found elsewhere in the order. In contrast, the ridge formed in Lythraceae
(Lagerstroemia and Lawsonia) and Onagraceae (Ludwigia) are formed by
the increase in thickness of the exine. Moreover, the ridge passes over the
colpi in Trapa, whereas in others it alternates with the colpi. The
distinctiveness of Trapa pollen was recognised by Erdtman (1971) who
examined the species and felt that the genus merited family status. Trapa
pollen shows a distant resemblance to Onagraceae in surface sculpturing
and in the nature of protruding apertures. Similarities to Onagraceae are
further evident in the very thick endexine and indistinguishable or at least
very thin foot layer.
Haloragaceae
The pollen of Myriophyllum tuberculatum of the Haloragaceae has
been studied as this family has been included in the Myrtales order by
different classificatory treatments such as (Emberger, 1960; Melchior,
1964; Soo, 1967; Takhtajan, 1980) etc. The pollen grains are isopolar,
radially symmetrical, porate, aspidote and crassimarginate, and the exine
sculpturing spinulate. These features seem to be deviating from that of the
core Myrtales especially the spinulate ornamentation. The porate, aspidote
aperture, though present in Onagraceae, the Onagraceae pollen do have a
number of distinctive features such as thick endexine, large central body,
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viscin threads etc. quite different from the Myriophyllum (Haloragaceae)
pollen and stands out from the other Myrtales.
5.3 Morphological evolution of pollen grains
A perusal of the literature of the pollen morphology of angiosperms
shows that schemes of evolution of palynological characters have varied
with individual workers. Of the different morphological characters of the
exine, the germinal apertures have been the focus of attention of
palynologists. Wodehouse (1935) has viewed phylogenetic changes in the
germinal aperture in relation to the function. Apart from the monocolpate
form, the trilete and 3-zonocolpate forms have been held by him to be of
basic significance in aperture evolution. Accordingly the major
evolutionary changes in aperture have been considered to have started
from the one-furrowed forms of primitive gymnosperms by the process of
“modification, protection or elimination of the wide open furrow”. For
evaluating the significance of pollen morphology in the monocotyledons,
Kuprianova (1948) considered the aperture form as a major feature.
According to Erdtman and Vishnu-Mittre (1957), pollen grains having
pore-like or illdefined proximal apertures have given rise to those with
zonal, distal or global apertures. Vishnu Mittre (1964) has viewed the
trilete form as the ancestral type from which inaperturate type followed by
aperturate ones evolved. Walker (1974) has contended the inaperturate
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pollen grain as primitive form from which evolved the aperturate ones, of
which the tricolpate type is the progenitor of all the apertural forms. Nair
(1970c) has proposed that the evolution of the dicotyledons has taken
place along two lines: one along the line of the Magnoliaceae and the other
along the Ranunculaceae, while the Monocots with the predominantly
monocolpate form evolved along an independent line. According to him
“the Magno-Ranalian complex have originated from the trilete or
trichotomocolpate form of progenitor characteristic of the preangiosperms
by which the trilete aperture is considered to be the most primitive type
and the other forms and the inaperturates advanced types in the scale of
morphological evolution of pollen. Forms such as colporate, porate,
pororate and spiraperturate are considered to have been evolved from the
colpate type by reduction and specialization from the aperturates. The
change in form has also been accompanied by increase in the number of
apertures. According to Nair (1985) the other palynological features such
as exine ornamentation, exine strata and pollen size also constitute various
structural entities of pollen.
Evolution of similar traits in different lineages can be considered as
evidence for convergent adaptive change (Brooks and McLennan, 1991).
The diversity of pollen surface patterns has been interpreted as a result of
an adaptation for rapid germination, protection from desiccation and
harmomegathic mechanisms within the closely related taxa (Nowicke and
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Skvarla, 1979). In addition to the germinal aperture which is of primary
importance in the morphology of pollen, there are also other
morphological characters in which evolutionary phenomina are expressed.
Wodehouse (1935) has observed gradual reduction in the excrescences in
the pollen grains of Asteraceae. In the case of saccate pollen forms those
with two or more sacci are considered to have originated from forms with
a single saccus. The pollen types with thick and heavily ornamented exine
are considered to be primitive, while those with thin and unornamented
exine advanced.
From the palynological data of myrtalean families in the South
Indian region the distribution of various aperture forms in the member
families of the order sensu Bentham and Hooker (1865) presented, it is
found that the order is fairly eurypalynous with different aperture forms
occurring. The different forms come under three major categories such as
colpate, colporate and porate, the distribution of which is represented
below.
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3-colpate Myrtaceae
Onagraceae
Colpate 3-syncolpate Myrtaceae
3-colporate Rhizophoraceae
Combretaceae
Myrtacaeae
Melastomaceae
Lythraceae
Onagraceae
Colporate 4-colporate Combretaceae
Myrtaceae
Melastomaceae
Lythraceae
Onagraceae
3-syncolporate Myrtaceae
Lythraceae
4-syncolporate Myrtaceae
Lythraceae
Para-syncolporate Myrtaceae
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2-porate Onagraceae
3-porate Lythraceae
Porate Onagraceae
4-porate Onagraceae
5-porate Onagraceae
It may be noted that the most primitive 3-colpate condition occurs
exclusively in the members of the Myrtaceae and one member of
Onagraceae, while the 3-colporate in all the six families and together with
4-colporate forms in most of them. The most advanced porate condition (2
to 5-porate) in members of the Onagraceae and in one genus of Lythraceae
(Sonneratia). It appears that aperture evolution has most operated in the
Lythraceae followed by Onagraceae. It is also interesting to note that
syncolpate, syncolporate and parasyncolporate conditions occur frequently
in the Myrtaceae and sporadically in the Lythraceae.
A tentative scheme of evolution of apertures in the myrtalean complex
is presented in the chart.
178
Fig. 7. Tentative scheme of evolution of apertures in the myrtalean complex
179
From the chart it may be seen that the most primitive 3-colpate
condition occurs only in the Myrtaceae and Onagraceae. Still more
advanced 3-colporate apertures in all the six families, Rhizophoraceae,
Combretaceae, Myrtaceae, Melastomaceae, Lythraceae and Onagraceae.
Relatively more advanced syncolpate form occurs in two species of
Myrtaceae; syncolporate in Myrtaceae and Lythraceae and parasyncolporate
in Myrtaceae alone. Still more advanced porate condition occurs in two
families (3-porate) in Lythraceae and (2 to 5-porate) in Onagraceae. Some
degree of heterogeneity is noticed in three families; in the Myrtaceae
possessing six different aperture forms (3-syncolpate, 3-colporate,
4-colporate, 3-syncolporate, 4-syncolporate, 3-parasyncolporate); Lythraceae
(3-colporate, 4-colporate, 3-syncolporate, 4-syncolporate, 3-porate) and in
Onagraceae (3-colporate, 4-colporate and 2 to 5-porate) conditions.
5.4 Systematic considerations
The myrtalean group of families is characterized primarily by two
distinctive wood anatomical features that are not commonly found together
in any other angiosperm groups (Conti et al, 1996). All plant families that
have been regarded as indubitable members of the Myrtales are
characterized by the combination of these anatomical features which is
uncommon in any other dicot families outside the Myrtales. They are
bicollateral vascular bundles in the primary stem and vestures in the
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bordered pits of the secondary xylem. The combined occurrence of these
two anatomical characters is very rare outside the Myrtales. This is
considered to strengthen the value of these characters for the delimitation
of the Myrtales (perceived from the anatomical point of view). The
families like Rhizophoraceae, Lecythidaceae etc. which are included by
some taxonomists in the Myrtales fail to gain anatomical approval (Vliet
and Baas, 1984). The members of the order consist of woody and
herbaceous plants characterized by production of tannins (Cronquist, 1981).
Leaves are typically opposite, simple and with entire margin. The flowers
are bisexual, mostly 4 or 5-merous, actinomorphic or weakly zygomorphic,
often distinguished by the presence of numerous stamens. The 2 to 5 carpels
are fused to form a syncarpous pistil and ovary with several locules; the
position of the ovary varies from perigynous to epigynous, often with a
short to long hypanthium. Pollen grains are predominantly tricolporate often
with pseudocolpi; pollination usually mediated by insects or birds. Other
notable features almost typical of the order include unilacunar nodes, starch-
accumulated plastids in the sieve tube elements, and chromosome numbers
in multiples of 11 or 12 (Raven, 1975).
This is one of the intensely investigated orders from various biological
angles, and despite this a number of systematic issues remain unsolved
concerning the circumscription, composition and interrelationships, both
intra- and interordinal. A great deal of controversy exists between and
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among the various systematic treatments both classical and modern.
Bentham and Hooker’s (1865) treatment of the Myrtales is known to be
the most comprehensive among the classical, and according to this the
order comprises six families such as the Rhizophoraceae, Combretaceae,
Myrtaceae, Melastomaceae, Lythraceae and Onagraceae. This composition
has been subjected to significant degree of shuffling and reorganization in
subsequent taxonomic treatments based on evidence from various
disciplines like morphology, embryology, anatomy, palynology,
phytochemistry and molecular biology (reviewed in Dahlgren and Thorne,
1984). Notwithstanding the wealth of taxonomic studies the order is
subjected with, considerable degree of uncertainty still exists. The
distribution of myrtalean families according to Bentham and Hooker and
other major treatments is furnished in Table - 7 below.
182
183
184
From the distribution of families in the order as projected in the
various treatments, it may be noted that the families of the Myrtales sensu
Bentham and Hooker that are most tampered with are Rhizophoraceae,
Myrtaceae and the Lythraceae, and the others are marginally shuffled. A
modest attempt is made here to view the merits and feasibility of the
existing major classificatory treatments of the order in the light of
evidence from available palynological information including the ones
gathered presently from a good sample of taxa of the order distributed in
the South Indian region which covers 108 species in 42 genera
representing the six families of Bentham and Hooker.
Some general conclusions can be drawn by comparing the
distribution of families in the various treatments. Almost all modern
authors agree in including families such as Myrtaceae, Heteropyxidaceae,
Psiloxylaceae, Lythraceae, Punicaceae, Sonneratiaceae, Combretaceae,
Trapaceae, Crypteroniaceae, Alzateaceae, Melastomaceae, Memecylaceae,
Onagraceae and Rhynchocalycaceae in the Myrtales. Most of them agree
in including Oliniaceae and Penaeaceae in the Myrtales with some
exceptions. Emberger (1960) excluded these two families from his
Myrtales, and has placed both in the order Thymelaeales with
Geissolomataceae, Thymelaeaceae and Elaeagnaceae; Melchior (1964)
and Soo (1975) have by and large followed Emberger’s treatment, but placed
Oliniaceae in the Myrtales. Takhtajan’s treatment (1980) conformed to those
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of Emberger (1960) and Melchior (1964) and Soo (1975) in keeping
Rhizophoraceae, Lecythidaceae and Haloragaceae in Myrtales. However, a
few of the still recent treatments as that of Cronquist (1981), Dahlgren et al
(1981) and Thorne (1983) have excluded these families from their Myrtales.
Thymelaeaceae has been retained in the order by Cronquist (1981).
Of the various modern treatments, the most comprehensive work
relying on evidences from morphological and chemical characters of
Myrtales and associated families is that of Dahlgren and Thorne (1984).
They did not, however, furnish an explicit phylogenetic analysis of
relationships in the group. The more recent work of Conti et al. (1996) has
attempted a cladistic analysis of circumscription of relationships of
Myrtales based on molecular data. They have prescribed a phylogenetic
analysis of molecular sequence data from the chloroplast gene encoding
rbcL of Myrtales and putatively related families. The main objective of
their work is (1) to establish whether the Mytales sensu Dahlgren and
Thorne (1984) constitute a monophyletic order (2) to determine whether
some controversial families (Thymelaeaceae, Rhizophoraceae,
Lecythidaceae, Haloragaceae) should be included or excluded from the
Myrtales and (3) to identify the sister group of the Mytales. Conclusion
from the phylogenetic analysis of rbcL sequence data is that no single
cluster of morphological or chemical characters can be used to
unequivocally determine the circumscription of Myrtales, based on
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morphological and anatomical characters, notably the combined
occurrence of bicollateral vascular bundles and vestured pits in the vessel
elements. Their findings based on the parsimony analysis of DNA sequence
data from the chloroplast gene has supported the monophyly of a myrtalean
clade consisting of 13 families: Alzateaceae, Rhynchocalycaceae,
Penaeaceae, Oliniaceae, Memecylaceae, Melastomaceae, Heteropyxidaceae,
Myrtaceae, Vochysiaceae, Onagraceae, Lythraceae, Trapaceae and
Combretaceae (Conti et al., 1996). Moreover the circumscription of
Myrtales defined by the rbcL tree largely correspond to that proposed by
Dahlgren and Thorne (1984) based on morphological and anatomical
characters, notably the combined occurrence of bicollateral vascular
bundles and vestured pits in the vessel elements. The finding based on the
rbcL data is shown to be strongly supportive of the exclusion of the
Thymelaeaceae and Lecythidaceae from the myrtalean complex.
Rhizophoraceae
This family is treated with a premier position in the distribution of
families of Bentham and Hooker’s Myrtales. The relationship of this
tropical family which includes four mangrove genera has been a matter of
great controversy. The questionable monophyly of this family, with the
debatable placement of Anisophyllea seems to further complicate the issue
in establishing its relationship to other families. Some of the modern
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treatments given in Table - 7, such as Emberger (1960), Melchior (1964),
Soo (1975) and Takhtajan (1980) have retained this family in their
Myrtales, while in the relatively more recent treatments (Cronquist, 1981;
Dahlgren and Thorne, 1984; Briggs and Johnson, 1979), this family is
excluded from the Myrtales. Characters suggesting inclusion of
Rhizophoraceae in the Myrtales include the well-developed hypanthium in
the genera with epigynous flowers, tricolporate pollen grains (a condition
predominant in the core families of the Myrtales) and abundance of tannin
(Dahlgren and Thorne, 1984). However, the lack of vestured pits and
internal phloem argues against a close affinity of this family with the
Myrtales. The unique characteristics of Rhizophoraceae seem to isolate
them from any other rosid group despite the palynological attribute of
tricolporate pollen grain consistently noted in all taxa of the family
presently studied. Cronquist (1981) has assigned the family to a separate
ordinal rank of its own, the Rhizophorales. Thorne placed Rhizophoraceae
in the Cornales with Haloragaceae, while Dahlgren argued that the lack of
iridoid compounds and abundance of tannins do not support the placement
(Dahlgren and Thorne, 1984). In a later revised classification, Thorne
(1992), as also Cronquist (1981) accommodated the family in the
Rhizophorales that was included in the super order Geranianae together
with Linales, Geraniales and Malpighiales.
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In the present study of the Rhizophoraceae (five genera and seven
species) of which five species are with mangrove habit and others
terrestrial. Palynologically all the taxa studied here are consistently
tricolporate, radially symmetrical and isopolar. However, as regards the
secondary character most of them possess punctate ornamentation which is
seldom known in any of the myrtalean taxa. Another palynological feature
almost consistently noticed here is the fused endoaperture which is
rectangular-lolongate in shape, a condition so far unknown in any of the other
families of Myrtales. Moreover, absence of pseudocolpi in all members of the
family studied here is a condition known to predominate in other families of
Myrtales and appears to be a factor against retention of Rhizophoraceae in the
Myrtales. In this connection the findings of Vezey et al. (1988) who studied
39 species of the family seems to be against the retention of Rhizophoraceae
in the myrtalean complex. Their main findings on palynological features are:
(1) fused endoapertures and (2) lack of pseudocolpi. Based on this and their
molecular study, they divided the Rhizophoraceae into two families such as
Rhizophoraceae and Anisophylleaceae, both being given a position outside
the Myrtales. The main objection raised by most of the modern taxonomic
treatments of the Myrtales for inclusion of Rhizophoraceae in the Myrtales is
the absence of the vestured pits, internal phloem and bicollateral vascular
bundles. This together with the aforesaid palynological features present in the
taxa of the Rhizophoraceae swings more against its retention in the myrtalean
189
complex, despite the solitary and seemingly favourable attribute of
tricolporate pollen grain which is not a single palynological character that can
be taken into consideration.
Combretaceae
This is one of the large families of the order Myrtales, almost
uniformly treated as a core member of the Myrtales in all taxonomic
treatments, both classical and modern. The character attributes studied
from various biological systems including pollen morphology show a high
degree of resemblance with other sister families of the order. In the present
study of 12 species representing six genera, the pollen is characterized by
tricolporate, heterocolpate, radially symmetrical and isopolar grains with
shape being prolate-spheroidal in majority of species. The heterocolpate
pollen grains of Combretaceae show broad similarities to the heterocolpate
pollen of Melastomaceae (Erdtman, 1971) and also to Lythraceae. One of
the genera namely, Strephonema is shown to be so distinct from all the
members of Combretaceae, and based on this a separate subfamilial status
(Strephonematoideae) has been suggested by Dahlgren and Thorne (1984)
and others. The genus differs from other members of Combretaceae due to
the absence of fibrous exotegmen in the seeds. Apart from this minor
difference, all the known treatments agree in considering Combretaceae as
a coherent family in the myrtalean group.
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As regards pollen morphology as well the genus Strephonema stands
out in having pollen which lacks pseudocolpi and having reticulate exine
surface in contrast to the condition in the rest of the members of the
family. However, a considerable number of features are common to the
subfamilies of Combretoideae and Strephonematoideae which include the
hair type, racemose inflorescence, obdiplostemony and unilocular ovary.
But Strephonema differs from other Combretaceae by having
hemiepigynous flowers and several wood anatomical features. These
together with palynological distinction may be argued in support of the
subfamilial status to Strephonema, a view highly favoured by Dahlgren
and Thorne (1984).
Myrtaceae
The family is a fairly distinct one, characterized in particular by
having gland-dotted leaves, stems and floral parts, inflorescence basically
paniculate, the flowers always epigynous, bisexual and actinimorphic, 4 or
5-merous, the androecium is usually polystemonous, the pollen grains
generally triangular, often syncolpate and lack pseudocolpi. Except for
Psiloxylaceae and Heteropyxidaceae, which Schmid (1980) included in
Myrtaceae the family shows no connections with other families of the
order Myrtales. A phylogenetic link with Lecythidaceae has been proposed,
but not supported (Dahlgren and Thorne, 1984). Bentham and Hooker’s
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Myrtaceae has been drastically reshuffled in most of the modern treatments.
In the largely accepted treatments, the Myrtaceae has been subdivided into a
number of families, most of them splinter ones and they are (in addition to
Myrtaceae) Lecythidaceae, Rhynchocalycaceae, Penaeaceae, Oliniaceae,
Crypteroniaceae, Psiloxylaceae and Heteropyxidaceae.
The Myrtaceae lineage as defined in the parsimony consensus rbcL
tree comprises a Heteropyxis/Psiloxylon subclade sister to the subclade in
which the inclusion of Vochysiaceae is only weakly supported, but would
imply a paraphyletic Myrtaceae sensu stricto. The taxonomic rank of
Psiloxylon and Heteropyxis has long been debated. Essentially the
discussion has focused whether these two genera should be treated as part
of Myrtaceae or as separate families. Melchior, 1964; Soo, 1975;
Cronquist, 1981; Schmid, 1980 and Dahlgren, 1981 (in Dahlgren and
Thorne, 1984) regards both Heteropyxis and Psiloxylon as segregate
monophyletic families. Psiloxylon differs from other parasyncolpate taxa
of Myrtaceae by having unusually large apocolpia and larger rugulate
elements in the equator; but otherwise similar to the rest of the families.
Heteropyxis is not easily distinguished from other genera of the syncolpate
and parasyncolpate group with intercolpar concavities. Myrtaceous pollen
as documemted by SEM (Patel et al, 1984) does not appear to have any
close similarity to taxa from other core Myrtales except for some
superficial similarities existing between Onagraceae pollen. (Erdtman,
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1971) has suggested similarity to Lythraceae through Cuphea. Both
Heteropyxis and Psiloxylon are narrow endemics, with the former
restricted to South- West Africa and the latter confined to the Mascarhenas
Island off the coast of Madagascar (these two genera are not represented in
the present study). The two genera differ in ovary, stigma and fruit
morphology (Schmid, 1980), and based on the phenotypic data and
embryological characters, the sister group relationship between
Heteropyxis and Psiloxylon is supported by other exomorphic features
(Conti et al., 1997).
Preliminary analysis of ndhF sequence across the order (Systma et
al., 1996) supports a second scenario, i.e. Heteropyxis and Psiloxylon
forms a sister clade to Myrtaceae sensu stricto where the Vochysiaceae is
sister to the larger clade. In view of the general exomorphic, embryologic
and molecular distinction combined with pollen morphological distinction,
separation of Psiloxylon and Heteropyxis from the Myrtaceae seems
relevant assigning both of them separate family rank.
The genus Barringtonia, Careya and Couroupita have been
associated with Myrtales sensu Bentham and Hooker. However, several
differences such as a number of embryological features, (summarised by
Tobe and Raven, 1983), presence of alternate leaves, bitegmic tenuinucellar
ovules, absence of internal phloem and vestured pits (Cronquist, 1981;
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Dahlgren and Thorne, 1984; Briggs and Johnson, 1979) strongly argue
against a myrtalean affinity to them. On these morphological grounds,
Cronquist (1981) has assigned a family status to these genera
(Lecythidaceae). Another alternative was to include the family in the order
Theales on the basis of numerous stamens and staminodia that are
symmetrically or asymmetrically disposed and developing in centrifugal
sequence as in most Theales. The tenuinucellate ovules with thicker
integument and more rapidly destroyed nucellus during its development
and absence of a parietal cell are attributes of Lecythidaceae which are
Thealean. The fruits are of various types and often very large, (Prance and
Mori, 1978) and the seeds are sometimes conspicuously large. The
endosperm is almost completely absorbed during seed development and
the embryo is large and rich in fat, as in myrtalean families.
The chemical contents of members of Lecythidaceae largely agree
with those of myrtalean families, but some agree with those in Theales
also. Lack of internal phloem, relatively more primitive vessels without
vestured pitting, presence of wedge-shaped phloem rays, alternate leaf
arrangement, polymerous, centrifugally developing androecium and
tenuinucellate ovules, all indicate Thealean affinity. A funicular aril, as found
in some Lecythidaceae has its counterpart in the Thealean Clusiaceae. The
pollen grains in certain genera are trinucleate when dispersed and the
tapetum is amoeboid which strongly deviate from the myrtalean pattern.
194
The pollen grains are tricolpate, often syncolpate (Erdtman, 1952) or
tricolporoidate, whereas the simply colpate condition is not known in the
Myrtales. Based on this in addition to the characters mentioned by
(Dahlgren, 1980; Thorne, 1981; Conti et al., 1996) have suggested the
inclusion of Lecythidaceae in Theales or more appropriately in its own
suborder. Analysis of rbcL sequence data suggests that Lecythidaceae are
best placed at the base of the Asteridae near Sapotaceae and Ebenaceae,
among others (Morton et al., 1995).
The present study of the genera and species that are held as the
present day Lecythidaceae revealed syncolpate, syncolporate and colporate
pollen and macroreticulate exine sculpturing and apertural verrucae in
some species of one genus thus appear to support the inclusion of these
genera into a separate family, Lecythidaceae as earlier suggested by
several taxonomists.
Melastomaceae
This family has been treated as a major core family of the Myrtales in
all the taxonomic treatments of the order both classical and modern. The
family is considered to possess stable character states with respect to various
biosystems which include exomorphology, anatomy, embryology, palynology
and molecular biology. Based on molecular study, Conti et al. (1997) have
projected a melastomaceous lineage consisting of a subclade formed of
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Oliniaceae, Peneaeaceae, Rhynchocalycaceae and Alzateaceae sister to a
subclade formed by Memecylaceae and Melastomaceae sensu stricto. Based
on phenotypic data, Johnson and Briggs (1984) held that the ancestor of
melastomaceous lineage is characterized by fixation of opposite phyllotaxy, a
character paralleled by the Lythraceae lineage. Support of individual
clades within the Melastomaceae lineage has implications in the
taxonomic ranking in the family. Earlier debates on the classification of
Melastomaceae has focused on whether Memecylaceae should be
recognized as a separate family as suggested by Johnson and Briggs
(1984), Renner (1993), Takhtajan (1997) or treated as a subfamily of
Melastomaceae as suggested by Emberger (1964) Soo (1975), Cronquist
(1981) and Dahlgren and Thorne (1984). The distinctiveness of
Melastomaceae sensu stricto is supported by molecular data and is in
strong support for the clade comprising the three African families
(Oliniaceae, Rhynchocalycaceae and Penaeaceae) plus Central/South
American. This is suggestive of a Gondwanian origin and vicariant
evolution of these families. Morphology of the clade is corroborated by
three phenotypic synapomorphies (Johnson and Briggs, 1984). However,
uniquely derived character states for the African clade and the low
measuring of branch support suggest that the relationship between the
three African families and Alzatea remain uncertain (Conti et al, 1997). It
should be noted, as phenotypic characters suggest, that a close relationship
196
of Crypteronia with the Melastomaceae lineage exists, but no clear
affinities within it evident (Dahlgren and Thorne, 1984).
Several taxa of the family have been studied here representing 8
genera. In all of them the pollen is tricolporate, heterocolpate, isopolar and
radiosymmetric. Palynological evidence is very much in favour of
assigning Melastomaceae and Memecylaceae in a comfortably single
family rank as against their suggested segregation on other grounds
(Dahlgren and Thorne, 1984).
Lythraceae
The Lythraceae sensu Bentham and Hooker has been subjected to a
great deal of taxonomic restructuring based on evidences from various
disciplines including palynology and molecular biology. During the present
study, 19 species representing 9 genera have been examined, and the taxa
showed greatest amount of variation in the order with diversity evident at all
levels, including shape of pollen, aperture system and exine sculpturing.
Infrageneric variation in palynological characters was remarkable as in
Cuphea. Major deviation of taxonomic treatment of both Bentham and
Hooker (1865) from the modern ones is the recognition of familial and/or
subfamilial rank given to genera such as Alzatea, Sonneratia, Duabunga and
Punica. In his taxonomic treatment, Dahlgren (1981) assigned familial rank
to Punica and Sonneratia. Similar family status for these genera was
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proposed by several modern treatments (Emberger, 1960; Melchior, 1964;
Soo, 1975; Hutchinson, 1973; Stebbins, 1974; Briggs and Johnson, 1979;
Cronquist, 1981; Takhtajan, 1981). However, in a later treatment Dahlgren
(1984) opted to assign these, only subfamilial status. From the results of Patel
et al. (1984)’s study it is pointed out that Lafoensia is similar to Sonneratia
(Sonneratiaceae), Diplusodon similar to Duabunga (Duabungaceae) and
Lagerstroemia bears resemblance to Punica (Punicaceae). These similarities
strongly support the view of Dahlgren and Thorne (1984) that Sonneratia,
Punica and Duabunga should be regarded as separate subfamilies
independently related to Lythraceae.
From the present examination of palynological features of Punica and
Sonneratia it may be noted that in Punica the pollen grains are tricolporate
without pseudicolpi, and are comparable to the pollen of Lagerstroemia
species. While, in Sonneratia the pollen is anguloaperturate, triporate with very
prominent psilate polar cushions and rugulate-verrucate mesocolpia, prominent
meridional ridges and apertural fields. Palynological attributes of Sonneratia
and Punica stand out from the rest of the genera of the Lythraceae, and more so
with Sonneratia, and hence on palynological grounds, familial or subfamilial
rank assigned to them appears very much justifiable. It may be noted that
palynological distinction in Sonneratia appears to be strong enough to
even this being given a separate family rank rather than subfamily.
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Onagraceae
This is a very distinctive family, and different from other Myrtales on
several features. Bentham and Hooker (1865) and most of the modern
treatments have included Onagraceae as a constituent family of the
myrtalean complex.The close relation with Lythraceae suggested has been
in view of the teeth structure and marginal ciliation of leaves (Hickey,
1981). Fibrous exotegmen of seeds and similar petal venation are some
other conspicuous attributes which may indicate close connection between
the two families (Dahlgren and Thorne, 1984). The relationship of
Onagraceae has been widely debated, the most parsimonious phenotypic
trees (Johnson and Briggs, 1984) have consistently supported a clade between
Onagraceae and Trapaceae, but no characters on the common branch were
largely homoplastic. The phenotypic analysis further showed Lythraceae
(including Sonneratiaceae, Punicaceae and Duabungaceae) are the sister clade
to Trapaceae + Onagraceae. Detailed morphological study of Lythraceae by
Graham et al (1993) favours the sister group relation between the Onagraceae
and Lythraceae. The interrelationships of Onagraceae were discussed by
Conti et al. (1993), and they stressed monophyly of the family. The
distinctiveness of Onagraceae is further supported by several morphological
synapomorphies pertaining especially to pollen morphology (pollen grains
with viscin threads and unique exine sculpturing and embryology, Oenothera
type embryo sac) Johnson and Briggs (1984).
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The genus Trapa formerly included in the Onagraceae by Bentham
and Hooker (1865) has temperate to tropical distribution. Out of the
existing three species of the genus, one species has been presently studied.
The plants are aquatic floating herbs. The pollen grains triangular and have
three meridional ridges. They can be interpreted as possessing intercolpate
depressions. It lacks viscin threads on the pollen grain; flowers are
epigynous; 4-nucleate Oenothera type embryo sac. In view of the
distinction it has with members of Onagraceae, especially absence of
viscin threads on the pollen grains which is a characteristic feature of
Onagraceae, the possession of meridional ridges formed by the exine
folding, their position over the colpus in contrast to the meridional ridges
formed by mere exine thickenings that alternate with colpus in
Onagraceae, a separate family rank given to Trapa (Trapaceae) can be
confidently vindicated.
Haloragaceae
This family has often been placed in Myrtales due to the possession
of opposite leaves with minute stipules, 4-merous, diplostemonous flowers
with four carpels, similar embryology and endospermous seeds (Orchard,
1975). Hickey and Wolf (1975) placed it in Hippuridales due to the
occurrence of Rosoid type leaf teeth. But these are met within some
Onagraceae and Lythraceae too (Hickey, 1981). Vestigial kind of stipules
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as in Myrtales are present in the Haloragaceae. The floral anatomy of the
family (Orchard, 1975) shows similarity to that in Cornales and Araliales
rather than myrtalean families. The pollen grains are shed in the tricellular
stage as in Araliales and unlike Myrtales. The endosperm formation in
Haloragis and in some species of Myriophyllum is of the cellular type that
is never found in Myrtales. In Polygonum type of embryo sac development
and the Caryophyllad type embryogeny, the family differs from all
Myrtales (Kapil, 1962; Kapil and Bala-Bava, 1968). The endosperm-rich
seeds of Haloragaceae are not common in Myrtales but, the seeds in
Araliales, though rich in endosperm the embryo is proportionately smaller
than in the former one.
Haloragaceae seems to be a fairly isolated family, and though it
possesses a number of myrtalean attributes, the dissimilarities count more.
The view of Orchard (1975) that Cornaceae is the closest family of
Haloragaceae is not supported by Dahlgren as the typical Cornales characters
such as the unitegmic, tenuicucellate ovules, lack of tannins, presence of
iridoids etc. are not met within the latter. Haloragaceae may better be placed
in an order separate from, but near Myrtales. Its position may be closer to
Araliales than to Cornales as in Engler and Prantle’s treatment. Thorne agrees
with Orchard in placing the family in Cornales and considers Gunneraceae
and Hippuridaceae as related families in the Haloragineae. In the present
study of a single taxon (Myriophyllum tuberculatum) of Haloragaceae the
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pollen grains are triporate, aspidote and with spinulate exine sculpturing, a
condition not observed in any of the myrtalean families included in this
study. So the separation of the family from Myrtales and its inclusion in a
separate order seems feasible.
5.5 Interrelationships, Evolution and Affinities of the Myrtales
The Myrtales are one of the few larger orders that have a rather
uncontroversial circumscription as regards the “nucleus” or “core
families”. The widely accepted such core families of the order are
Onagraceae, Trapaceae, Lythraceae (incl. Punicaceae and Sonneratiaceae),
Oliniaceae, Combretaceae, Alzateaceae, Penaeaceae, Rhynchocalycaceae,
Crypteroniaceae, Memecylaceae, Melastomaceae, Psiloxylaceae,
Heteropyxidaceae and Myrtaceae. All of these may not necessarily be
entitled familial status (Dahlgren and Thorne, 1984). In an attempt to
approach the interrelationships between families of the order, and its
evolution and affinities it may be profitable to deduce a probable original
state for their common ancestor. This has been done by comparing the
character states of myrtalean families inter se, and by examining those of
probably related order, Rosales being taken as the out group. Dahlgren and
Thorne (1984), in their attempt to trace the myrtalean ancestry have listed
a constellation of character attributes, mostly morphological and
anatomical. They have pointed out that myrtalean ancestors were probably
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woody plants with alternate or opposite leaves with teethy margins. Stems
had evolved bicollateral vascular strands, and the vessel elements had
alternate vestured pits. Moreover, they consider that the immediate
ancestral forms were tannin plants with a flavanoid spectrum. Though
none of the extant families exhibit this combination of character attributes,
they hold that the ancestral forms of Myrtales would have been fairly
similar to certain extant Lythraceae, and that the position of Lythraceae in
the order is central, and they show close relationship with several of the
families, including Penaeaceae, Rhynchocalycaceae and Onagraceae. A
number of primitive states are concentrated, to a higher degree than in
Lythraceae in Psiloxylaceae, Heteropyxidaceae, Myrtaceae and
Strephonematoideae, although each of these is specialized in various
respects. It may be noted that varied rates of evolutionary specializations
seen throughout in Myrtales occur in Psiloxylon, which has retained the
presumably ancestral condition (Dahlgren and Thorne, 1984).
Johnson and Briggs (1984) have given a cladistic presentation of the
probable evolution of Myrtales. Supporting their contention, Dahlgren and
Thorne (1984) viewed that an evolutionary line that probably diverged
very early from the myrtalean ancestors is represented in Psiloxylaceae,
Heteropyxidaceae and Myrtaceae. These share some conspicuous features
such as the characteristic shape and aperture condition (syncolpate of the
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pollen grains, (Patel et al., 1984) and the presence of schizogenous cavities
with essential oils visible as pellucid dots on the green parts.
The great concordance in pollen morphology among Psiloxylaceae,
Heteropyxidaceae and many Myrtaceae suggests that this rather peculiar
type evolved from protomyrtaceous ancestors, and later in many
Myrtaceae gave rise to superficially simpler kinds (Dahlgren and Thorne,
1984). The fact that this kind of pollen is known already in the Cretaceous,
before any other / certain Myrtales, also indicate that this group of families
may have differentiated from the myrtalean ancestors very early.
Onagraceae seem to deviate rather strongly from other Myrtales, and
probably may have evolved as a lateral evolutionary line at an early stage.
The evidence is somewhat contrary to in this respect (Dahlgren and
Thorne, 1984). The family is an unusually distinct one having the
combination of epigynous flowers, pollen with viscin threads, Oenothera
type embryo sac formation, and tissues with Calcium oxalate raphides. The
latter three characters are absent in nearly all other Myrtales, and it is
likely that all these character states are derived ones. Thus it is likely that
other families such as the Trapaceae could have evolved from an
Onagarceous evolutionary line after these attributes had been acquired. In
pollen grain shape and pollen wall structure Onagraceae shows some
general similarity (present observation) to Myrtaceae, Heteropyxidaceae
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and Psiloxylaceae; but more conspicuous are a number of characteristics
shown by Onagraceae and Lythraceae. Wood, with libriform and septate
fibres, leaves with lateral teeth, petals with similar pinnate venation etc.
(Corner, 1976). With the exception of the leaf teeth, these attributes seem
to represent a derived state. Convergent evolution of some of the derived
states is also unlikely; and more likely is the alternative that Onagraceae
diverged from proto-lythraceae ancestors after the wood and the seed coat
structure had already evolved.
The Onagraceae are palynologically very distinct in the order. The
pollen are special by virtue of the often triangular shape with three or more
protruding “papillose” apertures, the mechanism of tetrad cohesion,
(Skvarla et al., 1975), and the fine structure of the exine, in particular the
ektexine, which is granular, “beaded” delicately branched (Skvarla et al.,
1976), and especially by their constant presence of viscin threads (present
study, Skvarla et al., 1978). The relative number per pollen grain and the
surface structure of these threads are variable in Onagraceae which may be
considered to be interesting for the division of the family. It is possible
that the characters, to be associated with the pollen grains of the family,
along with many other distinctive features is suggestive that the family
was derived early from ancestral Myrtales (as suggested elsewhere). On
the whole the exine structure as well as pollen shape and aperture of
Onagraceae can suggest its closest resemblance in Myrtaceae as well as
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Psiloxylaceae and Heteropyxidaceae, a view upheld by Dahlgren and
Thorne (1984).
The remaining families of the Myrtales, (sensu Dahlgren and Thorne,
1984) form somewhat a coherent group, most families being characterized
by so-called “heterocolpate” pollen grains in which pseudocolpi are
present between the true apertures. These features are extremely rare in
angiosperms outside the Myrtales, and it would be highly likely that
pseudocolpi evolved independently in several phyletic lines within the
order. Hence, as Dahlgren and Thorne (1984) have contented, it can be
presumed that the genetic/gametic constitution for pseudocolpi (whether
expressed or not) may have evolved once in the common ancestor of these
families, but may have become lost subsequently that is, this attribute has
not come to expression in some lines. Thus, as in Lythraceae, pseudocolpi
occur in some, but not in all genera (Patel et al., 1984; present
observation). In some genera they are indistinct or inconsistently present.
In Lythraceae the genus Lythrum has pseudocolpi of the same number as
the true aperture, while in most herbaceous genera with pseudocolpi, the
number is twice as many as the number of the apertures (present study).
Pseudocolpi are absent or very indistinct in the pollen of Alzateaceae
and Lythraceae subfamily Punicoideae, Sonneratioideae and
Duabungoideae, in the Combretaceae subfamily, Strephonematoideae, and
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in Trapaceae (Patel et al., 1984), where the intercolpate depressions
dubiously correspond to pseudocolpi. The first five families which are all
few in species content show strong affinity (in various respects) to families
or subfamilies where, the pollen grains possess pseudocolpi, and hence it
is likely that their common ancestors had heterocolpate pollen.
The Trapacae are often associated with Lythracae by Miki (1959)
who derived Trapa from Lythrum through Hemitrapa. The family is
unique in the order in many respects. However, a specialized floating
aquatic, this deviates from the other Myrtales in many exomorphological
and embryological features. It is possible that Trapa may have diverged
strongly from other Myrtales and also from any plausible ancestral types.
The pollen morphology of Trapa may indicate affinity with the
heterocolpate condition if the intercolpate areas between the meridional
crests are considered homologous to intercolpate depressions, and hence to
pseudocolpi. But this is a disputed possibility.
Within the subfamily Lythroideae, the polymerous large-flowered
genera such as Lafoensia and Lagerstroemia have numerous stamens and
unspecialized pollen grains lacking pseudocolpi. These features are
generally considered to be primitive, a view challenged by Dahlgren and
Thorne (1984), who consider that increase in floral size have favoured an
increase in stamen number. Lack of pseudocolpi is found in a number of
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lythraceous genera, and does not seem to characterize natural group of
genera in subfamily Lythroideae. Dahlgren and Thorne (1984) have
considered reasons for the absence of pseudocolpi, and maintained this
feature to be a derived condition in the family. They contented that in
contrast to all other myrtalean families, the pseudocolpi, when present,
tend to be double the aperture number, a condition which should be
considered another derived character state.
Oliniaceae agree with Combretaceae in several features e.g. epigyny
and the frequent occurrence of small petal scales and common basic
chromosome number (x = 12). A number of families mostly with
heterocolpate pollen grains have seeds without a fibrous exotegmen.
Although Oliniaceae are not known in this respect, they probably belong
to the group sharing a number of features with Penaeaceae and
Rhynchocalycaceae and these families are Melastomaceae, Memecylaceae,
Crypteroniaceae, Rhynchocalycaceae, Penaeaceae and Alzateaceae.
Alzateaceae probably belongs to this group although their pollen grains
lack pseudocolpi.
The two unigeneric families, Alzateaceae and Rhynchocalycaceae,
share a number of wood anatomical characters (Vleit and Baaas, 1984)
which make them closely allied, and in which they differ from, especially
Penaeaceae, which have also retained a primitive wood anatomy, also
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closely connected with Oliniaceae, Rhynchocalycaceae and Alzateaceae in
various floral features. Many taxonomic experts, especially Dahlgren and
Thorne (1984) consider it difficult to speculate about the interrelationships
and evolutionary sequences for the Melastomaceae - Memecylaceae –
Crypteroniaceae – Penaeaceae – Oliniaceae - Rhynchocalycaceae –
Alzateaceae group, and refer to alternative interpretation prescribed by
others like Johnson and Briggs (1984). They are of the view that small
deviations in interpretation and small deviations from the most
parsimonious evolutionary courses may strongly change their evolutionary
model.
Families that are allegedly related to or in various respects
conspicuously similar to Myrtales are Thymelaeaceae, Haloragaceae,
Rhizophoraceae, Lecythidaceae, Elatinaceae etc. The Haloragaceae,
however has often been placed in Myrtales by virtue of its opposite leaves,
4-merous, basically diplostemonous flowers, similar embryological
features etc. Many features are in accordance with those of Myrtales
barring internal phloem and vestured pitting. The consensus in the circle of
the renowned taxonomists is that Haloragaceae seem to comprise a fairly
isolated family, although they possess a number of myrtalean attributes,
which however are counter balanced by several dissimilarities. Porate
pollen grains and the spinulate exine sculpturing in them differ which may
be suggestive that the family is not strongly allied to Myrtales. Thus it may
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be proper that Haloragaceae be better treated in an order separate from, but
near Myrtales.
There are a few families which are sometimes associated with, but
apparently distantly related to Myrtales. They are Rhamnaceae,
Marcgraviaceae, Theaceae, Clusiaceae, Myrsinaceae Malpighiaceae,
Columelliaceae, Loganiaceae etc. But there is no unanimity regarding this
among leading taxonomists.
Concerning the relationship and affinity of the Myrtales with other
orders there is no agreement. The position of the Myrtales has varied in
different classifications, and is still a matter of doubtful opinion. Cronquist
(1981) and Takhtajan (1980) are somewhat constrained by their division
into the subclass Dilleniidae and Rosidae of the majority of orders of
dicots. Myrtales in both classifications are placed in the Rosidae, where
they form the main order. It is generally agreed that the order is more or
less related to Rosiflorae including Rosales and Saxifragales. Other orders
sometimes recognized and associated with Myrtales are Elaeagnales and
Thymelaeales. Two attributes to which most leading modern taxonomists
attach great importance are occurrence of internal phloem and the presence
of bicollateral vascular bundles and vestured pitting in the vessel elements.
When the Myrtales are strictly circumscribed, these characteristics become
critical.
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Another order where opposite leaves are combined with internal
phloem and vestured pits is Gentianales, within which especially
Loganiaceae show some resemblance to Myrtales. But, because floral
morphology, embryology and chemistry are vastly different, Dahlgren
(Dahlgren and Thorne, 1984) does not consider any relationship between
Loganiaceae and Myrtales. The major orders with which affinities for
Myrtales have been contemplated by various taxonomic experts are
Rosales, Cunoniales, Saxifragales, Rutiflorales, Theales and Cornales.
However, arguments for and against these affinities also exist among them.
Numerous studies (morphological, anatomical, embryological, cytological,
palynological, phytochemical and molecular) have been conducted by a
host of workers on the Myrtales, and yet a convincing consensus
concerning inter relationships among the families of the order and its
phylogeny and evolution and affinities still remain elusive.
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