B.Sc. B.Ed. Semester – IV

Course Instructor

Dr. Gautam Kumar

Plant Biodiversity

PTS 251

‘Gymnosperm’ comes from the composite Greek ‘gymnos’ means naked and ‘sperma’ means seed, literally meaning ‘naked seeds’.

Gymnosperms are flowerless plants that produce cones and seeds.

Seeds are not encased within an ovary. The unenclosed condition of their seeds (called ovules in their unfertilized state).

They seeds sit exposed on the surface of leaf-like structures called bracts.

Gymnosperm

Gymnosperms are vascular plants of the subkingdom Embyophyta and include conifers, cycads, ginkgoes, and gnetophytes.

Gymnosperms are abundant in temperate forest and boreal forest biomes with species that can tolerate moist or dry conditions.

Gymnosperms are believed to be the first vascular plants to inhabit land appearing in the Triassic Period around 245-208 million years ago.

The development of a vascular system capable of transporting water throughout the plant enabled gymnosperm land colonization.

Gymnosperm wood is considered softwood, unlike the hardwood of some angiosperms.

Today, there over one thousand species of gymnosperms belonging to four main divisions:

i) Coniferophyta

ii) Cycadophyta

iii) Ginkgophyta

iv) Gnetophyta

Divisions

Coniferophyta

The Coniferophyta division contains conifers, which have the greatest variety of species among gymnosperms.

Most conifers are evergreen (retain their leaves throughout the year) and include some of the largest, tallest and oldest trees on the planet.

Examples Pines, Sequoias, Firs, Hemlock, and Spruces. Conifers are an important economic source of

timber and products, such as paper, that are developed from wood.

The word conifer means "cone-bearer," a distinct characteristic common to conifers. Cones house the male and female reproductive structures of conifers.

Most conifers are monoecious, meaning that both male and female cones can be found on the same tree.

Another readily identifiable trait of conifers is their needle-like leaves.

Different conifer families, such as Pinaceae (pines) and Cupressaceae (cypresses), are distinguished by the type of leaves present.

Pinus sp. (Pine) have single needle-like leaves or needle-leaf clutters along the stem.

Cupressus sp. (Cypress) have flat, scale-like leaves along the stems.

Agathis sp.have thick, elliptical leaves.

Nageia sp. have broad, flat leaves.

Agathis sp.

Pinus sp. Cupressus sp. (Cypress)

Nageia sp.

Conifers are conspicuous members of the taiga forest biome and have adaptations for life in the cold environment of boreal forests.

The tall, triangular shape of the trees allows snow to fall from the branches more readily and prevents them from breaking under the weight of the ice.

The needle-leaf conifers also have a waxy coat on the leaf surface to help prevent water loss in the dry climate.

Taiga forest biome

Triangular shape of conifers

Needle-leaf of conifers

Cycadophyta

The Cycadophyta division of gymnosperms include cycads.

Cycads are found in

tropical forests and subtropical regions.

These evergreen plants

have a feather-like leaf and long stems that spread the large leaves out over the thick, woody trunk.

Though cycads may resemble palm trees but they are not related. Cycads can live for many years and have a slow growth process eg.

King Sago palm,

Cycas sp.

Unlike many conifers, cycad trees either produce only male cones (produce pollen) or female cones (produce ovules).

Female cone-producing cycads will only produce seeds if a male is within the vicinity.

Cycads rely mainly on insects for pollination, and animals aid in dispersal of their large, colorful seeds.

Female cone of Cycas sp.

Cycad seeds can be dangerous to pets and humans if ingested

Ginkgophyta

Ginkgo biloba is the only surviving member of the Ginkgophyta and therefore called living fossil.

Today, naturally-growing ginkgo plants

are exclusive to China. Ginkgoes can live for thousands of

years and are characterized by fan-shaped, deciduous leaves that turn yellow in autumn.

Ginkgo biloba are quite large, with the tallest trees reaching 160 feet. Older trees have thick trunks and deep roots.

Ginkgoes grow in well sunlit areas that

receive lots of water and have plenty of soil drainage.

Ginkgo biloba

Like cycads, Ginkgo plants produce either male or female cones and have sperm cells that use flagella to swim toward the egg in the female ovule.

They produce chemicals, including several flavinoids and terpenes with antioxidant, anti-inflammatory and antimicrobial properties.

These durable trees are fire-resistant, pest-resistant and disease-resistant.

Ginkgo male cone

Ginkgo female cone

Gnetophyta

Gnetophyta has a small number of species (65) found within three genera: Ephedra, Gnetum and Welwitschia.

Ephedra sp.

Many of the species from the genus Ephedra are shrubs that can be found in desert regions of the Americas or in the high, cool regions of the Himalayan mountains in India.

Certain Ephedra species have medicinal properties and are the source of the decongestant drug ephedrine. Ephedra species have slender stems and scale-like leaves.

Ephedra sp.

Ripe female cones of Ephedra sp.

Gnetum sp. Gnetum species contain some shrubs and

trees, but most are woody vines that climb around other plants.

They inhabit tropical rain forests and have broad, flat leaves that resemble the leaves of flowering plants.

The vascular tissue structure of these plants is also similar to that of flowering plants.

The male and female reproductive cones are contained on separate trees and often resemble flowers, though they are not.

Female cone of Gnetum sp. with seed

Male cone of Gnetum sp.

Seed of Gnetum sp.

Welwitschia sp. Welwitschia has a single species,

W. mirabilis.

These plants live only in the African desert of Namibia.

They are very unusual in that they have a large stem that remains close to the ground, two large arching leaves that split into other leaves as they grow, and a large, deep taproot.

This plant can withstand the extreme heat of the desert with highs of 50°C as well as the lack of water (1-10 cm yearly).

Male cones are brightly coloured and both male and female cones contain nectar to attract insects

Welwitschia sp.

Male cones of Welwitschia sp.

Life cycle of gymnosperms

In the gymnosperm life cycle, plants alternate between a sexual and an asexual phase. This type of life cycle is known as alternation of generations.

Gamete production occurs in the sexual phase or gametophyte generation of the cycle.

Spores are produced in the asexual phase or sporophyte generation.

Unlike in non-vascular plants, the dominant phase of the plant life cycle for vascular plants is the sporophtyic generation.

In gymnosperms, the plant sporophyte is recognized as the bulk of the plant itself, including roots, leaves, stems and cones.

The cells of the plant sporophyte are diploid and contain two complete sets of chromosomes.

The sporophyte is responsible for the production of haploid spores through the process of meiosis.

Containing one complete set of chromosomes, spores develop into haploid gametophytes.

The plant gametophytes produce male and female gametes which unite at pollination to form a new diploid zygote.

The zygote matures into a new diploid sporophyte, thus completing the cycle.

Gymnosperms spend most of their life cycle in the sporophyte phase and the gametophyte generation is totally dependent upon the sporophyte generation for survival.

Reproduction in gymnosperms

Female gametes (megaspores) are produced in gametophyte structures called archegonia located in ovulate cones.

Male gametes (microspores) are produced in pollen cones and develop into pollen grains.

Female cone

Male cone

Some gymnosperm species have male and female cones on the same tree (monoecious), while others have separate male or female cone producing trees (dioecious).

For pollination, gametes must come into contact with one another, which typically occurs via wind, animal or insect transfer.

Fertilization in gymnosperms occurs when pollen grains contact the female ovule and germinate.

Sperm cells make their way to the egg inside the ovule and fertilize the egg.

In conifer and gnetophytes, sperm cells have no flagella and must reach the egg via the formation of a pollen tube.

Upon fertilization, the resulting zygote develops within the gymnosperm seed and forms a new sporophyte.

In cycads and ginkgoes, the flagellated sperm swim toward the egg for fertilization.

Kingdom: Plantae

Clade: Tracheophytes

Division: Cycadophyta

Class: Cycadopsida

Order: Cycadales

Suborder: Cycadineae

Family: Cycadaceae

Genus: Cycas

Cycas sp. is the only genus recognised in the family Cycadaceae.

Cycas is a palm-like, evergreen plant.

The plants grow under xerophytic conditions.

Plant body consists of a columnar aerial trunk with a crown of pinnately

compound leaves as its top.

Cycas sp.

Distribution of Cycas

Cycas is distributed in Japan, Australia, India, Indochina, China, Mauritius, Africa, Nepal, Bangladesh, Sri Lanka and Myanmar.

In India, Cycas grows naturally in Orissa, Assam, Meghalaya, Tamil Nadu, Karnataka and Andaman and Nicobar Islands.

Cycas is represented by 15 species. According to Willis (1966) there are 20 species of the genus. Schuster (1932) recognized only 8 species, mentioning for the rest as the forms, varieties or sub-species of the other species.

Besides Cycas circinalis, C. beddomei, C. rumphii and C. pectinata, which occur in the wild state in India. C. revoluta and C. siamensis are such species which are cultivated commonly in the Indian gardens. Cycas revoluta is the most commonly cultivated species of the Indian gardens.

i) Normal tap-roots

Normal tap-roots are positively geotropic, grow deep into the soil and generally possess no root hairs.

Their function is to fix the plant in the soil and to absorb water and other minerals.

From the normal roots some lateral branches are developed near the ground surface.

Two types of roots are present in Cycas: i) Normal tap roots forming a tap root system

ii) Coralloid roots

Normal tap-roots in Cycas sp.

These lateral roots get infected with some bacteria, fungi and algae, and are called coralloid roots.

They grow- first horizontally in the soil and become swollen at their tips.

ii) Coralloid roots

Coralloid roots in Cycas sp.

They divide repeatedly to form big bunches of greenish or brownish structures, which are coral like in appearance.

They divide dichotomously, come out of the soil on the ground surface and are phototrophic in nature.

Young plants bear more coralloid roots than the older ones.

Cyanobacterial zone

Coralloid root

Coral exoskeleton in sea

Coralloid Root T.S. of coralloid root

Coralloid roots are coral-like, dichotomously branched, fleshy and arise from the lateral branches of normal roots.

Due to the presence of endophytic algae, these roots become swollen, appear like a coral and hence named coralloid.

Coralloid roots are apogeotropic in nature (growth or orientation away from the earth; negative geotropism).

Coralloid roots contain symbiotic Cyanobacteria (blue-green algae), which fix nitrogen and, in association with root tissues, produce such beneficial amino acids as asparagine and citrulline.

Coralloid root in Cycas revoluta

Cortex is parenchymatous and divisible into outer cortex and inner cortex having a middle algal zone.

Endodermis, pericycle and vascular bundles are same as in normal root. Xylem is exarch and triarch.

Normally, the secondary growth is absent.

Coralloid roots are green in colour due to the presence of an algal zone.

Members of Myxophyceae (Nostoc, Anabaena) and some bacteria are

present in this zone.

Outer cortex

Inner cortex

Algal zone

Transverse section (T.S.) of coralloid root is circular in outline and the outermost layer is epiblema.

Cork as well as cork cambium develops at maturity.

Root hairs are normally absent.

Algae:

Noxtoc punctiforme

Anabaena cycadeae

Oscillatoria sp. Members of Bacillariophyceae (diatoms)

Bacteria:

Pseudomonas sp.

Azotobacter sp.

Algae and bacteria reported from algal zone of

coralloid root

The roots of cycads are colonized by the photosynthetic bacteria cyanobacteria.

These microbes produce certain poisons and neurotoxins that accumulate in the plant seeds. The toxins are thought to provide protection against bacteria and fungal parasites..

These occur regularly in alternate bands.

The larger leaf bases represent the bases of foliage leaves, while the smaller ones are the bases of scaly leaves in male plants and scales and megasporophylls in female plants.

Stem is erect, stout and unbranched.

It is tuberous when young but columnar, erect and stout at maturity.

Branching in stem is also not rare after the plants have reached a certain age.

The aerial part of the trunk remains covered by a thick armour of large and small rhomboidai leaf bases.

Stem

Rhomboidal leaf bases

on bark of Cycas sp.

Stem of Cycas sp.

The age of the plant can be calculated by counting the number of crowns of leaves and megasporophylls which are produced every year.

At the apex of stem is present a crown of leaves arranged spirally.

Two types of leaves are present in Cycas: i) Green, assimilatory or foliage leaves

ii) Scaly leaves or cataphylls

Leaves

i) Foliage Leaves or Assimilatory Fronds

These are green, large, pinnately compound and stout leaves with a spiny petiole and large, strong rachis.

They are produced at the apex of the stem in the form of crown. The rachis bears many leaflets.

With the help of a transversely expanded rhomboidal leaf base, a leaf remains attached with the stem.

Two rows of strong and stiff spines are present on the petiole. These

spines gradually transform into two rows of pinnae towards the upper side of the leaf.

Cycas Leaflet

Cycas leaf is very large and may reach up to 3 metres in length in some species such as C.

thouarsii.

Two rows of pinnae on the leaves may be alternate or opposite. The number of pinnae varies in different species. As many as hundred pairs of pinnae may be present in a mature leaf.

Each pinna is sessile, elongated, ovate or

lanceolate in shape with a spiny or acute apex. Pinnae are repeatedly and deeply dichotomized in C. micholitzii. Each pinna or of leaflet contains a midrib without any lateral branching.

Forked pinnae of

C. micholitzii

Forking of the midvein of the leaflet has been reported in C.

circinalis.

Margins of the leaflets are revolute in C. revoluta and C. beddomei, while in C. rumphii and C. circinalis they are flat.

ii) Scaly leaves or cataphylls

These are dry, brown-coloured, somewhat triangular leaves with their one end pointed.

They are present at the apex of the stem and remain covered with several ramental hairs

Scaly leaves of Cycas sp.

Reproduction in Cycas

Vegetative Reproduction

The most common method of vegetative propagation in Cycas is by bulbils.

The bulbils develop from the axil of the scaly leaves.

They are more or less oval structures with a broad base narrowing towards the apex. Several scaly leaves are arranged spirally and compactly over a dormant stem in a bulbil.

On detachment from the stem, a bulbil starts germination by producing many roots towards the lower side and a leaf towards the upper side.

A bulbil from male plant will develop only into the male plant, while from the female plant will form only the female plant because Cycas is a strictly dioecious plant.

Bulbil in Cycas sp.

Sexual Reproduction

Cycas is strictly dioecious, i.e., male and female sex organs are

borne on separate plants.

After several years of vegetative growth the plants start to form sex organs.

Generally, Cycads of more than 10 years of age produce the sex organs.

The male plants develop male cones or male strobili bearing microsporophyll’s, while the female plants produce a loose collection of megasporophylls.

The male cone is terminal while the megasporophylls are produced in succession with the leaves at the top of the stem.

Male Cone

The male cone or male strobilus is a large, conical or ovoid, compact, solitary and shortly-stalked structure, which is generally terminal in position.

It sometimes attains a length of as much as 1.5 metre. In the centre of the cone is present a cone axis.

Several perpendicularly attached microsporophyll’s are arranged around the cone axis in closely set spirals.

Male reproductive structures of Cycas

At the base of male cone many young leaves are present. All the microsporophyll’s in a male cone are fertile except a few at its basal and apical parts.

Male cone of Cycas sp.

The terminal growth of the stem is checked for sometime when a male cone appears at its apex. It is because of the fact that the apical meristem is used up during the development of the male cone.

Cones of some species of Cycas are amongst the largest cones in the plant kingdom.

Longitudinal section of Cycas

male cone

Cone axis

Microsporophyll

Apophysis

Microsporangium

Microsporophylls, Microsporangia and Microspores

Microsporophylls are flat, leaf-like, woody and brown-

coloured structures with narrow base and expanded upper portion.

The upper expanded portion becomes pointed and is called apophysis. Narrow base is attached to the cone axis with a short stalk.

Each microsporophyll contains two surfaces, i.e., an adaxial or upper surface and an abaxial or lower surface.

On the adaxial surface is present a ridge-like projection in the middle and an apophysis at the apex. Microsporophyll of

Cycas sp.

On the abaxial surface thousands of microsporangia are present in the middle region in the groups of 3-5. Each such group is called a sorus. In between these groups many hair-like, very soft, one or two-celled structures are present.

In a microsporophyll, there many microsporangia are present on the abaxial side.

Many pollen grains or microspores are present in each sporangium.

Each microspore or pollen grain is a rounded, unicellular and uninucleate structure surrounded by an outer thick exine and inner thin intine.

Microsporangia of Cycas sp.

Pollen grain of Cycas sp.

True female cone or strobilus is absent Cycas.

Female reproductive organs are present in the form of megasporophylls.

Many megasporophylls are present around the apex of the monopodial trunk of the female plant above each crown of foliage and scaly leaves.

Similar to foliage leaves, megasporophylls also remain spirally

Female reproductive organs of Cycas

Female cone of Cycas sp.

Vegetative leaves and fertile megasporophylls are produced in an

alternate succession without showing any effect on apical men stem. .

arranged at the apex of the stem but their number is very large and thus they appear like a rosette.

Megasporophyll

Each megasporophyll is considered a

modification of foliage leaf. It reaches up to 30 cm or more in length in different species. It is a flat body consisting of an upper dissected or pinnate leafy portion, middle ovule-bearing portion and proximal petiole. Petiole varies in length in different species.

The middle part is comparatively wider than petiole and bears ovules arranged in two pinnate rows. The number of ovules varies between 2-12 in different species. The ovules are green when young but at maturity they are fleshy and bright orange or red-coloured structures.

Megasporophyll of Cycas sp.

Cycas thouarsi contains the largest ovule amongst the living gymnosperms (7 cm in length).

Cycas is used as a source of food in Japan, Australia, South East Asia, southern and eastern parts of India and some other countries. It is used in the preparation of starch and alcoholic drinks. The starch, extracted from its stem, is called ‘sago’.

In Japan, seeds and stem of Cycas revoluta are used for preparing wine. The juice obtained from young leaves of Cycas circinalis is used in skin

diseases, vomiting of blood and stomach disorders. The decoction of young red seeds of C. circinalis is used as a purgative

and emetic. To relieve the headache, giddiness and sore throat, the seeds of Cycas

revoluta are prepared in the form of a tincture and used. Cycas revoluta and C. circinalis plants are grown for ornamental

purposes in various parts of the world. The wood of Cycas revoluta is used for preparing small boxes and

dishes. Cycas leaves, being very large, are used for preparing baskets, mats,

etc. Cycas circinalis seeds are used in Democratic Kampuchea as a fish-

poison.

Economic Importance of Cycas

References