formation of the pollen-aggregating threads in strelitzia reginae

Annals of Botany 77 : 243–250, 1996

Formation of the Pollen-aggregating Threads in Strelitzia reginae

EVA KRONESTEDT-ROBARDS

Department of Biology, Uni�ersity of York, York YO1 5DD, UK

Received: 23 January 1993 Accepted: 17 October 1995

Morphogenesis of the specialized thread-forming (TF) cells in the Strelitzia reginae anther was investigated;particular attention was given to the cell walls and the degree of vacuolation. The mass of both cell wall and cytoplasmincreased until just before dehiscence. However, cell growth and degradation were largely synchronous processes inthe TF cells : before any wall thickening could be observed, degradation of primary cell wall material was alreadyinitiated. This degradation continued, with the result that the mature thread cells were eventually fully separated fromtheir surrounding cells.

Four stages of development, mainly relating to the degree of cell separation, were established. At stage 1, TF cellsbegan to separate from the subepidermis, while at stage 2 some initial cell wall thickening was taking place. The wallsof the TF cell were, at stage 3, thickened considerably (about 1 µm), especially along the radial axes. The texture ofthese walls was loose due to the presence of large intermicrofibrillar regions, and the previously vacuolated cells werefilled with cytoplasm. Longitudinal sections revealed conical gaps in the thick cell wall over the plasmodesmata. Justbefore dehiscence (late stage 3), the TF cells separated from each other and the subepidermis to such an extent thatonly plasmodesmata and fibrillar wall remnants kept the files of TF cells in place. The released uniseriate threads wereclassified as stage 4. (Occasionally the threads were multicellular but only where the transverse walls had not separatedfrom each other.) The threads had thinner cell walls than the TF cells at stage 3 and were vacuolated.

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Key words : Bird-of-Paradise flower, cell wall degradation, Crane flower, cell wall formation, morphogenesis,ontogeny, specialized threads, Strelitzia reginae, ultrastructure.

INTRODUCTION

Pollen dispersal units usually consist of separate grains butin a few angiosperms, e.g. some asclepiads and orchids, thepollen is removed from the anthers aggregated in masses.Such an arrangement facilitates simultaneous transfer ofhigher numbers of grains than would otherwise be possiblein that plant. Strelitzia reginae, which is bird-pollinated, haslarge dispersal units where multicellular threads formaggregates with the pollen (cf. Hesse and Waha, 1983).These threads are derived from the epidermis in the stomium,the cells of which separate from each other and from thesubepidermis at dehiscence (Palla, 1891; Hesse, 1981;Kronestedt and Bystedt, 1981).

The theca consists of the following cell types in Strelitzia :thread-forming (TF) cells, ordinary epidermis, endothecium(absent in the stomium region), initially up to six layers ofparenchyma cells (middle layers, which mainly becomeobliterated during the course of anther development),raphide idioblasts, tapetum, and microsporocytes}micro-spores. Wall thickenings develop in the TF cells, during theprocess of maturation, in the ordinary epidermal cells andin the endothecium. The TF cells differ from ordinaryepidermal cells with regard to their position in the anther,shape, wall texture and amount of cytoplasm. The shape ofthe cross-sectioned specialized cells is oval while that of theordinary epidermal cells is square. Further, the wall of theTF cells gives a looser appearance and the cells containmuch more cytoplasm than the ordinary epidermis (Krone-

stedt and Bystedt, 1981; Kronestedt and Walles, 1981,1983). Results from these studies suggested that twodiametrically opposite processes take place in this specializedepidermis : (a) the degradation of cell wall material, with theultimate result of the cells separating from each other; and(b) a considerable wall thickening. The present investigationwas undertaken to clarify the developmental sequence ofthese events by following themorphogenesis of the epidermisof the stomium. The inflorescensce of Strelitzia , a cincinnuswhere the sequential order of the flower opening can easilybe determined, is ideal for such ontogenetic studies.

MATERIALS AND METHODS

Inflorescences of different ages of Strelitzia reginae Ait.(Bird-of-Paradise flower, Crane flower) were collected fromgreenhouses at the Department of Botany, University ofStockholm and from Haga Park in Stockholm. The lengthof the anthers used in the investigation varied between ! 10and 56 mm. Anthers less than 10 mm were so thin andslender that they were difficult to measure with accuracy;they were thus assigned to one group (i.e. 10 mm). Fixationwas undertaken at room temperature with 2±5% glutar-aldehyde in phosphate buffer, pH 7±0, overnight, followedby 1% osmium tetroxide in the same buffer for 2 h. Thesamples were dehydrated through an acetone series andembedded in Spurr’s resin. Sections were made with anLKB Ultrotome III ; semithin sections were stained with

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244 Kronestedt-Robards—Pollen-aggregating Threads in Strelitzia reginae

toluidine blue O. The ultra-thin sections were contrastedwith a saturated solution of uranyl acetate in ethanol and0±2% lead citrate, and then examined with a Zeiss EM 10-A at 60 kV.

Material for scanning electron microscopy (SEM) wasfixed for 2 h in 2±5% glutaraldehyde and 2% para-formaldehyde in 0±05 phosphate buffer (‘half strengthKarnovsky’). The specimens were postfixed for 2 h in 1%OsO

%in the same buffer and dehydrated in an acetone series

before drying in a critical point drier (CPD), Polaron E3000. The samples were sputter coated with gold in aPolaron SEM coating unit E 5000 and observed in aCambridge 600 SEM.

A digitizing pad attached to a PC (ABC 80 Luxor) wasused in order to obtain the relative areas, and thereforevolumes, occupied by walls and vacuoles in the cells ; inaddition the proportion of lipid bodies and starch grainsrelative to the cytoplasm was estimated. Areas of thesectioned cell components were traced on the pad and arepresented, in this report, as means of the measurementsgiven by the computer. In all, 181 cells were analysed (54 forstage 1, 31 for stage 2, 50 for stage 3 and 46 for stage 4).

RESULTS

The investigated material was divided into four groupsdepending on the degree of separation of the TF cells.

Stage 1. The formation of separation zones along thetangential walls between the TF cells and the subepidermis

Flower buds were taken from an erect inflorescence, orthey constituted the youngest buds in a bent inflorescencewhere the oldest flower was at anthesis. The perianth waswhite and the length of the anthers ranged from " 10 to45 mm.

In cross-section, the length of the TF cells was 60–150 µmand the height about 10 µm. Already at this stage, thesewere larger and more oval than the cells of the ordinaryepidermis, but otherwise the ultrastructure of the two celltypes did not seem to differ (Figs 1 and 2).

The tangential walls of the TF cells in the youngest buds(" 10 to about 32 µm in length) were in close contact withthe underlying cells, as no intercellular spaces were yetformed. The initiation of these spaces took place by theformation of slot-like cavities, which became wider andoften contained a fibrillar material when they had enlarged.The formation of cavities continued within the middlelamella, proceeding from the region of intercellular spaces(Fig. 5) ; the alignment of the cavities often followed thedirection of the microfibrils in the cell wall. The materialcovering the distal region of the cells where these jointogether, here called ‘distal cell junctions’, remainedunbroken at this stage (Fig. 1). The wall material constituted5–6% of the cell volume, and the thickness of both theradial and the distal walls was 0±1 µm. A slightly electronopaque zone in the middle of the distal cell wall wassometimes discernable. The cuticle was thin (about 0±02 µm).

F 1–2. Stage 1. Fig. 1. The profiles of the TF cells were larger andin general more oval than the ordinary epidermal cells. The centralvacuole was large. Some of the intercellular spaces towards thesubepidermal cells still contained material, in which there were slot-likecavities (arrows). The material covering the ‘distal cell junctions ’(arrow heads), had no cavities yet. Bar¯ 2 µm. Fig. 2. Ordinaryepidermal cells with a large central vacuole and thin cell walls. Thematerial covering the ‘distal cell junctions ’ (arrow head) had no

cavities. Bar¯ 2 µm.

Plasmodesmata were found both in the radial and thetangential cell walls.

The cells were highly vacuolated (73%) with only a thin

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Kronestedt-Robards—Pollen-aggregating Threads in Strelitzia reginae 245

L

F. 3. Stage 2. The intercellular spaces were conspicuously widenedand the TF cells had nearly separated from the subepidermal cells.Meanwhile the radial cell walls of the TF cells had thickened and thecentral vacuole decreased. Cavities had formed in the material covering

the ‘distal cell junctions ’ (arrow head). Bar¯ 2 µm.

layer of cytoplasm lining the cell wall. Lipid droplets werecommon both in cytoplasm and vacuoles. The plastids(about 1 µm long) sometimes contained crystals. Theyoungest TF cells (in anthers up to 32 mm long) did notshow any sign of starch.

The cells of the ordinary epidermis (Fig. 2) were smallerand square in appearance on the transections (about7¬7 µm); their distal cell walls were 0±1 µm thick while theother walls were slightly thinner. About 6 % of the totalarea}volume was occupied by wall. The material coveringthe ‘distal cell junctions’ was unbroken and no intercellularspaces were formed. The cells were highly vacuolated (83%of the area}volume) and some plastids contained crystals.

The profile of the subepidermal cells was 60–125¬25 µm.The cell wall thickness was the same as that of the TF cellsand lipid droplets occurred in the cells. Enlarged intercellularspaces were seen between the underlining parenchyma cells.

Stage 2. Separation zones progressed and wall thickeninginitiated

Material was taken from one of the youngest buds in aninflorescence, where one flower had opened. The perianthwas white and the anther length 45 mm.

The intercellular spaces were considerably larger thanpreviously (Fig. 3) and the number of cavities in the middlelamella between the TF cells and the subepidermis hadincreased. Cavities were present also in the material coveringthe ‘distal cell junctions’ ; similar material appeared tocontinue along the outside of the exterior wall, with anelectron opaque zone between this layer and the cell wallproper. The radial walls were thickened to 0±3 µm (Fig. 3),but the distal ones remained about 0±1 µm; the wallconstituted about 9% of the total cell volume.

The cells were vacuolated (69% of the protoplast). Thevacuome (i.e. the vacuolar system) constituted 8% of theprotoplast. The plastids were ! 2 µm long and containedstarch grains. The wall of the subepidermal cells was thesame thickness as at the previous stage.

Stage 3. Late separation stage but threads not yet released

The flower buds were at the point of anthesis. The sepalswere light yellow–orange and the petals white with slightlyblue margins. The anther length was 45–51 mm.

The TF cells were 50–190 µm long, 10–13 µm wide and20 µm high. The intercellular spaces beneath them hadenlarged further so that the cells were connected with eachother and with the subepidermis only by fibrils and

F. 4. Stage 3, with TF cells ready to be released. Only fibrils andplasmodesmata (small arrows) connected the considerably thickenedwalls of the TF cells with each other and with the subepidermis. Thecuticle (asterisk) was distended. The material covering the ‘distal celljunctions ’, and that covering the exterior of the distal wall, held cavities(arrow head). The cells were filled with cytoplasm, and numerouslipidic (L) and electron opaque bodies were present in the cells. Theplastids contained large starch grains (S) and occasionally a crystal

(long arrow). Bar¯ 2 µm.

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F. 5. Stage 1. The formation of cavities continued within the middle lamella, proceeding from the region of intercellular spaces and giving riseto separation zones along the base of the differentiating TF cells (one of which is marked TF on the micrograph). S, subepidermal cell. Bar¯

2 µm.

F. 6. Stage 3. Profiles of slot-like cavities in different directions have separated the thickened cell wall of the TF cells and that of the subepidermalcell (asterisk). The texture of the thickened TF walls appears loose and the layering of the fibrils undulated. Bar¯ 1 µm.

F. 7. Stage 3. The orientation of the microfibrils was identical to that of the cortical microtubules. The surface of the plasmalemma was rough,making numerous finger-like extensions (small arrows) into the periplasmic space. Bar¯ 0±25 µm.

plasmodesmata (Figs 4 and 6). In addition to the cavities inthe middle lamella, there were also cavities within the outerthird of the proximal wall (that is, towards the middlelamella) ; these were parallel to the microfibril bundles. Thematerial covering the ‘distal cell junctions’ also containedcavities and remained, at later stages, only in the form of a

meshwork. The outermost layer of the distal cell wall wasthicker than at the previous stage and contained numerouscavities (Fig. 4). The cuticle had distended from this surface.

The cell wall increased from 25% of the protoplast earlyin stage 3 to 40 % later. The radial walls were much thickerthan the distal ones, i.e. about 1 µm(maximum measurement

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F. 8. Stage 3, longitudinal section. Less deposition of material over the plasmodesmata gave rise to the conical depressions in the walls. Asterisksindicate the transversal walls—the thickest walls of the TF cells. N, nucleus. Bar¯ 2 µm.

1±9 µm) between the cells and 0±7–1 µm distally (Fig. 4). Lessthickening took place around the plasmodesmata, whichgave rise to infundibular (conical) depressions in the walls(as seen from longitudinal sections; Fig. 8). The plasmo-desmata were sometimes branched, from a single point,and}or widened in the middle lamella region. The transversewalls were 3 µm (Fig. 8) ; cavities were also observed in thetransverse wall, in the 0±4–0±8 µm zone closest to the middlelamella. The texture of the walls appeared loose due to thelayering of the fibrils, which were often aggregated intoundulating bundles (Fig. 6). The orientation of the micro-fibrils was similar to that of the cortical microtubules (Fig.7). The surface of the plasmalemma had numerous finger-like extensions, about 21 nm in diameter (i.e. the same asthat of the microtubules ; Fig. 7). Between the cell wallproper and the plasmalemma there was a layer varyingbetween 50 and 100 nm in breadth (Figs 7–10). Somevesicles had apparently fused with the plasmalemma,presumably releasing their contents to the periplasmic zone;this material and that of the vesicles appeared similar intexture (Figs 9–10).

The cells were rich in cytoplasm—the vacuome onlyamounted to about 17% of the whole cell at the point ofdehiscence. The amount of lipid in the cytoplasm hadincreased (from 2–3% of the protoplast at stage 1 to 5–17%at stage 3) ; the osmiophilic nature of these bodies wasshown from uncontrasted sections. Electron opaque bodiesof similar size to the lipid droplets were also abundant ; theyhad a pink colour in toluidine blue stained semi-thinsections. The plastids had an average length of 2 µm and

contained large starch grains and crystals (Fig. 4). Intra-plastidal membranes were few; sometimes there was electronopaque material between them.

The cross-sectioned ordinary epidermal cells were 65–94 µm long with a height of 12–20 µm and a width of10–15 µm. Their wall thickening (about 1 µm in thickness)formed a spiral along the cells and had a denser appearancethan the wall thickening of the TF cells. The primary wall(0±13 µm) was electron opaque and the cuticle remained thin(0±1 µm). No slot-like cavities were present in the ‘distal celljunctions’ or in the middle lamella, but sometimes in-tercellular spaces were enlarged. Plasmodesmata were seenbetween the ordinary epidermal cells and the subepidermis.The cells were highly vacuolated with only a thin layer ofcytoplasm along the cell wall. Crystals were often present inthe plastids of the ordinary epidermis, and the cells containedlipid droplets. The subepidermal cells had walls which wereof the same thickness as in stage 1, and they containedamyloplasts and lipids.

Stage 4. Free threads

The thecas had opened and exposed threads among thepollen grains ; the anther length of these mature flowers was51–56 mm. The sepals were orange and the petals blue.

The TF cells had developed into curled threads (Figs11–12) by separating from the subepidermal layer. Theaverage diameter of the thread cells was 11 µm. Lipidicmaterial was present between the threads and in peripheralcavities (Figs 13–14). The cell wall thickness was about

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F 9–10. Stage 3. Vesicles (large asterisks), in TF cells, apparentlyfusing with the plasmalemma and releasing their content to theperiplasmic zone (small asterisks). The arrow head in Fig. 9 indicatespart of a plasmodesma, and in Fig. 10 a conical depression of the wall

(continuing into a plasmodesma). Bars¯ 0±25 µm.

0±75 µm. The inner part of the wall (about 0±1–0±2 µm) wasdenser than the outer part, which had a loose texture.Transverse cell walls (1–3 µm in thickness) were sometimesobserved (Fig. 13 shows a lateral part) ; such walls largelyhad cavities along their midlamellar region.

When viewed in sections, the thread cells were seen to behighly vacuolated (Fig. 14) with few inclusions. Identifiedinclusions were mitochondria, lipid droplets, other electronopaque bodies, and free starch grains.

The ordinary epidermis remained in its original position,as did the endothecium. The spiral thickening of theordinary epidermis maintained a width of about 1 µm; theprimary cell wall and the cuticle were each 0±1 µm.Plasmodesmata were present in the radial and tangentialwalls, and the cells had large central vacuoles.

DISCUSSION

Significant changes in the anthers of the Strelitzia reginaeflower buds took place just before dehiscence (called stage 3in this investigation). These changes included separation ofthe thread-forming (TF) cells from the subepidermal cells

and from each other, a conspicuous thickening of the cellwalls of the epidermis (and endothecium), and a considerableincrease of cytoplasm in the TF cells (discussed below).

Cell separation

Already before the cell wall thickening had started,disintegration of wall material occurred in the middlelamella region below the TF cells, a process which thereaftercontinued between the radial walls. The outcome of this wasthat before the dehiscence of the anthers, files of TF cellswere made free to such an extent that they were kepttogether only by plasmodesmata and fibrillar wall remnants.Some transverse walls still remained in contact with eachother, making the uniseriate free threads multicellular.Zones of cavities were, however, found in these walls (closeto the middle lamella), which thus presented possiblebreakage zones. The ordinary epidermal walls did not showany breakdown of wall material, not even in the (pre-sumably) pectic material overlying the ‘distal cell junctions’.

Formation of intercellular spaces, mainly in pea, has beenstudied by e.g. Roland (1978), Kollo$ ffel and Linssen (1984)and Jeffree, Dale and Fry (1986). Roland determined the‘splitting layer ’, differing from the pectic middle lamella, inthe hypocotyl ; Jeffree, Dale and Fry suggested thatintercellular spaces are formed schizogenously at predictablepositions in the developing leaves ; and Kollo$ ffel and Linssenproposed that, in developing cotyledons, the localization ofthe intercellular spaces is predetermined by intra-wallstructures limiting the schizogenous process.

The process of cell separation in the specialized epidermisof Strelizia reginae has been suggested to be due to digestionby pectinase; this was determined using cytochemicalmethods (Vennigerholz and Walles, 1987). The presence ofpectin in the middle lamella, the subcuticular layer and overthe ‘distal cell junctions’ was demonstrated with hydroxyl-amine-ferric chloride. The proportion of pectic substances ishigh in the primary wall of Zingiberales (Jarvis, Forsyth andDuncan, 1988), the order to which Strelitzia belongs. Thesesubstances act as a cement, keeping individual cells together,but they are also the most readily extractable polymers inthe cell wall (Selvendran, Stevens and O’Neill, 1985). Thesoftening of a number of fruit types during ripening has alsobeen attributed to enzymatic hydrolysis of cell wallpolysaccharides, largely the pectins. Polygalacturonases,widely distributed in plants, have been claimed to be ofmajor importance, causing a progressive breakdown anddissolution of the middle lamella and in some cases agradual separation of wall fibrils (Ben-Arie, Kislev andFrenkel, 1979; Crookes and Grierson, 1983).

Wall thickening

After the initiation of cell separation, considerable cellwall thickening began. Deposition of wall material was notuniform but was thicker along the radial walls than thedistal ones. The wall of the released threads was thinnerthan that of the mature TF cells. This may be explained bywall material being metabolized during the threads’ lifeoutside the plant body; that these are living structures can

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F. 11–14. Stage 4 with free, curled threads. Fig. 11. Light micrograph (fresh material) of a thread around a pollen grain. P, Pollen grain; asteriskindicates thread. Bar¯ 50 µm. Fig. 12. Scanning electron micrograph of intertwined threads (asterisks) and part of a pollen grain. Bar¯ 50 µm.Fig. 13. Lipidic material between the threads (asterisk), and in wall cavities. Bar¯ 0±5 µm. Fig. 14. Profile of a thread (highly vacuolated). The

asterisk indicates part of a transversal cell wall. Bar¯ 4 µm.

be seen from the cytoplasmic streaming inside them. Themore compact structure of the inner wall in the threads alsoindicates that a transformation of material may have takenplace. Only the transverse walls, where the cells kept incontact in the thread, remained as thick as before dehiscence.Deposition of wall material was reduced over the plasmo-desmata causing infundibular depressions along the longi-tudinal cell walls—and thus the location of plasmodesmata

could easily be determined. In contrast to the epidermalwalls, the cell wall of the subepidermis stayed the samethickness throughout development and did not develop anysecondary thickening.

Cellulose microfibrils were clearly deposited in parallel tothe underlying microtubules, a much discussed phenomenonsince Ledbetter and Porter (1963) first postulated a role ofmicrotubules in the orientation of cellulose microfibrils. The

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orientation of microfibrils affects the mechanical propertiesof plant walls. Arched patterns were revealed in the walls ofthe specialized epidermis, with microfibrils in layers sepa-rated by large intermicrofibrillar regions giving the im-pression of a loose texture. This differed strongly from thetexture of the ordinary epidermis. As to the nature of thecell wall, Hesse and Waha (1983) thought its structure anddensity to resemble that of a cellulosic cell wall, and theyfound that the threads were completely dissolved afteracetolysis.

Cytoplasmic components

Both the starch and the lipidic material constitute apotential nutrient supply for the free-living threads (butwere also found in the subepidermal cells). The plastids wereamyloplasts with only a small amount of membrane,especially in the fully differentiated TF cells, and with largestarch grains. Palla (1891), in fact, regarded the starch to bethe component first used for cell growth. The distinctincrease in lipids, during the ontogeny of the TF cells, alsocould represent a rich source of energy.

The vacuome was well established in the youngest TFcells. The amount of cytoplasm, however, increased duringthe course of morphogenesis and largely filled the cells justbefore dehiscence. Meanwhile the vacuome increased in theordinary epidermal cells, the subepidermal cells and the cellsof the middle layer. Cycles of differentiation and dedifferen-tiation have been described from secretory systems such asthe tapetum in Pinus during meiosis (Rowley and Walles,1985). Such dynamic changes have been observed inStrelitzia, apart from in the TF cells, in the developingnectary gland and in the transmitting tissue (unpubl. res.)during morphogenesis, and in the tapetum (Kronestedt-Robards and Rowley, 1989).

The surface of the plasmalemma had tubular evaginationsinto the periplasmic space. The dimensions of these were thesame as those of the microtubules. They resembled theplasmalemmasomes that have been described by Harris(1981) and then dealt with in a series of papers (e.g. Harrisand Chaffey, 1986; Kandasamy, Kappler and Kristen,1988). These evaginations were interpreted to increase themembrane surface area in order to facilitate short-distancetransport during a limited amount of time (whereas ‘ transfercells ’ are built up for long-term transport).

The present investigation demonstrates the timing of thewall thickening in relation to the cell separation, in theepidermis of the theca of Strelitzia. These events took placein the opposite order to what might be expected. Further,the maturation process of the TF cells can obviously be

related to microspore development, as investigated byKronestedt-Robards and Rowley (1989). It is then foundthat the first release of the threads coincided with the onsetof pollen grain dispersal, which is something that could beexpected.

LITERATURE CITED

Ben-Arie R, Kislev N, Frenkel C. 1979. Ultrastructural changes in thecell walls of ripening apple and fruit. Plant Physiology 64 :197–202.

Crookes PR, Grierson D. 1983. Ultrastructure of tomato fruit ripeningand the role of polygalacturonase isoenzymes in cell walldegradation. Plant Physiology 72 : 1088–1093.

Harris N. 1981. Plasmalemmasomes in cotyledon leaves of germinatingVigna radiata L. (mung-bean). Plant, Cell and En�ironment 4 :169–175.

Harris N, Chaffey NJ. 1986. Plasmatubules—real modifications of theplasmalemma. Nordic Journal of Botany 6 : 599–607.

Hesse M. 1981. Viscinfa$ den bei Angiospermen—homologe oderanaloge Gebilde? Mikroskopie 38 : 85–89.

Hesse M, Waha M. 1983. The fine structure of the pollen wall inStrelitzia reginae (Musaceae). Plant Systematics and E�olution141 : 285–298.

Jarvis MC, Forsyth W, Duncan HJ. 1988. A survey of the pectic contentof nonlignified monocot cell walls. Plant Physiology 88 : 309–314.

Jeffree CE, Dale JE, Fry SC. 1986. The genesis of intercellular spacesin developing leaves of Phaseolus �ulgaris L. Protoplasma 132 :90–98.

Kandasamy MK, Kappler R, Kristen U. 1988. Plasmatubules in thepollen tubes of Nicotiana syl�estris. Planta 173 : 35–41.

Kollo$ ffel C, Linssen PWT. 1984. The formation of intercellular spacesin the cotyledons of developing and germinating pea seeds.Protoplasma 120 : 12–19.

Kronestedt E, Bystedt P-A. 1981. Thread-like formations in the anthersof Strelitzia reginae. Nordic Journal of Botany 1 : 523–529.

Kronestedt E, Walles B. 1981. Structural studies on the specializedepidermis in the anthers of Strelitzia reginae. Journal of Ultr-astructure Research 76 : 315–316.

Kronestedt E, Walles B. 1983. Wall growth and separation of thread-forming cells in Strelitzia reginae. Journal of UltrastructureResearch 85 : 110.

Kronestedt-Robards EC, Rowley JR. 1989. Pollen grain developmentand tapetal changes in Strelitzia reginae (Streliziaceae). AmericanJournal of Botany 76 : 856–870.

Ledbetter MC, Porter KR. 1963. A ‘microtubule ’ in plant cell finestructure. Journal of Cell Biology 19 : 239–250.

Palla E. 1891. Ueber die Entwicklung und Bedeutung der Zellfa$ den imPollen von Strelitzia reginae. Berichte der deutschen botanischenGesellschaft 9 : 85–91.

Roland JC. 1978. Cell wall differentiation and stages involved withintercellular gas space opening. Journal of Cell Science 32 : 325–336.

Rowley JR, Walles B. 1985. Cell differentiation in microsporangia ofPinus syl�estris. III. Late pachytene. Nordic Journal of Botany 5 :255–271.

Selvendran RR, Stevens BJH, O’Neill MA. 1985. Isolation and analysisof cell walls. In: Brett CT, Hillman JR, eds. Biochemistry of plantcell walls. Cambridge: Cambridge University Press, 39–78.

Vennigerholz F, Walles B. 1987. Cytochemical studies of pectin digestionin epidermis with specific separation. Protoplasma 140 : 110–117.

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formation of the pollen-aggregating threads in strelitzia reginae

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