the ultrastructure and ontogeny of pollen in...

J. Cell Sci. 5, 459-477 (1969) 459Printed in Great Britain

THE ULTRASTRUCTURE AND ONTOGENY

OF POLLEN IN HELLEBORUS FOETID US L.

III. THE FORMATION OF THE POLLEN GRAIN WALL

P. ECHLIN AND H. GODWINBotany School, University of Cambridge, England

SUMMARY

The first recognizable elements of the pollen grain wall of Helleborus foetidus are initiated inthe cellulosic primexine which is formed immediately outside the microspore cytoplasm whilethe pollen grains are still in the tetrad configuration and enveloped in a thick layer of callose.Elements of the primexine give rise to the precursors of the rod-like bacula of the matureexine. The bacula increase in electron density due to the rapid deposition of sporopollenin,and begin to expand laterally at the outer side to form the tectum. There follows a lateralexpansion on the inner side to form the foot-layer. Further deposition of sporopollenin is con-tinued and all elements of the pollen grain wall expand outwards and laterally as the pollengrain enlarges. The enveloping callose disappears and the pollen grains are free in the thecalcavity. A secondary exine is deposited below the primary exine, particularly around the furrows.Initially this process involves a number of thin electron-transparent lines or lamellae about4 nm thick that appear to arise from the cytoplasm and provide a locus around which sporo-pollenin is deposited. As the deposition proceeds, the lamellae thicken and finally merge witheach other to form the secondary exine. No sign of the lamellae can be seen in the mature pollengrain wall. Towards the end of secondary exine formation the deposition of sporopollenin doesnot appear to be centred on thin lamellae, but appears as small granules which graduallycoalesce. The secondary exine remains discontinuous in the region of the furrow, but becomesconsolidated in the inter-furrow regions.

As the pollen grain matures the sporopollenin, which is electron-dense when initially de-posited, becomes progressively less so.

The final stage in development is the deposition of the cellulosic intine, which forms insidethe secondary exine and is associated with increased dictyosome activity and randomlyoriented microtubules.

INTRODUCTION

A description has already been given of the morphological events which occurduring the early stages of development of the pollen grain of Helleborus foetidus(Echlin & Godwin, 19680, b). These studies, together with the results of other in-vestigators, have shown that the early stages of wall formation during which theposition of the vertical elements of the wall are located and the ultimate exine patterndetermined, occur while the pollen grain is enveloped in a largely impermeable layerof callose.

Although numerous studies have been made on the form and structure of the wallof the mature pollen grain, relatively few investigations have been made on the ultra-structural features accompanying the morphological changes which occur duringdevelopment of the wall. The principal studies in this direction have been the work of

460 P. Echlin and H. Godwin

Heslop-Harrison on Silene (Heslop-Harrison, 1963 a, b) and Lilium (1968 a, c), Skvarla& Larson on Zea (1966), Larson & Lewis on Parkinsonia (1961, 1962), Rowley onCommelina (1959), Godwin, Echlin & Chapman on Ipomoea (1967) and Angold onEndymion (1967 a, b). This present paper describes an attempt to follow by electronmicroscopy the structural changes in both the cytoplasm and wall accompanyingpollen grain ontogeny during the latter stages of development, from the period ofprobacula initiation through exine development to the formation of the cellulosicintine. Preliminary reports of this work have appeared elsewhere (Echlin, 1968,1969a, b; Echlin & Godwin, 1968c).

MATERIALS AND METHODS

Intact flowers of Helleborus foetidus L. (Stinking Hellebore) were removed from plants grownin a Cambridge garden, and the stage of development of the anthers established by light-microscopic examination of acetocarmine squashes. Intact anthers of the required stage wereremoved from the flowers, immediately placed in 5 % glutaraldehyde in o-i M Sorenson'sphosphate buffer (pH 7-0) and cut in half. The anthers remained in the fixative for 15 h at 4 °C.The tissue was then thoroughly washed in Palade's veronal-acetate buffer (pH 70) and post-fixed for 2 h at 4 °C in 1 % osmium tetroxide in Palade's veronal-acetate buffer (pH 7#o).Although this particular fixation regime proved to be adequate for all stages of development,an attempt was made to obtain more critical fixation of tetrads within the callose. The pro-cedure closely followed the fixation regime previously outlined and involved a series of differentconcentrations of glutaraldehyde for varying times, followed by the appropriate post-fixation.

Although the findings are based on material fixed by the glutaraldehyde/osmium method,it was considered necessary to prepare material using permanganate as a fixative, as this hasbeen the principal technique used by other workers who have studied pollen ontogeny at theselater stages. Accordingly, anthers from the appropriate stages in development were placedeither in 2 % potassium permanganate in distilled water or in 2 % potassium permanganate ino-i M phosphate buffer (pH 7-0) for 2 h at room temperature.

All the tissues fixed by the various methods were dehydrated in a graded ethanol series,passed through 3 changes of 1:2 epoxypropane and embedded in Araldite without plasticizer.Thin sections were briefly stained with either lead citrate or potassium permanganate whereappropriate, dried, coated with a thin layer of evaporated carbon and examined in an AEI EM 6electron microscope.

The majority of the micrographs in this investigation were prepared from material treatedby the 5 % glutaraldehyde 1 % osmium tetroxide technique outlined above.

Samples of anther segments were acetolysed using a mixture of 1 part concentrated sulphuricacid and 10 parts glacial acetic acid. Small aliquots of pollen were placed in this mixture forvarying time periods of between 15 s and 1 h and at temperatures ranging from 150 to 100 °C.After treatment the samples were thoroughly washed in distilled water and placed in 4 %aqueous osmium tetroxide for 90 min at room temperature. The dehydration and subsequentembedding in resin followed the method outlined above.

RESULTS

The previous studies had shown that the initiation of probacula in the cellulosicprimexine represented the first definitive sign that wall formation had occurred.Whereas the primexine is fibrous in appearance in the electron microscope, theprobacula, although of comparable electron density, are finely granular. They arespaced at regular intervals in the primexine (Fig. 7). During the early stages, theprobacula progressively increase in electron density, and although more electron-densethan the surrounding primexine, are not as electron-dense as the exinous elements of

Ontogeny of Helleborus pollen grain wall 461

the mature wall. The probacula gradually develop into the bacula, which are thevertical elements in the pollen grain wall as viewed in transverse section (Fig. 8). Asdevelopment proceeds, the bacula show a slight swelling at their distal end, and this isfollowed by a lateral extension of the distal end to form the tectum (Fig. 8). Followingthese changes at the distal end, the proximal end of the bacula shows similar changesand there is a lateral extension to form the foot-layer (Fig. 10). A synchronous appear-ance of the initial elements of the tectum and the foot-layer appears to be the exceptionrather than the rule, and it is more usual to find much of the tectum formed with onlyminimal formation of a discontinuous foot-layer (Fig. 9). In some micrographs it ispossible to see what appear to be isolated pieces of tectum apparently unsupported bya baculum (Fig. 8). There is no evidence to suggest that the tectum develops in isola-tion, and serial sections indicate that such pieces of tectum are in fact subtended bybacula which are out of the plane of sectioning.

The formation of the initial elements of the pollen grain wall does not proceed atthe same rate at all sites around the periphery of the microspore. Initially, growthoccurs in the inter-furrow regions, and this is well advanced, usually to the bacula,tectum and foot-layer stage, before there is any noticeable wall formation in theregions where the furrow will form. The sheets of endoplasmic reticulum runningparallel to the cell surface which marked the furrow region at an earlier stage ofdevelopment (Echlin & Godwin, 19686) are still present at this stage.The formationof both the tectum and foot-layer appears not to be preceded by precursor structures,as appears to occur in bacula formation, but there is an immediate and progressiveaccretion of moderately electron-dense material at the base and apex of the bacula.The fibrous primexine still remains very much in evidence, and may be easily dis-tinguished from the more finely granular callose, which still envelops the developingmicrospore (Figs. 3, 9). Towards the end of this initial phase the callose, although stillcompletely surrounding the microspores, begins to show signs of dissolution in theregions immediately to the outside of the developing tectum (Figs. 1, 10). The foot-layer, bacula, and tectum when seen in cross-section have approximately the samedimensions, and are of uniform electron density. In surface view the tectum and foot-layer consist of processes that radiate in all directions from the bacula, and soon thereis as much tectum above the bacula as there is foot-layer formed at its base.

During the stage of development in which the bacula, tectum and foot-layer areformed, the microspore cytoplasm shows few distinctive features. The mitochondriaare dense and ill-defined and the majority of the ribosomes appear free in the cyto-plasm. The nucleus is large, with prominent nucleoli. The cell membrane is highlyconvoluted and there appear to be a large number of vesicles in the peripheral cyto-plasm. In size and shape these are similar to the vesicles derived from the proximal faceof the dictyosomes, that are also a prominent feature of the peripheral cytoplasm at thisstage. In some places the cell membrane appears to be discontinuous and there arespaces which open into the inter-baculoid cavity. Such profiles may well representthe fusion of Golgi-derived vesicles with the cell membrane, with the consequentevacuation of the vesicle contents into the space immediately outside the cytoplasm.This activity may account for the apparent space which invariably appears between the

462 P. Echlin and H. Godzoin

developing wall and the underlying microspore cytoplasm. This space is not a con-sequence of faulty fixation as adjacent cells in the same block do not show this pheno-menon. The microtubules which were in evidence at the early stage when the primexinewas formed and the probacula were initiated are no longer visible. The grey spheroidalbodies, like those previously reported as a characteristic feature of the tapetal cyto-plasm at this stage, are occasionally found in the microspore cytoplasm (Fig. 1).

Although the development of the wall is a continuous process, the initial stages ofbacula, tectum and foot-layer formation constitute a recognizable stage in develop-ment. For this reason it has been proposed to refer to this part of the exine as primaryexine, and the part of the exine which develops later as secondary exine.

The phase of primary exine formation is followed by the development of substantialamounts of underlying secondary exine, particularly in the region of the furrow. Thedeposition of sporopollenin in and on the wall is closely correlated with the dissolutionof the callose and the release of the microspores from the tetrad. In certain unspecifiedregions of the foot-layer, thin electron-transparent lines appear surrounded by electron-dense material (Fig. 3). This thin layer is apparent only when surrounded by electron-dense material, and has the dimension (3-5-4-0 nm) of the inner electron-transparentregion in the so-called 'unit membrane'. These thin white lines (or 'tapes') are seento arise from the cytoplasm (Fig. 4) and are considered to be of a membranous origin.They appear to serve as a locus around which the initial material of the secondaryexine is deposited. The tapes have been seen only in longitudinal section, and it isthought that they form a thin layer in the developing secondary exine, rather like athin tape wound round a spheroidal object.

The tapes first appear towards the conclusion of foot-layer deposition, at a stagewhen the tectum is well developed and has more than doubled its original thickness.

Electron-dense material appears to accrete around each tape to form an elongatepara-cylindrical strand, and it would appear that each strand consists of only a limitedamount of electron-dense material (Fig. 2).

Because of its resistance to acetolysis the electron-dense material is thought to becomposed principally of sporopollenin. It is worth noting that as the exine develops inH. foetidus there is a considerable change in its electron density (compare Figs. 2and 6). The most recently deposited sporopollenin is invariably electron-dense and thewhole wall of the pollen grain shows a gradual decrease in electron density as itmatures. The significance of this will be discussed later.

The thin primary exine layer is completed in the furrow region, but unlike the forma-tion of the wall in the inter-furrow region, is rather sparsely sculptured and shows veryshort bacula with swollen heads and a minimal foot-layer (Fig. 2).

It will be convenient to distinguish between the development in the region of thefurrow and in the inter-furrow region. In the inter-furrow region the tectum and foot-layer continue to thicken, while the bacula show only a small increase in their girth.By the time the pollen grain may be considered mature, the tectum and foot-layer areof equivalent thickness, and 2-3 times thicker than the bacula (Fig. 12). The tectumdevelops irregularities on the outer surface, which appear as minute papillae in surfaceview.


The secondary exine continues to develop initially in gaps in the foot-layer andsubsequently below the foot-layer and in the mature grain a thin electron-transparentzone appears between the secondary exine and the primary exine (Fig. 12). Theappearance of the tapes around which sporopollenin appears to accumulate is only atransitory phenomenon in the inter-furrow region, and the electron-dense accretionssoon coalesce to form a homogeneous layer which is approximately half the thicknessof the overlying foot-layer.

In contrast to the relatively meagre formation of secondary exine in the inter-furrowregion there is massive formation in the region of the furrow (Fig. 2). The only furtherdevelopment of the primary exine appears to be centred on the increase in size of theouter part of the bacula. A tectum does not form in the furrow region, although theswellings on individual bacula eventually do approach the same size as the thickness ofthe tectum in the inter-furrow region (Fig. 2). In the inter-furrow region there is acomplete perforate tectum and a continuous foot-layer, and as one approaches theregion of the furrow so the perforations increase in size and the tectum graduallydisappears, until all that is left in the middle of the furrow are thickened bacula.

Two outstanding features in the development of the secondary exine in the furrowregion are the persistence of the tapes and accompanying electron-dense material, andthe greatly increased thickness of secondary exine which continues to develop andwhich does not appear to be associated with the tapes (Fig. 2). As the pollen grainmatures the electron-dense material surrounding the tapes gradually coalesces to forma homogeneous layer and the tapes themselves eventually disappear. Thus, the amountof secondary exine deposited by accretion around tapes varies according to the locationof the site on the periphery of the pollen grain.

The greatest increase in thickness of the secondary exine in the furrow region is ina layer immediately inside the layer containing the tapes (see Fig. 2). This regionincreases in size by the aggregation of small electron-dense granules into increasinglylarger masses. Discontinuities are found in this region, and for a long time duringdevelopment the underlying cytoplasm of the microspore is separated from the thecalcavity by only the microspore cell membrane (Fig. 2). As the grain matures so thisdiscontinuous wall layer shows signs of consolidation, but it never becomes as compactand apparently impenetrable as the secondary exine in the inter-furrow region. At thetime of maturity the secondary exine, measured at its maximum width in the furrowregion, is 8-10 times thicker than the secondary exine in the inter-furrow region(Fig. 2).

During the phase of secondary exine formation the pollen grain cytoplasm also showssome distinctive changes. The cell membrane remains highly convoluted and a largenumber of Golgi-derived vesicles may be seen in the peripheral cytoplasm. Thereappears to be a small, but nevertheless progressive increase in the number of greybodies and mitochondria in the pollen grain cytoplasm (Fig. 2). These two organellesappear to be randomly distributed and there does not appear to be a greater number ofmitochondria and grey bodies in the furrow region, where the bulk of the secondaryexine is deposited, than in the inter-furrow region which has only a thin secondaryexine.

464 -P. Echlin and H. Godwin

It will be remembered that microtubules were a prominent feature of the peripheralcytoplasm at the time of primexine formation, and that as the initial elements of thewall were deposited, so these structures became less frequent. With the onset ofsecondary exine formation, and in particular its formation in the furrow region, thesestructures reappear in the cytoplasm.

Concurrently with the changes in the pollen grain, other changes are occurring inthe region immediately outside the pollen grain. The cellulosic primexine has finallydisappeared, as has the callose layer which has shrouded the developing pollen grainsince the time of microsporogenesis (Figs. 2, 12). The pollen grains with their dis-tinctly sculptured walls are free in the thecal cavity, although they are mixed with theproducts and remains of the disintegrating tapetum (Fig. 6). The inter-baculoid cavity,which forms a continuous space extending over the whole surface of the pollen grain,appears to be in open contact with the thecal cavity.

The final stage in the development of the pollen grain wall is the formation of thecellulosic intine, which is deposited after the major features of exine patterning havebeen defined and the bulk of the secondary exine has been deposited.

The intine is characterized by a fibrillar appearance which is quite distinct from thehomogeneous appearance of the exine. The first indication of intine formation is in thefurrow region, where there is an apparent thickening of the highly convoluted cellmembrane, followed by a further increase in the number of Golgi profiles and vesiclesin the periphery of the pollen cytoplasm (Fig. 5). It is difficult to define exactly thefirst appearance of the intine and there is evidence which suggests that it is depositedbefore secondary exine formation is completed. This results in an interbedding of thesecondary exine and the intine and, while this may be seen at any point around thepollen grain wall, it is particularly noticeable around the periphery of the furrow(Fig. 6). The effect may be seen better in pollen grains which have undergone brief(30 s) acid acetolysis at room temperature.

Although the exact timing may be in doubt, there is little doubt concerning the modeof deposition. An electron-transparent gap is seen between the cytoplasm and thesecondary exine, and from the appearance of the cell membrane it is evident that thisgap has formed by the evagination of vesicles from within the pollen grain cytoplasm.The electron-transparent gap is now progressively filled with a finely fibrillar material,and in some instances it is possible to see the same finely fibrillar material in smallvesicles within the pollen grain cytoplasm (Fig. 5). There are microtubules at thisstage of development, in the outer 0-5 /im of the peripheral cytoplasm, and they runat various angles to the surface of the pollen grain. This appearance of microtubulesmay frequently be the first sign of intine formation. However, farther into the pollengrain there are progressively fewer microtubules except in the perinuclear region,where as the pollen grain approaches maturity, they appear in distinct groups andassume a configuration tangential to the nuclear membrane.

The morphological changes that occur in the tapetum during the development ofthe pollen grain have already been described (Echlin & Godwin, 1968 a). Suffice it tosay that the appearance of bulk secondary exine formation in the pollen correspondswith the maximal appearance of Ubisch bodies upon the surface of the tapetal cells.


By the time intine formation is initiated the tapetal cells show considerable cellulardegeneration, with the concomitant release of the bulk of the remaining organelles intothe thecal cavity. This material, which is called tryphine, frequently has a much greaterelectron density than the exine of the mature pollen grain wall: it covers the pollengrains, frequently also filling the inter-bacular cavity.

DISCUSSION

The primexine is the first-formed component of the pollen grain wall, and therehas been some discussion concerning its chemical nature. From a comparative studyof the effects on other cells in the anther of Helleborus foetidus of the chemical fixativesand stains used in this present work and because of its fibrous appearance in theelectron microscope, it is considered that the primexine is cellulosic. More definiteinformation concerning the chemical nature of this material has come from recentwork on Lilium by Heslop-Harrison (1968 c), who has shown that when the walls ofmicrospores, removed mechanically from the tetrad during the period of early exinepatterning, are exposed to cellulase digestion, the exine patterning disappears. Water-keyn (1968), in a careful light-microscope study of the early stage of wall formation inIpomoea purpurea, concludes that the primexine is of a mucilaginous nature, consistingof non-cellulosic polysaccharides. He considers that this layer becomes birefringentand dichroic only after fixation, at which time it may possess an oriented ultrastruc-ture. It is difficult to reconcile this finding with the observations obtained by electronmicroscopy, as in the studies on H. foetidus it is possible to distinguish quite easilybetween the appearance in electron micrographs of the finely granular callose (astraight-chain ^-1-3 glucan) and of finely fibrillar cellulose (a straight-chain /?-i-4glucan).

Heslop-Harrison (1968 a) considers that in Lilium longiflorum the probacula in theearly stages of formation are associated with lamellate sheets or concentric cylindersof a lipoproteinaceous material of unit-membrane dimensions. In a later paper (1968 c)he was able to show in L. longiflorum that the initial material of the probacula is notresistant to acetolysis, but that as the probacula consolidate, a network is formed whichis acetolysis-resistant. This material, because of its differential stainability and apparentabsence of lignin, is termed 'protosporopollenin'. On release from the tetrads theprotosporopollenin is thought to be rapidly converted to sporopollenin, which givesa positive reaction for lignin, a property lost later in development. In H. foetidus theelectron opacity of the probacula is identical with that of the pro-Ubisch bodies foundin the tapetum, and the similar grey bodies found in the developing pollen grain cyto-plasm. It is known from previous studies (Echlin & Godwin, 1968 a) that the pro-Ubisch bodies become enveloped with sporopollenin, and in the present paper thegrey bodies in the pollen grain cytoplasm are closely associated with sporopolleninsynthesis. Larson (1965), using permanganate fixation, described conspicuous andnumerous bodies with electron-dense outer coats in the developing pollen of a numberof different plant species. These bodies reacted to fixation in the same way as theUbisch bodies also described in the same plants, and similar bodies have been recog-nized as containing lipids (Frey-Wyssling, Grieshaber & Muhlethaler, 1963).

30 Cell Sci. 5

466 P. Echtin and H. Godwin

In H. foetidus the lateral spreading of electron-dense material at the base and apexof each baculum is not unlimited and it appears as if each baculum contained informa-tion or metabolites for only a finite accretion of sporopollenin. From an examinationof tangential sections it is apparent that the top and base of each baculum increasesradially by about 4 times the radius of the central part of the baculum. The morpho-logical appearance of the primary exine requires an explanation of the fact that elements,especially the bacula, do not grow beyond certain dimensions, and Godwin (1968 a)has suggested that if access of material to the primary exine is from the inter-bacular•cavities, then as these become more occluded the rate of accretion will slow•down.

At the stage in wall development when tectum and foot-layer are being deposited,the first signs of the dissolution of callose may be seen. Callose is a very labile, hygro-scopic substance, and is one of the first of the components in the anthers to showfixation damage. It is therefore difficult to decide whether the small electron-transparentregion immediately outside the primexine-primary exine region is the result of fixationdamage or of callose dissolution. It is not clear whether this space between the calloseand the primexine is formed before or after the formation of the elements of the•primary pollen grain wall.

Echlin & Godwin (19686) have previously commented on the presence of the largenumber of microtubules in the periphery of the microspore cytoplasm during the•early stages of wall development. At that time it was conjectured that the microtubuleswere concerned in some unspecified way with formation and/or location of the pro-bacula, but this remains unconfirmed. It now seems that the microtubules, along withvesicles derived from the dictyosomes, are concerned with the deposition of the cellu-losic primexine. The dictyosome activity is thought not to be limited to the formation•of the cellulosic primexine, but to play an important role in the formation of theprimary exine.

A significant feature of the formation of early secondary exine in H. foetidus is theaccretion of electron-dense (sporopollenin) material around thin electron-transparentlamellae, 4-0 nm in diameter. Godwin et al. (1967) and later Godwin (1968 a) havedefined the term 'primary exine' to include those wall elements which originate in theprimexine, and the term ' secondary exine' to those layers which arise later in develop-jnent usually as a result of accretion around thin tapes or lamellae. In H. foetidus thetapes frequently occur in or below the foot-layer, so that it is impossible to draw aJiard and fast line between primary and secondary exine formation.

A number of papers have appeared confirming the existence of the thin electron-transparent regions around which sporopollenin may accrete. Godwin et al. (1967),Rowley & Southworth (1967), Rowley & Dunbar (1967), Dunbar (1967), and Rowley{1967) point out a number of ways in which such membranes or tapes may originate.Although such membranous structures may either pre-exist in the cytoplasm or repre-sent an outwards extension of the pollen-grain cell membrane, it is not known whetherthey represent sites for the de novo synthesis of sporopollenin or templates on whichsporopollenin precursors accrete and condense. Dickinson & Heslop-Harrison (1968)consider that all layers of the exine of Lilium longiflorum originate by a process in-


volving the 'association or apposition of lamellae'. The dimensions and stainingcharacteristics of these lamellae are similar to those of the lamellae or tapes seen duringearly stages of secondary exine formation in H. foetidus, but there is at present noevidence to suggest that lamellae are associated with primary exine or with the laterstages of secondary exine deposition in H. foetidus. Angold (1967 a, b) showed inEndymion non-scripta L. that tapes were seen during only the earliest stages of second-ary exine development, whereas the studies of Godwin et al. (1967) showed that inIpomoea purpurea L. the tapes were present until the final stages of secondary exinedeposition. It would thus appear that the extent to which tapes are seen associatedwith the formation of the exine varies according to the plant species concerned.

In the studies where permanganate was used as a fixative, the exine invariablyshows an even density when viewed in the electron microscope, irrespective of thedevelopmental stage of the pollen grain. This is not so when osmium is used as afixative, and as indicated previously the variation in electron density may be relatedto the maturity of the exine. In order to understand better the nature of the fixationby osmium derivatives, it will be necessary to review the chemistry of sporopolleninand osmium tetroxide.

The early work of Zetzsche (1932) which was carried out principally on spore coatsof Lycopodium clavatum suggested that sporopollenin was likely to be polyterpenoid innature. A recent more extended study by Shaw & Yeadon (1966) has shown this notto be so. These workers were able to show, using mild degradation techniques, thatthe spore walls of Lycopodium and Pinus are made up of 10-15 % cellulose, 10 % xylan-hemicellulose fraction, 10-15% lignin and about 60% lipid. The lipid fraction iscomposed primarily of simple mono- and di-carboxylic acids which have a maximumchain length of 16 carbon atoms. Following the work of Shaw & Yeadon, Heslop-Harrison (1968 c) has found that in the early stages of wall formation, the exine showsa transitory yet positive response to some of the common tests for lignin, revealingthe presence of free aldehydes. But as maturation proceeds, this reactivity for ligninis progressively lost, being masked by the accretion and condensation of other wallcomponents.

Some more recent work on Lilium henryi by Brooks & Shaw (1968) modifies theconclusions of the earlier studies, as these authors now have substantial evidence thatthe pollen grain exine is derived by an oxidative polymerization process from caro-tenoids and carotenoid esters present in the anthers, and that it is improbable thatlignin forms any part of the pollen exine. The grey spheroidal bodies which are foundin the periphery of H. foetidus microspore cytoplasm during exine formation mayrepresent depots of carotenoids, as the grey bodies are morphologically similar tostructures found in other tissues which are known to contain carotenoids.

Heslop-Harrison (1968 c, d) has followed the development of the pollen grain walland the maturation of the tapetum of Lilium longiflorum in relation to the synthesisand appearance of carotenoids as measured by their absorbance at 450 nm, and doesnot consider these carotenoid pigments contribute to sporopollenin synthesis, as thesynthesis is completed before these globules are released.

Heslop-Harrison makes a valid point that, although coloured carotenoids do not30-2


appear to be involved in sporopollenin synthesis in Lilium, this does not preclude theconcept that colourless precursors of related molecules are involved.

The initial studies of Criegee & Richter (1936) showed that osmium tetroxidereacts stoichiometrically with olefins to form a stable osmic acid ester of the glycolderived from the olefin by oxidation. More recent work by Korn (1966) has shown thatosmium tetroxide reacts with the double bonds of olefinic groups in lipids to formstable osmic acid esters. The product of such reactions is a dimer, and it is possiblethat polyunsaturated fatty acids might form polymeric structures linked by diesters ofosmic acid.

This affinity by osmium tetroxide for hydrocarbon double bonds would help to•explain why the most recently deposited sporopollenin is very much more electron-dense than the earlier deposited material. The results of Brooks & Shaw (1968)suggest that sporopollenin is formed by the oxidative polymerization of carotenoidsand carotenoid esters. According to Karrer & Jucker (1950) there are approximatelyeighty natural carotenoids, all of which are closely related chemically and belong tothe class of polyisoprenes whose chief characteristic is the possession of a large numberof conjugated double bonds. Thus, the constituent molecules of sporopollenin wouldbe expected to contain many reactive double bonds which would bind covalently withosmium tetroxide. As polymerization of the sporopollenin proceeds, these previouslyavailable double bonds are themselves involved in cross-linking to form larger poly-mers, leaving fewer available sites for binding osmium tetroxide. The net result ofsuch a polymerization would be a progressive decrease in the electron density of thepollen grain wall. It thus appears possible to gauge the state of maturity of a pollen£rain wall by the electron density of the sporopollenin which becomes progressivelyless electron-dense as it ages. The initial electron transparency of the probacula is dueto the absence of any reactive components, but as protosporopollenin is formed onthese structures so there is a progressive increase in osmophilia, and as the proto-sporopollenin is polymerized into sporopollenin so the osmophilia decreases.

Mepham & Lane (1968, 1969) consider that the exine of Tradescantia bracteata iswholly derived from a secretion of the pollen protoplast and that the tapetum makesno direct contribution to its development. Although this finding has already beendiscussed in some detail by Godwin (19686) and Echlin (1969 a) it is appropriate thata further brief comment be made in this present paper.

Although the hypothesis put forward by Mepham & Lane remains a possibility,there is at present no published evidence (i.e. electron micrographs) to show that theelectron-transparent structures seen in the lipid material which fills the inter-bacularcavities of Tradescantia bracteata do in fact pass through the secondary exine and theintine into the pollen grain cytoplasm. Nor does the recent study by Horvat (1966)on the formation and development of the exine in Tradescantia paludosa provide suchevidence. Although Horvat's micrographs show that there are abundant lipid depositsin the inter-bacular cavity, there are no such deposits in the pollen grain cytoplasm.In an earlier study by Rowley (1959), in which the preservation was less good, it waspossible to demonstrate the presence of lipids in both the inter-bacular cavity andthe pollen grain cytoplasm of T. paludosa. However, neither Rowley nor Horvat was


able to show the presence of electron-transparent lamellae in the inter-bacular lipidand did not comment on the possibility that the whole of the exine may originate fromwithin the pollen grain.

The electron-transparent structures which Mepham & Lane have shown to existin the inter-bacular lipid of Tradescantia bracteata may occasionally be seen in thecorresponding material that fills the inter-bacular cavity of H. foetidus. There is, how-ever, no evidence to suggest that these lamellae extend through the pollen grain walland into the pollen grain cytoplasm. The variability in appearance of these lamellarstructures in H. foetidus leads one to suggest that they may be an artifact of faultyfixation rather than real structures.

Before the hypothesis of Mepham & Lane could be adopted it would be necessaryto have, first, electron micrographs showing the stages of exine ontogeny whilst thecallose special wall still surrounds the individual microspores and the whole tetradand, secondly, electron micrographs that are truly radial and show lamellae con-tinuously extending through the inter-bacular cavities, secondary exine, intine and intothe microspore cytoplasm. Mepham & Lane provide neither, and Horvat's figures areexcellent evidence for the contention that the section displaying lamellae, on whichMepham & Lane rely, is very far from radial. The complete series of ontogeneticstages produced by Horvat show well-developed bacula before dissolution of thecallosic special wall, but it is not until after this stage that lipids appear in the inter-bacular cavities or when the tapetal material first comes into direct contact with thedeveloping exine. It is impossible from the published micrographs of Mepham & Laneto differentiate satisfactorily between the lipid material thought to be extruded by thepollen grain cytoplasm, and the periplasmodial lipid deposited at the pollen grainsurface.

Thus although the mode, amount and timing of exine deposition is likely to varybetween different plants, there is little evidence to support the view by Mepham &Lane that the exine of the pollen grain is formed entirely by secretions from withinthe microspore.

A very recent paper by Heslop-Harrison (1968^) shows that the deposits of lipoidalmaterial on the outside of the pollen grain originate from the tapetum and not fromthe developing microspore. The continuous inter-bacular cavity which extends overthe entire surface of the pollen grain may provide a reservoir for sporopollenin build-ing materials. Such a mechanism adequately explains the growth of the surfacefeatures of the pollen grain which are in open contact with the thecal cavity and hencethe tapetum. In H. foetidus, however, there is clearly a need to explain accumulationof sporopollenin of the secondary exine from the microspore side of the wall. Thestructure that has already been described for the furrow regions in H. foetidus pollenseems to offer a possible route through which materials might enter the microsporefrom outside.

Morphological and cytochemical investigations fail to reveal substantial amounts ofreserve materials in the pollen grain cytoplasm up to and during the stage of wallsynthesis, and it would appear that the metabolites for the synthesis of the bulk ofthe pollen grain wall come from outside the pollen grain. This material could be

47° P- Echlin and H. Godwin

derived either from the autolysing tapetum or from the dissolution of the callose, andwould be able to pass into either the microspore cytoplasm for subsequent secretionthrough the cell membrane, or be deposited and subsequently polymerized in situ atprecise locations in the pollen grain wall. The synthetic sites would be located withinor near the wall of the pollen grain, either in the form of thin electron-transparentlamellae (tapes) which would give rise to the banded appearance of the early secondaryexine, or small loci which would give rise to the granular appearance of the latesecondary exine in the furrow region.

From the time of tetrad formation in H. foetidus until maturity, the pollen grainincreases in diameter by a factor of three, while the primary exine increases in thick-ness by between 3 and 4 times. These changes in size are not accompanied by anygreat alteration in the shape of the parts, which implies that there has been a fairlyrapid and consistent deposition of sporopollenin during maturation and that anyextension of the exine is immediately compensated for by accretion of wall material.

During the final stages of secondary exine formation microtubules reappear princi-pally at the periphery of the cell. They are randomly arranged and in close associa-tion with Golgi-derived vesicles which contain material morphologically identicalto that in the intine. The arrangement of the cellulose microfibrils in the intine isentirely random, and is similar to the texture of primary cell walls. Ledbetter &Porter (1963) were able to show a similar fibrillar arrangement in some of the de-veloping root cells of Phleum, and considered that in primary cell walls where thecellulose texture is random the microtubules in the cortical cytoplasm showed asimilar random arrangement.

In a study on the exine of Passiflora caerulea, Larson (1966) found considerableevidence for secondary exine-intine interbedding, similar to the situation found inH. foetidus. Similarly, it is possible to find pockets of intine in the secondary exine ofCommelinantia anomala (Rowley & Dahl, 1962). The significance of the exine-intineinterbedding is not at all certain. Larson (1966) suggests that an intermingling of thetwo layers would allow a greater plasticity during growth of the pollen grain.

The tryphine which is deposited during the final period of wall maturation is thebreakdown product of the tapetum and was presumably deposited there at the timeof the final dissolution of this tissue. It is possible to distinguish recognizable cyto-plasmic elements among the debris, notably strands of endoplasmic reticulum and thegrey spheroidal bodies which in the intact tapetal cells gave rise to the Ubisch bodies.The tryphine is only transitory, as scanning and transmission micrographs of matureuntreated grains do not reveal the large deposits of material which are seen in and onthe surface of the pollen grain wall of immature grains. There is no advantage to ourpresent study in speculating upon the possible function of tryphine in H. foetidus,interesting though such conjectures may be in other species.

The authors wish to express their thanks to Mr Brian Chapman for his technical abetment,to Mr Paul Curtis for his invaluable help with the photography and to Miss Ruth Bravermanfor her patient secretarial assistance.


REFERENCES

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BROOKS, J. & SHAW, G. (1968). Chemical structure of the exine of pollen walls and a newfunction for carotenoids in nature. Nature, Lond. 219, 532-533.

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{Received 11 February 1969)

Fig. 1. Two developing pollen grains at stage of primary exine formation. The furrowregion is less sculptured than the inter-furrow region. The inset (x 51000) showsa group of microtubules in a perinuclear position, x 16000.

Fig. 2. Nearly mature pollen grain wall in region of furrow. Note the tape-associatedsecondary exine, and the underlying granular secondary exine. The cytoplasm containsgrey bodies and mitochondria, as well as numerous Golgi-derived vesicles andribosomes. x 32000.

Ontogeny of Helleborus pollen grain wall

w• 1

(I

•j

473

rI

1 //m.

1 fim ;

474 P- Ecklin and H. Godwin

Fig. 3. Young pollen grain wall showing tape formation in the foot-layer. Note thethickened cell membrane and the Golgi-derived vesicles, x 80000.Fig. 4. Young pollen grain wall, showing membranous elements arising from theunderlying cytoplasm and going into the foot-layer, x 88000.Fig. 5. Peripheral cytoplasm during intine formation. Note fibrous intine, highly con-voluted cell membrane, and numerous Golgi-derived vesicles, x 29000.Fig. 6. Pollen grain wall in the furrow region, showing fibrous intine inserted below andbetween the amorphous exine. x 45000.


1

0-5//m 0-5//m


Figs. 7-12. Stages of development of the pollen grain wall in the inter-furrow region.Note the gradual disappearance of the fibrous primexine and the granular callose. Allphotographs x 51000.

Fig. 7. Pro-bacula formation.Figs. 8-10. Stages in the formation of the primary exine.Figs. 11, 12. Stages in secondary exine formation.


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