The Changes in the Tapetum of Pinus banksiana Accompanying Formation and Maturation of the Pollen

As meiosis is completed, and following the synthesis of lipidduring meiotic prophase, the tapetum begins to form precursorsof sporopollenin. These accumulate in cisternae of the endoplasmicreticulum, resembling large dictyosome vesicles. They are releasedfrom the tapetal protoplasts intact, but rupture in the loculus.The liberated precursors polymerize either on lipid dropletsin the expanded tapetal walls, forming the orbicules, or onthe lipid layer surrounding the loculus, forming the secondcomponent of the peritapetal membrane. On rupture of the callosewall condensation also proceeds on the walls of the meiospores,already coated with a thin layer of sporopollenin synthesizedby the spore itself. The tapetal protoplasts expand considerablyduring synthesis of the precursors. Wide channels also formbetween the protoplasts, and the nuclei undergo irregular divisions. Ribosomes are conspicuous in the tapetal cytoplasm during thesporopollenin synthesis, but protein levels are low. It is proposedthat protein is exported to the loculus and untimately incorporatedinto the developing microspores. In the final phase of microsporogenesis the tapetum fragments,and parts move into the loculus.Protein levels in the tapetumare now high, possible indicating the massive synthesis of hydrolaseswhich accomplish the dissolution of the tissue. Removal of thelipid component of the peritapetal membrane precedes the desiccationof the anther. The surfaces of the mature pollen lack organizedor irregular deposits of tapetal debris.     

Development and cell surface of a non-syncytial invasive tapetum inCanna: Ultrastructural,freeze-substitution,cytochemical and immunofluorescence study

Summary The anther ofCanna indica L. ×C. sp. hybrid contains a hitherto uncharacterized non-syncytial, invasive category of tapetum. With the onset of prophase I the tapetal walls are dissolved and the released protoplasts migrate into the loculus, where they stay discrete. Concomitant with the dissolution of walls the tapetal protoplasts develop a 17 nm thick extracellular granulo-fibrillar cell coat. This feature develops in the synchronous phase of tapetal development. The cell coat reacts positively with ruthenium red, potassium ferrocyanide, ConA-FITC and in the Thiéry reaction. Immunofluorescence microscopy using anti-tubulin revealed that even after the migration of tapetal cells into the loculus, the microtubules retain a predominant orientation in the cell cortex, probably derived from that in the original tapetal walled cells. This order is lost during late post-meiotic stages when the cells distort and can produce amoeboid processes. The microtubule orientation is correlated with that of the cell coat fibrils. Tapetal cells vary in ultrastructure and the density of cell coat fibrils after their migration into the loculus, but the cell coat persists until the cells degenerate. It is surmised that development of the cell coat relates to the lack of cell fusion and that the cortical microtubules help to sustain cell form. During post-meiotic stages the free tapetal cells develop massive peripheral arrays of interconnected ER cisternae, probably as part of a secretory apparatus which matures when the spores are producing their ornamented walls. Buds grown in colchicine solution showed accumulation of sporopolleninlike granules in all extracellular spaces of the anther cavity.     

Secretory tapetum of Brassica oleracea L.: polarity and ultrastructural features

Summary The ultrastructure of the secretory, binucleate tapetum of Brassica oleracea in the micro spore mother cell (MMC) stage through to the mature pollen stage is reported. The tapetal cells differentiate as highly specialized cells whose development is involved in lipid accumulation in their final stage. They start breaking down just before anther dehiscence. Nuclei with dispersed chromatin, large nucleoli and many ribosomes in the cytoplasm characterize the tapetal cells. The wall-bearing tapetum phase ends at the tetrade stage. The dissolution of tapetal walls begins from the inner tangential wall oriented towards the loculus and proceeds gradually along the radial walls to the outer tangential one. The plasmodesmata transversing the radial walls between tapetal cells persist until the mature microspore, long after loss of the inner tangential wall. After wall dissolution, the tapetal protoplasts retain their integrity and position within the anther locule. The tapetal cell membrane is in direct contact with the exine of the microspores/pollen grains and forms tubular evaginations that increase its surface area and appear to be involved in the translocation of solutes from the tapetal cells to the microspores/ pollen grains. The tapetal cells exhibit a polarity expressed by spatial differentiation in the radial direction.     

Ontogeny of intruding non-periplasmodial tapetum in the wild chicory,Cichorium intybus (Compositae)

The tapetal development ofCichorium intybus L. is investigated using LM and TEM and discussed in relation to the development in other species. During the second meiotic division the tapetal cells become binucleate and lose their cell walls. They intrude the loculus at the time of microspore release from the meiotic callose walls, which means that a locular cavity is never present in this species. During pollen development they tightly junct the exine, especially near the tips of the spines. During the two-celled pollen grain stage they degenerate and most of their content turns into pollenkitt. Until anther dehiscence they keep their individuality, which means that these intruding tapetal cells never fuse to form a periplasmodium. The ultrastructural cytoplasmatic changes during this development are discussed in relation to possible functions.     

The tapetum inSchizaea pectinata (Schizaeaceae) and a comparison with the tapetum inPsilotum nudum (Psilotaceae)

A combination tapetum consisting of a cellular, parietal component and a plasmodial component occurs inSchizaea pectinata. A single, tapetal initial layer divides to form an outer parietal layer which maintains its cellular integrity until late in spore wall development. The inner tapetal layer differentiates into a plasmodium which disappears after the outer exospore has developed. In the final stages of spore wall development, granular material occurs in large masses and is dispersed as small granules throughout the sporangial loculus. No tapetal membrane develops. Comparisons are drawn with the combination tapetum found inPsilotum nudum.     

Tapetal Organization During Sporogenesis in Psilotum nudum

Stages in the differentiation of the tapetum of Psilotum nudumare described. Two concurrently occurring components of thetapetum can be recognized. A plasmodial tapetum with associatedfunctional nuclei develops within the sporangial loculus duringthe early stages of differentiation, appears to remain viablefor several months, that is during the entire period of sporogenesis,and undergoes reorganization on three occasions. During MeiosisI groups of spore mother cells are enclosed in clear areas withinthe plasmodium: by the end of Meiosis II each tetrad is isolatedin a plasmodial chamber; and, finally, mature spores are enclosedwithin individual tapetal chambers. Typically enlarged cellsare present during the development of a cellular, parietal tapetum.A sporopollenin-containing layer or tapetal membrane characteristicof a secretory tapetum develops on the inner tangential walland lines the surface of the loculus. This tapetal membranepersists even after dehiscence of the sporangium. These observationsare discussed in relation to previously published conflictingdata and may be relevant to the arguments concerning the relationshipof the Psilotaceae to the Filicales. Psilotum nudum, light microscopy, parietal tapetum, plasmodial tapetum, tapetal membrane, tapetal reorganization, sporogenesis, sporopollenin     

A Comparative Study of Anther and Pollen Development in Male Fertile and Male. Sterile Green Onion (Allium fistulosum L.)

Anther and pollen development in male-fertile and male-sterile green onions was studied. In the male-fertile line, both meiotic microspore mother ceils and tetrads have a callose wall. Mature pollen grains are 2-celled. The elongated generative cell with two bended ends displays a PAS positive cell wall. The tapetum has the character of both secretory and invasive types. From microspore stage onwards, many oil bodies or masses accumulate in the cytoplasm of the tapetal cells. The tapetum degenerates at middle 2-celled pollen stage. In male-sterile line, meiosis in microspore mother cells proceeds normally to form the tetrads. Pollen abortion occurs at microspore with vacuole stage. Two types of pollen abortion were observed. In type I, the protoplasts of the microspores contract and gradually disintegrate. At the same time the cytoplasm of microspores accumulates oil bodies which remain in the empty pollen. The tapetal cells behave normally up to the microspore stage and early stage of microspore abortion, but contain fewer oil bodies or masses than those in the male-fertilt line. At late stage of microspore abortion, three forms of the tapetal ceils can be observed: (1) the tapetal cells with degenerating protoplasts become flattened, (2) the tapetal cells enlarge but protoplasts retractor, (3) the cells break down and tile middle layer enlarges. In type Ⅱ, the cytoplasm degenerates earlier than the nucleus of the microspores and no protoplast is found in the anther locule. There are fibrous thickenings iii the endothecium of both types. It is difficult to verify whether the tapetum behavior and pollen abortion is the cause or the effect.     

The loculus content and tapetum during pollen development in Lilium

 The ratio of loculus volume to the volume of the entire anther began to increase from the microspore mother cell stage and reached 32.3% at anthesis. The content of the loculus was examined in Lilium during pollen development and two waves could be distinguished. From the premeiotic stage until the vacuolated microspore stage, the loculus consisted of neutral polysaccharides, pectins and proteins. These substances originated from tapetal activity from the premeiotic stage until the young microspore stage. Dictyosomes and rough endoplasmic reticulum seemed to be involved in tapetal secretion, although, in some mitochondria, vesicles progressively developed as early as premeiosis and increased until the young microspore stage, which could reveal their involvement in the secretion process. At this stage, numerous cytoplasmic vesticles containing material similar to the locular material fused with the plasma membrane of the tapetum so that vesicle content was in contact with the loculus. It seems that tapetal and callose wall degradation at the late tetrad stage may also have contributed to the production of material in the loculus. From pollen mitosis to anthesis, the anther loculus contained mainly the pollenkitt which was synthesized in the tapetum between the young microspore stage and the vacuolated microspore stage. At the young microspore stage, proplastids divided and developed into elaioplasts and smooth endoplasmic reticulum (SER) increased dramatically. Pollenkitt had a double origin: some droplets were extruded directly from the plastid stroma through the plastid envelopes; the others were unsaturated lipid globules, which presumably derived from the interaction between SER saccules and plastids. Received: 2 September 1997 / Revision accepted: 12 March 1998     

Anther, pollen and tapetum development in safflower, Carthamus tinctorius L

In safflower, the anther wall at maturity consists of a single epidermis, an endothecium, a middle layer and the tapetum. The tapetum consists mainly of a single layer of cells. However, this single-layer appearance is punctuated by loci having ‘two-celled’ groupings due to additional periclinal divisions in some tapetal cells. Meiotic division in microsporocytes gives rise to tetrads of microspores. The primexine is formed around the protoplasts of microspores while they are still enveloped within the callose wall. Just prior to microgametogenesis, the microspores enlarge through the process of vacuolation, and the exine wall pattern becomes established. Microgametogenesis results in the formation of 3-celled pollen grains. The two elongated sperm cells appear to be connected. The exine wall is highly sculptured with a distinct tectum, columellae, a foot layer, an endexine and a thin intine. Similar to other members of the Asteraceae family, the tapetum is of the invasive type. The most novel finding of this study is that in addition to the presence of invasive tapetal cells, a small population of ‘non-invasive’ tapetal cells is also present. The tapetal cells next to the anther locules in direct contact with the microspores become invasive and start to grow into the space between developing microspores. These tapetal cells synthesize tryphine and eventually degenerate at the time of gametogenesis releasing their content into the anther locules. A smaller population of non-invasive tapetal cells is formed as a result of periclinal divisions at the time of tapetum differentiation. These cells are not exposed to the anther locules until the degeneration of the invasive tapetal cells. The non-invasive tapetal cells have a different cell fate as they synthesize pollenkitt. This material is responsible for allowing some pollen grains to adhere to each other and to the anther wall after anther dehiscence. This observation explains the out-crossing ability of Carthamus species and varieties in nature.     

A Tapetal Membrane in Psilotum nudum (L.) Beauv

Ultrastructural studies (both SEM and TEM) on Psilotum nudumreveal that a tapetal membrane develops on the loculus sideof the inner tangential wall of a cellular, parietal tapetumwhich invests each sporangium within a synangium. The membraneconsists of orbicular projections of a homogeneous nature, supportedon a lamellated base; the whole structure is resistant to acetolysisand persists even at the dehiscence of the synangium. Pro-orbiculeswere not identified and the orbicles were not released intothe sporangial loculus. Spheroids, approx. 3 µm in diameter,are found in membrane-enclosed chambers within the plasmodialtapetum. Their formation is described as well as their finalstructure which reflects the structure of the mature sporodermfrom the outer exosporal layer outwards. Differentially stainingbodies occurring in the parietal tapetum and in the adjacentcells of the sporangium wall are implicated in the formationof the tapetal membrane. The bodies occur at the time when thesporangium wall is yellow and when plastids within the cellsof the sporangium wall develop large numbers of osmiophilicplastoglobuli. parietal tapetum, plasmodial tapetum, Psilotum nudum, spheroids, sporopollenin, tapetal membrane     

Microspore and tapetal development in Echinodorus cordifolìus (Alismaceae)

In the microspore tetrad period the exine begins as rods that originate from the plasma membrane. These rods are exine units that on further development become columellae as well as part of the tectum, foot layer and “transitory endexine”. The primexine matrix is very thin in the future sites of the pores. At these sites the plasma membrane and its surface coating (glycocalyx) are without exine units and adjacent to the callose envelope. The exine around the aperture margin is characterized by units of reduced height. After the exine units and primexine matrix have become ca 0.2 μm in height a fibrillar zone forms under the aperture margin. It is the exine units around the aperture that are templates for exine processes on apertures of mature pollen. Oblique sections of the early exine show that the tectum consists of the distal portions of close-packed exine units. The exine enlarges in the free microspore period but initially its substructure (tectum, columellae, foot layer and transitory endexine) is not homogeneous and unit structures are visible until after the vacuolate microspore period. There are indications of a commissural line/plane (junction plane) which separates the foot layer from the endexine during early development. Our observations of development in Echinodorus pollen extend a growing number of reports of “transitory endexines” in monocot pollen. The exine unit-structures become 0.2 μm or more in diameter and many columellae are composed of only one exine unit. Spinules become exceptionally tall, many protruding ca 0.7 μm above the level of the tectum as units only ca 0.1 μm in diameter. The outer portion of the tectum fills in around spinules and by maturity they are microechinate with their bases spread out to ca 1 μm or more. Unit structures can be seen with SEM in mature pollen following oxidation by plasma ashing and in the tapetum these units are arranged both radially, as in spinules, and parallel with the tapetal surfaces. There are clear indications of such an arrangement of units in untreated fresh pollen. Units comprising the basal part of the exine are not completely fused by sporopollenin accumulated during development. This would seem to be a characteristic feature, based on published work, of the alismacean pollen. Our use of a tracer shows, however, that there is considerable space within or between exine structure of mature Echinodorus pollen. Based upon the ca 0.1 μm size of exine-units formed early in development and exine components seen after oxidative treatment it seems that the early (primary) accumulated sporopollenin has greater resistance to oxidation than sporopollenin added, secondarily, around and between units later in development. Both primarily and secondarily accumulated sporopollenin are resistant to acetolysis but published work indicates that acetolysis alters exine material. At the microspore tetrad time and until the vacuolate stages tapetal cells are arranged as in secretory tapetums. During early microspore stages there are orbicules at the inner surface of tapetal cells. At free microspore period tapetal cells greatly elongate into the loculus and surround the microspores. By the end of the microspore vacuolate period tapetal cells release their cellular contents and microspores are for a time enveloped by tapetal organelles and translocation material.     

Development and Histochemical Observations of Tapetum and Peritapetal Membrane in Anther of Pulsatilla chinensis

Tapetum of Pulsatilla chinensis is of secretory type. Its development proceeds rapidly in following sequence: (1) The stage of initiation-differentiation. At this stage cytological and histochemical features have been described in detail in this paper. (2) The stage of growth- synthesis: This stage appears to be the most important anabolic phase during the development of the tapetum. The salient features are that the tapetal cells become relatively enlarged and form two polyploid nuclei or aberrent polyploid nuclei resulting in synthetizing maximum proteins, fluorescing substances and maximum fluorescent Pro-Ubisch bodies in the tapetal cytoplasm. (3) The stage of secretion-disorganization: After the disintegration of the tapetal wall the enlarged naked cells appear at once. This is an important secretion period in which Pro-Ubisch bodies as well as all other fluorescing substances, carbohydrate or some enzymes are released into anther loculus. The naked cell layer becomes disorgnized until the beginning divition of the pollen grains into two ceils. As to peritapetal membrane of P. chinensis, mainly based on the membrane being on the outer side of the tapetum enclosing both the pollen, tapetal cytoplasm and Ubisch bodies, and the cellular configurations facing the pollen, Authors postulate that peritapetal membrane might be survival of the cytoplasmic membrane of tapetal cells. However, the peritapetal membrane of P. chinensis is similar to that of plasmodial, tapetum reported in certain Compositae and that of secretory tapetum reported in Pinus banksiana. Heslop-Harrison and Gupta et al. had conceded that the tapetal and peritapetal membrane belong to the general class of sporopollenin. On the contrary in P. chinensis the sporopollenin property of peritapetal membrane is only confined to its inner surface. But the thin mem- brane itself with the reticulate sporopollenin attched on its inner side appears negative staining reactions for sporopollenin though it has an ability to resist the acetolysis as well. In P. chinensis the Ubisch body is short necked flask shaped and their size is very similar. Ubisch body is either single or 2–5 in a group, resulting in compound bodies. When the Pro-Ubisch body is still within the tapetal cell it shows positive fluorescent reaction, while it eomletely unstains with Teluidine blue O. So Authors infer that the sporopollenin precur- sors may have permeated through Pro-Ubisch bodies. Finally, How sporopollenin precursor is synthesized in the tapetal cells, transported to pollen locula and polymerized into the sporopollenin on pollen, Ubisch body and peritapetal membrane? Future works along these problems may yield fruitful results.     

凤仙花花药发育特征

凤仙花花药发育比较特殊: 在造孢细胞时期,花药横切面中央是体积较大、细胞内含物较多的细胞团、包括造孢细胞和绒毡层细胞。花药药壁细胞的细胞质较稀少,与中部细胞界限明晰。花粉母细胞时期的花药药壁由约6层细胞组成,但细胞的界限不明显;绒毡层细胞显示变形流入药室中。到四分体时期,绒毡层细胞进一步退化。开花时,成熟花药的药壁细胞由一层表皮细胞、两层药室内壁细胞和一层中层细胞组成。对凤仙花花药绒毡层的特殊性质进行了讨论。     

Isolation,sorting, and characterization of uni- and binucleate tapetal protoplasts from anthers of normal and Texas cytoplasmic male-sterileZea mays L.

Summary A cytological study of Texas cytoplasmic male sterile (Tcms) and normal (N) anther tapetal protoplasts ofZea mays was undertaken to determine whether there were any differences prior to Tcms male cell abortion not noted in previous published studies. Squash preparations, tapetal protoplast separation via flow cytometry, image analysis, and electron microscopy were utilized. Chemically preserved tapetal protoplasts from both lines were prominently angular in shape and typically smaller than any other cell type in the anthers. The tapetum from both lines consisted of a mixture of uninucleate and binucleate protoplasts. The Tcms tapetum consistently had a higher proportion of binucleate protoplasts during all stages of microsporogenesis prior to abortion. The size of Tcms uniand binucleate tapetal protoplasts was more variable than the N tapetal protoplasts and was largest during the microspore stage when male cells abort. Tapetal nuclear size in both lines was less variable. Uni- and binucleate tapetal protoplasts from each line could be separated from the other anther cells and from each other by filtration and then by flow cytometry, based on intensity of nuclear fluorescence. These results suggest that Tcms uninucleate tapetal protoplasts have a higher level of DNA than N uninucleate tapetal protoplasts. Both fluorescence microscopy and electron microscopy confirmed pure populations of intact uni- and binucleate tapetal protoplasts using flow cytometry. The results from this study indicate that the methodology presented here could be used for a variety of further studies to better understand the cellular and molecular basis of male sterility in maize, and in other taxa, where the tapetum is the primary target that leads to male sterility.Abbreviations AO acridine orange - Bi binucleate protoplast - D dyad - DAPI 4,6-diamidino-2-phenylindole - FC flow cytometry - M meiocyte - MI microspore - MMC mithramycin - N normal anther tapetal protoplast - PI propidium iodide - PS protoplast sorting - RT room temperature - SM sporogenous mass - Tems Texas cytoplasmic male sterile anther tapetal protoplast - Uni uninucleate protoplast     

Microsporogenesis in Taxus baccata L.: The Formation of the Tetrad and Development of the Microspores

Following meiosis II in Taxus microsporangia a small proportionof the tetrads regularly degenerated. Despite frequent inequalityin the frequency of ribosomes between the spores of a tetrad,partial degeneration within a tetrad was never observed. Theinitial wall of the young spores was found to resemble the wallof the mother cell in containing a fibrillar layer, and thetwo walls may possess similar isolating properties. The symmetryof the tetrad was regularly iso-bilateral. The formation ofthe sporoderm began as the spores were released into the loculusby the rapid dissolution of the wall of the mother cell. Osmiophilicdroplets emerged from the spore protoplast and entered the wall.The fibrillar layer ceased to be recognizable and the dropletscoalesced to form an outer layer on which up to six sporopolleninlamellae, probably of tapetal origin, were deposited. The accretionof a single layer of sporopollenin droplets, in no recognizablepattern, gave rise to the outer verrucose part of the exine.Cytochemical tests showed that the tapetum was rich in acidphosphatases from the beginning of meiosis. Towards the endof its degeneration the tapetum intruded into the loculus andcould therefore be regarded as partly invasive. Taxus baccata, microsporogenesis, tetrad symmetry, sporoderm     

Cytoskeleton,cell surface and the development of invasive plasmodial tapetum inTradescantia virginiana L.

Summary The anther tapetum inTradescantia virginiana L. is of the invasive plasmodial type: the cells lose their walls during early spore meiosis and develop long invasion processes which invade the loculus to penetrate spaces between the sporogenous cells. Fusion to form a syncytium is delayed and conventional ultra-thin sections and the Thiéry reaction reveal the presence of a loose fibrillar extracellular cell coat on the free surfaces of tapetal cells and their invasion processes. Cell fusion involves formation of apposition areas characterized by an absence of cell coat and the local appearance of microtubular arrays. Conspicuous membrane sacs, associated closely with microtubules, were found to migrate to and accumulate at the plasma membranes near the fusion sites and sporogenous cells. Microtubules are always present in the cortical regions of the tapetal cells and their invasion processes. It is surmised that microtubules are not responsible either for initiating or guiding tapetal invasion of the loculus; instead they may help to sustain the form of the invasion processes, help in the migration of membrane sacs, and participate in cell fusion. The cell coat disappears with syncytium formation towards the end of meiosis, and the developing spore cells become surrounded by a perispore membrane, which, derived from the original plasma membranes and augmented by membrane sacs, forms labyrinthine membrane reservoirs that are described further in the accompanying paper.     

侧柏小孢子囊壁绒毡层和中层细胞的发育

侧柏[Platycladus orientalis (L.)Franco]小孢子囊壁包括3层细胞：表皮、中层和绒毡层。中层细胞为1层扁平的细胞。绒毡尾细胞属于分泌型。成熟的绒毡层细胞除了有单核和双核细胞外,还有三核和四核等多核细胞,细胞核有圆形和长椭圆形2种形态。绒毡层细胞的洒色质伴随着小孢子母细胞减数分裂有一个浓缩和伸展的时期,这个时期影响营养物质向小孢子囊内部转运,绒毡层细胞发育的初期就为造孢细胞提供营养,后期解体时,分泌的乌氏体不是散乱地而是有组织地向花粉粒的表面转移。中层和绒毡层细胞最终作为营养被全部吸收利用。     

CYCLIC INVASION OF TAPETAL CELLS INTO LOCULI DURING MICROSPORE DEVELOPMENT IN NYMPHAEA COLORATA (NYMPHACEAE)

In work with Nymphaea colorata Peter three distinct intervals were recorded during which tapetal cells (protoplasts) protruded into anther locules either as bridges and partitions or as invasive cells between or around tetrads of microspores. Before these intervals and between and after them, tapetal cells, while variable in shape, were noninvasive. Observations were based on sections of over 60 fixed and epoxy-embedded anthers covering the relatively brief interval from the end of meiosis through the vacuolate microspore stage. The progression of development, from early microspore stages through the microspore vacuolate period, is illustrated by transmission electron micrographs showing change in proexine and exine size and complexity. Our results indicate cycles of tapetal cell differentiation and dedifferentiation in this species.     

Colchicine inhibits plasmodium formation and disrupts pathways of sporopollenin secretion in the anther tapetum ofTradescantia virginiana L.

Summary During an earlier investigation, microtubules were observed at the periphery of invasion processes in the developing syncytial tapetum ofTradescantia virginiana L. They were also associated with membranous sacs that accumulate adjacent to tetrads, with putative fusion sites where the tapetal plasmodium is initiated, and, in postmeiotic stages, with the perispore membrane that encloses the developing spore cells. Colchicine was administered to developing flower buds to investigate the roles of these microtubules. The results indicate that microtubules neither initiate nor guide the tapetal invasion of the loculus. The treatments, however, resulted in absence of cell coat from invasion processes and prevention of cell fusion. They also inhibited polarized migration of membrane sacs and removed the associated microtubules. The development of an organized secretory apparatus at the perispore membrane was disrupted, with subsequent disordered deposition of sporopollenin in the extracellular spaces of the partially-fused plasmodium. The results suggest that microtubules participate in the formation and internal spatial organization of the tapetal plasmodium, and establishment of a secretory surface that normally produces sporopollenin at the tapetum-microspore interface.     

The rôle of the tapetum in the formation of sporopollenin-containing structures during microsporogenesis in Pinus banksiana

Summary In the microsporangium of Pinus the outer layer of the peritapetal membrane and the pro-orbicular cores are not only formed in a similar manner, but are composed of apparently identical materials. Precursors for this lipoidal material are produced by the tapetal protoplasts, as are the precursors of sporopollenin. Production the precursors is sequential and appears to involve different cytoplasmic structures.The sporopollenin synthesised by the tapetum condenses upon the pro-orbicular cores, the peritapetal membrane, the exine initials and, on fragmentation of the tapetum, parts of the disintegrating cytoplasm. The evident unpolarised nature of the tapetal protoplasts, and the sequential nature of the synthesis of the lipoid and the sporopollenin by them, may point to orbicule formation in gymnosperms being a necessary by-product of the development of the peritapetal membrane.