Entwicklungsgeschichte und Ultrastruktur von Pollenkitt und Exine bei nahe verwandten entomophilen und anemophilen Angiospermensippen derAlismataceae,Liliaceae, Juncaceae,Cyperaceae, Poaceae undAraceae

Some closely related members of the monocotyledonous familiesAlismataceae, Liliaceae, Juncaceae, Cyperaceae, Poaceae andAraceae with variable modes of pollination (insect- and wind-pollination) were studied in relation to the ultrastructure of pollenkitt and exine (amount, consistency and distribution of pollenkitt on the surface of pollen grains). The character syndromes of pollen cementing in entomophilous, anemophilous and intermediate (ambophilous or amphiphilous) monocotyledons are the same in principal as in dicotyledons. Comparing present with former results one can summarize: 1) The pollenkitt is always produced in the same manner by the anther tapetum in all angiosperm sub-classes. 2) The variable stickiness of entomophilous and anemophilous pollen always depends on the particular distribution and consistency of the pollenkitt, but not its amount on the pollen surface. 3) The mostly dry and powdery pollen of anemophilous plants always contains a variable amount of inactive pollenkitt in its exine cavities. 4) A step-by step change of the pollen cementing syndrome can be observed from entomophily towards anemophily. 5) From the omnipresence of pollenkitt in all wind-pollinated angiosperms studied one can conclude that the ancestors of anemophilous angiosperms probably have been zoophilous (i.e. entomophilous) throughout.

     

Ultrastruktur und Entwicklungsgeschichte des Pollenkitts vonEuphorbia cyparissias,E. palustris undMercurialis perennis (Euphorbiaceae)

Development, fine structure and distribution of pollenkitt is investigated inEuphorbia cyparissias, E. palustris, andMercurialis perennis. The predominantly anemophilousM. perennis produces a great amount of strictly homogeneous pollenkitt, which is deposited in the exine caves. In contrast to this and to all other angiosperms so far investigated, bothEuphorbia species produce large quantities of an extremely inhomogeneous and particular pollenkitt. Its ultrastructure is quite different, both during its development and after its deposition on the exine surface: Lipid particles with different electron density and size are wrapped in a strictly homogeneous electron transparent matrix. This can be considered as new and additional proof for the secondary entomophily ofEuphorbia.

     

Ultrastruktur und Verteilung des Pollenkitts in der insekten- und windblütigen GattungAcer (Aceraceae)

Several insect- and wind-pollinated species of the genusAcer have been investigated and compared in regard to pollen stickiness. The considerable amount of very inhomogeneous pollenkitt inA. negundo remains on the loculus wall or is deposited inside the exine cavities; thus the pollen is powdery. The small amount of non-homogeneous, granular pollenkitt inA. campestre mostly disappears into the exine cavities; only small droplets appear on the tectum surface; the pollen stickiness therefore is only moderate. On the other hand,A. pseudoplatanus andA. opalus produce an average amount of granular pollenkitt, which is deposited partially inside the exine and partially as a slender film on the tectum surface; in both the pollen is sticky.A. platanoides contains a great deal of granular and homogeneous pollenkitt; it does not only fill up the exine cavities but also extends as a thick, ± homogeneous, non-granular layer of pollenkitt over the tectum surface; therefore, the pollen is very sticky.—The characteristics of pollen agglutination together with other aspects of floral biology illustrate the wide spectrum between unequivocal entomophily and anemophily within the genusAcer: WhileA. negundo is anemophilous andA. platanoides is entomophilous, the remaining species investigated have a pollination syndrome with entomophilous and anemophilous features and are thus amphiphilous. The evolution withinAcer is tending not only towards dioecy, but also towards anemophily.

Herrn Prof. Dr.L. Geitler zum 80. Geburtstag gewidmet.     

Entwicklungsgeschichte und Ultrastruktur von Pollenkitt und Exine bei nahe verwandten entomophilen und anemophilen Sippen derOleaceae,Scrophulariaceae, Plantaginaceae undAsteraceae

Different members of the Angiosperm familiesOleaceae, Scrophulariaceae, Plantaginaceae andAsteraceae with variable modes of pollination were studied in relation to ultrastructure of pollenkitt and exine. In entomophilous representatives the mostly electron-dense and homogeneous pollenkitt is located on the outside layer of the exine, making the pollen sticky. In related anemophilous taxa the pollenkitt mostly is electrontransparent and inhomogeneous; it becomes inactive by remaining in the loculus or sinking to the bottom of the exine caves; this makes the pollen powdery. According to such criteria,Fraxinus ornus andRhinanthus serotinus are entomophilous, whileFraxinus excelsior is anemophilous. The pollen ofLathraea squamaria eventually becomes powdery and suited for wind dispersal. The broad transition field between entomo- and anemophily is demonstrated byPlantago andArtemisia. Plantago lanceolata is anemophilous, whilePl. serpentina andPl. media are amphiphilous.Artemisia nitida andA. mutellina are entomophilous,A. absinthium, A. pedemontana andA. gabriellae are amphiphilous, but tend towards entomo- (A. absinthium) or anemophily (A. gabriellae), whileA. chamaemelifolia and especiallyA. vulgaris are anemophilous.

     

Entwicklungsgeschichte und Ultrastruktur von Pollenkitt und Exine bei nahe verwandten entomophilen und anemophilen Angiospermensippen:Ranunculaceae,Hamamelidaceae, Platanaceae undFagaceae

Transfer of fresh pollen either by wind or by insects is strongly related to the degree of its stickiness. The sticky substances involved should be understood as pollenkitt. Ultrastructural investigations indicate that the sticky pollen of entomophilous angiosperms (in this publicationHamamelis vernalis andH. virginiana, Corylopsis platypetala) results from the deposition of usually electron-dense, homogeneous pollenkitt mainly on the outside layer of the exine. In contrast, the pollen of anemophilous angiosperms (in this publicationParrotia persica, Platanus orientalis, Thalictrum flavum andQuercus robur), but also of some entomophilous angiosperms with special pollination ecology (e.g.Aquilegia vulgaris) is rather dry and powdery. In these cases the pollenkitt often is electron-transparent and inhomogeneous and is mainly removed into the cavities of the exine. Ultrastructure and deposition of pollen and pollenkitt in ambophilous angiosperms (in this publicationThalictrum minus, Castanea sativa) are ± intermediate.

     

The Transfer of Pollenkitt in Smyrnium perfoliatum (Apiaceae)

In Smyrnium perfoliatum the formation of pollenkitt within asecretory tapetum, and the subsequent breakdown of the cellorganelles, is followed by the transformation of pollenkittlumps into pollenkitt droplets. These droplets move within alocular fluid towards the pollen exine, where they enter theexine cavities after passing a fibrillar layer (remnants ofthe primexine-matrix) in between the tectum elements. This isfollowed by the fusion of pollenkitt droplets, forming a distinctlayer at the bottom of the exine cavities. Smyrnium perfoliatum L., Apiaceae, tapetum, pollenkitt formation, organelle disintegration, transformation, pollenkitt deposition, primexine matrix     

The tapetum: Its form,function, and possible phylogeny inEmbryophyta

It appears that the tapetum is universally present in land plants, even though it is sometimes difficult to recognize, because it serves mostly as a tissue for meiocyte/spore nutrition. In addition to this main function, the tapetum has other functions, namely the production of the locular fluid, the production and release of callase, the conveying of P.A.S. positive material towards the loculus, the formation of exine precursors, viscin threads and orbicules (= Ubisch bodies), the production of sporophytic proteins and enzymes, and of pollenkitt/tryphine. Not all these functions are present in all land plants:Embryophyta. Two main tapetal types are usually distinguished in theSpermatophyta: the secretory or parietal type and the amoeboid or periplasmodial type; in lower groups, however, other types may be recognized, with greater or lesser differences. A hypothetical phylogenesis of the tapetum is proposed on the basis of its morphological appearance and of the nutritional relations with meiocytes/spores. The evolutionary trends of the tapeta tend towards a more and more intimate and increasingly greater contact with the spores/pollen grains. Three evolutionary trends can be recognized: 1) an intrusion of the tapetal cells between the spores, 2) a loss of tapetal cell walls, and 3) increasing nutrition through direct contact in narrow anthers.     

Pollenkitt formation in Ilex paraguariensis A.St.Hil. (Aquifoliaceae)

 We investigated the cellular and organelle transformations during the formation of the pollenkitt in the secretory tapetum of Ilex paraguariensis. After the dissolution of the callose surrounding the young microspores, the elaioplasts of the tapetum produce many globules of saturated and unsaturated lipids (plastoglobules). Further on, oleosomes with unsaturated lipids, synthesized in the endoplasmic reticulum, accumulate in the tapetal cytoplasm. In contrast to other species, the plastoglobule production seems to precede the oleosome synthesis. The tapetum shows signs of cellular maturation in the late vacuolated microspore stage, when the plastoglobules and oleosomes coalesce and form the pollenkitt mass. In mature stages of the tapetum the pollenkitt is released into the loculus. Finally, it is mainly deposited on the exine, according to the entomophilous character of this species. The mode of pollenkitt formation in Ilex para guariensis and its transfer to the pollen surface is slightly dissimilar to other Angiosperms. Received October 24, 2002; accepted December 2, 2002 Published online: March 20, 2003     

Pollenkitt is lacking inGnetum gnemon (Gnetaceae)

The anther tapetum of the gymnospermGnetum gnemon produces no lipid osmiophilous droplets as pollenkitt forerunners. Thus—in contrast to entomophilous and anemophilous angiosperms—no pollenkitt is produced at all. From lack of pollenkitt in other gymnosperms, it appears to be restricted to angiosperms. This can be considered as new and additional proof for the hypothesis that the angiosperms are one coherent phylogenetic group, and that the development of pollenkitt in their ancestors was one of the main prerequisites for the adaptive switch to entomophily.     

Pollenkitt is lacking inGnetatae:Ephedra andWelwitschia; further proof for its restriction to the angiosperms

Three members of theGnetatae (Ephedra campylopoda, E. americana, Welwitschia mirabilis) were investigated by TEM and SEM with respect to their anther tapetum and pollen development. In all three species pollenkitt is lacking. The pretended pollen stickiness thus does not depend on pollenkitt. Considering former observations one can now clearly state that pollenkitt is missing in all recent gymnosperm classes (both anemophilous and ± entomophilous). Pollenkitt thus is restricted and ± omnipresent within the angiosperms, where it represents one of the most important components of the entomophily syndrome. This can be regarded as important proof for the hypothesis that the angiosperms are a single coherent phylogenetic group.     

Developmental stages of the pollen wall and tapetum in some Crocus species

The developmental stages of the pollen wall and tapetum, together with exine morphology were studied in a number of Crocus species, by light and scanning electron microscopy. Gametogenesis was characterized by: 1) development of a thick intine, 2) single mitosis, and 3) terminal amylolysis. The tapetum was of the secretory type. In C. cartwrightianus cv. albus, abnormal sporogenesis and gametogenesis produced vacuolate pollen grains with a reduced-or no intine layer, and rich with starch granules; the tapetum was either of the parietal-or amoeboid type. The exine was echinate and the pollen grains had different types of aperture: furrows, colpi or pores. The ornamentation varied from microreticulate to irregularly perforate. The exine framework was overlaid by a pellicle resistant to chloroform-carbon disulphide, on which a layer of pollenkitt was deposited. The results are discussed from both cytological and evolutionary viewpoints.     

Zur Skulptur der Pollen-Exine bei drei Centrospermen (Gisekia,Limeum, Hectorella), bei Gyrostemonaceen und Rhabdodendraceen

The exine-sculpturing of pollen fromGisekia africana (Gisekiaceae),Limeum argute-carinatum (Molluginaceae) andHectorella caespitosa (Hectorellaceae) with supratectate spinulae and an anulopunctate tectum is in accordance with the manyCentrospermae investigated so far with the SEM. Pollen ofRhabdodendron macrophyllum and three species fromGyrostemonaceae do not exhibit these surface details. WhileRhabdodendron with a finely reticulate exine probably fits intoRutaceae, the unique columellaless exine ofGyrostemonaceae has no direct counterpart, neither inCentrospermae nor inCapparales, the two orders to which this family was allied. The genera ofGyrostemonaceae can be distinguished by different arrangements of minute pollen surface details.

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Zur Skulptur der Pollen-Exine bei drei Centrospermen (Gisekia, Limeum, Hectorella), bei Gyrostemonaceen und Rhabdodendraceen

     

Zur Frage der Anheftung des Pollens an blütenbesuchende Insekten mittels Pollenkitt und Viscinfäden

The attachment of pollen grains among themselves, on the loculus wall, and on flower-visiting insects is quite different in entomophilous angiosperms using pollenkitt and those using viscin threads as pollen adhesives. The sticky and viscous pollenkitt makes the pollen grains adhere, while the thin, non-elastic, non-sticky, and flexible viscin fibers fasten them like ropes on insect hairs or bristles. Nectar vomited by honey-bees, sticky stigma secretions or other additional sticky substances further improve the pollen adherence to flower-visiting insects.

Herrn Univ.-Prof. Dr.Walter Leinfellner zum 70. Geburtstag gewidmet.     

Ultrastruktur und systematische Bedeutung des Pollens beiBocageopsis,Ephedranthus, Malmea undUnonopsis (Annonaceae)

The four genera investigated show solitary sulcate pollen grains. The structure and sculpture of the sporoderm is very similar inBocageopsis andUnonopsis, and supports the close relationship of both genera; their flower morphology also is very similar. In contrast, the sporoderm ofEphedranthus, and especially that ofMalmea, is different in some characters and suggests more remote relationships; this is also confirmed by differences in flower morphology. Within this group of genera a distinct exine progression can be recognized from non-columellate (Malmea) to granular (Unonopsis guatterioides), further to somewhat irregularly (Bocageopsis, someUnonopsis species), and finally to very regularly columellate (Ephedranthus). The sculpture of the tectum varies from a reticulum with large lumina (Malmea) to one with small performations (Ephedranthus). Within theAnnonaceae the genusMalmea is among the most primitive in respect to pollen structure. The sulcus of the four genera is very large and runs over 1/3 of the pollen grain. It is characterized by a reduction of the exine and a bulgy thickening of the intine.

     

车桑子小孢子发生与雄配子体发育

为了解干热河谷区车桑子(Dodonaea viscosa)胚胎学特征及其结籽率低的原因,采用常规石蜡切片法和电镜扫描技术对车桑子小孢子发生、雄配子体发育和花粉的形态特征进行了观察。结果表明,车桑子花药具有4个花粉囊。完整的花药壁从外到内依次为表皮、药室内壁、2～3层中层细胞和绒毡层;绒毡层类型是腺质绒毡层。花药成熟期,中层、绒毡层均退化消失。小孢子母细胞进行同时型胞质分裂;四分体为四面体型结构。成熟的花粉为二细胞型。花粉近球形,外壁密布颗粒状纹饰,具有3条不构成合沟的萌发沟。雄性生殖发育过程出现的异常现象可能是干热河谷地区车桑子结籽率低的原因之一。     

Zur Ultrastruktur und Funktion pollenverbindender Fäden beiEricaceae und anderen Angiospermenfamilien

Viscin threads and other pollen connecting threads of some angiosperm families were investigated, especially those ofEricaceae. According to the definition adopted, viscin threads are ± long exinous processes which consist of exinous material and connect pollen grains or tetrads. Such viscin threads are found within theOnagraceae, Caesalpiniaceae, Ericaceae, andMimosaceae only. While they differ in structure and composition, they always consist of sporopollenin and exhibit a very strong stickiness, even after all viscid substances have been removed by acetolysis. In contrast, the pollen connecting scleroprotein threads ofOrchidaceae and the cellular threads ofStrelitzia reginae Aiton. (Musaceae) are not connected with the exine surface, are destroyed by acetolysis, and thus do not correspond to viscin threads.

     

Embryology ofCrocus thomasii (Iridaceae)

The embryology ofCrocus thomasii is described. Male meiosis is of simultaneous type, and gives rise to starchy microspores which develop into lipoid pollen grains; these are two-celled and show a spinulate acolpate, abaculate exine lacking apertures. The tapetum is glandular and its cells become bi- or sometimes multinucleate. The ovule is anatropous and bitegmic; the inner integument forms the micropyle. Megasporogenesis is heteropolar with starch accumulation in the functional chalazal megaspore. Embryo sac development conforms to thePolygonum type. The endosperm development is nuclear. The embryo develops according to the Caryophyllad type. In the ripe seed it is differentiated and enveloped by a starchy cellular endosperm. The embryological characters observed strongly favour a close relation betweenC. thomasii andC. sativus.     

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.     

Ultrastructural expression of cytoplasmic male sterility in sugar beet (Beta vulgaris L.)

Summary The development of microspore mother cells (MMC) and tapetum in male-fertile and male-sterile anthers of Beta vulgaris L. was compared at the electron microscope level. These studies were complemented by morphometric analyses of mitochondria in both tissues through successive stages of microsporogenesis. The earliest irregularities in the ultrastructure of male-sterile anthers were noted within the tapetum at the tetrad stage. These disturbances were initially expressed by a slight reduction in mitochondrial size and the appearance of concentric configurations of endoplasmic reticulum. As development proceeded, a further decrease in mitochondrial size become more conspicuous and was accompanied by a reduction in ribosome population and a failure of the tapetum to produce Ubisch bodies. This failure to produce Ubisch bodies is reflected in the underdevelopment of sterile microspore exine.     

Orbicules and the ektexine are homologous sporopollenin concretions inSpermatophyta

During microsporogenesis sporopollenin becomes accumulated independently by the only two sporopollenin-producing cell types in the higher plants—the anther tapetum and the microspores—not only within the exine, but also within the orbicules. In Spermatophytes usually only a single orbicule type is found, but sometimes, e.g., inEuphorbia palustris, two different types exist within a single species. Very interestingly, most orbicules are strikingly similar in their structure, sculpture, shape and dimension to the respective ektexine: e.g., smooth sporopollenin globules are found near the totally smooth exines ofEupomatia laurina andPentaphragma sinense respectively, while inDelonix elata andEuphorbia palustris the orbicules often look like some incomplete tectum pieces. All these parallelisms can be attributed to the homologous and therefore highly similar genetic information to form sporopollenin in the sporogeneous tissue and the anther tapetum. The orbicules should be seen neither as sporopollenin storage, as sporopollenin transfer vehicle, nor as sporopollenin surplus.