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 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.

     

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.

     

Entwicklungsgeschichte und Ultrastruktur des Pollenkitts beiTilia (Tiliaceae)

The pollenkitt ofTilia platyphyllos andT. tomentosa is produced during the tetrade-stage by the tapetum plastides. After tapetum degeneration pollenkitt and accompanying substances are deposited predominantly in the intrabacular spaces of the exine, and only in small quantities on the surface of the exine itself.

     

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.

     

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.     

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.     

Correlation between pollen morphology and pollination mechanisms in the Hydrocharitaceae

The pollen morphology of 11 genera and 11 species of the Hydrocharitaceae and one species of the Najadaceae was studied using scanning and transmission electron microscopies, and the exine structures and sculptures are discussed in relation to pollination mechanisms and the molecular phylogeny. The pollen grains of the Hydrocharitaceae are spherical, inaperturate, and form monads or tetrads, while those of the Najadaceae are elliptical, inaperturate, and form monads. The entomophilous genera Egeria, Blyxa, Ottelia, Stratiotes, and Hydrocharis share pollen grains that have projections like spines or bacula. The anemophilous genus Limnobium has reticulate pollen grains. The hypohydrophilous genera Thalassia and Najas are characterized by pollen grains with reduced exine structures. The pollen-epihydrophilous genera Elodea and Hydrilla have tightly arranged small spinous pollen grains, and the male flower-epihydrophilous genera Enhalus and Vallisneria have reduced reticulate or gemmate exines. Character state reconstruction of the exine structures and sculptures using a molecular phylogenetic tree suggests that variation in the exine is generally correlated with the pollination mechanism; the selective pressures acting on the pollination mechanisms have reduced the exine structure in hypohydrophilous plants and resulted in various exine sculptures that are adapted to the different pollination mechanisms in entomophilous, anemophilous, and pollen-epihydrophilous plants.     

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.     

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.

     

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.

     

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

     

Allergenicity of Acroptilon repens and Juglans regia pollen in rats

Pollen proteins that are located in the cytoplasm or on the surface of the exine can function as allergens and evoke immune system responses in sensitive patients, leading to allergic rhinitis and asthma. In this research, the pollen allergenicity and ability to induce IgE response of the pollen of two plant species were studied in rats. Acroptilon repens is an herbaceous, invasive plant with entomophilous pollen, while Juglans regia which is a tree crop produces anemophilous pollen. Immunoblot analysis using sera of sensitised rats revealed IgE reactivity to three protein bands including the 70, 41 and 25.12 kDa bands present in the A. repens pollen extract, while only one single immunogenic band of 11 kDa was detected in J. regia pollen extract. Both pollen extracts increased the eosinophil content and caused some clinical signs of allergy in treated rats. The results showed that both entomophilous and anemophilous pollen can be allergenic.     

Notes on the floral morphology in Vivianiaceae (Geraniales)

The functional floral morphology of the three genera of Vivianiaceae (= Ledocarpaceae, Geraniales), Rhynchotheca, Viviania and Balbisia, is compared. Likely pollination mechanisms are inferred from morphology and field observations. The flowers of Viviania are nectariferous and apparently zoophilous with nectar as the (primary) pollinator reward. Balbisia has pollen flowers without nectaries, its showy corolla indicates that it is also zoophilous with pollen as sole pollinator reward; bees were observed as flower visitors. One taxon (B. gracilis) may be anemophilous. Rhynchotheca has flowers without petals, with large, pendulous anthers and lacks nectaries. It shows synchronous mass flowering in its natural populations and is evidently anemophilous. A comparison with other Geraniales shows that nectar flowers with small anthers are likely the ancestral condition in Vivianiaceae. This suggests that the pollen flowers with larger anthers of Balbisia and Rhynchotheca may represent an apomorphic condition. The documentation of pollen flowers and anemophily in Vivianiaceae expands the range of known floral and pollination syndromes in Geraniales.     

Pollen morphology inParis and its related genera

Pollen grains ofParis, Medeola, andScoliopus have been examined with light, scanning electron, and transmission electron microscopies.P. quadrifolia L.,P. verticillata Bieb.,P. delavayi Franch. andP. polyphylla Smith have monosulcate pollen characterized by psilate, foveolate or reticulate exine sculpture. In contrast to the former species,P. japonica (Fr. et Sav.) Franch. andP. tetraphylla A. Gray have monosulcate pollen with gemmate (rarely rugulate) exine.Medeola has monosulcate pollen with reticulate exine that is distinct from that ofParis. Scoliopus has monosulcate pollen characterized by a peculiar reticulate exine pattern. The palynological evidence suggests thatParis andTrillium are closely related to each other, andMedeola andScoliopus should be separated fromParis andTrillium.     

POLLEN MORPHOLOGY AND ULTRASTRUCTURE OF THE CABOMBACEAE: CORRELATIONS WITH POLLINATION BIOLOGY

Of all species comprising the two genera of the Cabombaceae, only Brasenia schreberi J. F. Gmel. and Cabomba caroliniana Gray have been critically investigated with regard to their pollination biology. Brasenia schreberi has been shown to be anemophilous, while C. caroliniana has an entomophilous (myophilous) pollination syndrome. In the present paper, a number of pollen and pollen-related characters, including pollen size, shape, quantity, terminal settling velocity, pollen-ovule ratios, and overall exine architecture of B. schreberi and C. caroliniana are evaluated. Pollen from both species is elliptic, monosulcate, and has a tectate-columellate sporoderm with supratectal surface ornamentation. Grains of B. schreberi are small, produced in copious amounts, and settle relatively slowly. Flowers of this species have large pollen-ovule ratios. The exine of B. schreberi pollen is scabrate, relatively thin, has a uniformly thick sexine composed of a two-zoned (homogeneous/granular) tectum and distinct columellae, and a homogeneous nexine. Pollen of C. caroliniana is relatively large, produced in small quantities, and has a rapid terminal settling velocity. Flowers exhibit small pollen-ovule ratios. Exine organization of C. caroliniana pollen is typically two times thicker than that of B. schreberi; ornamentation is striate. Nonapertural sexine regions have a thick tectum and well-defined columellae, with both sexine components traversed by a dense system of channels. The nexine is relatively thin. All of the palynological characters examined correlate well with the anemophilous and entomophilous syndromes of B. schreberi and C. caroliniana, respectively. Moreover, several other parameters of exine ultrastructure from each species exhibit positive correlations with the respective pollination mechanisms, including: tectum thickness, columellae diameter, tectum-nexine ratios, and the consistency, distribution, and total amount of pollenkitt present. Overall exine ultrastructure is also discussed from a historical perspective as well as with respect to its phylogenetic significance.     

Taxonomic,evolutionary, and functional considerations ofCompositae pollen ultrastructure and sculpture

Two basic patterns of exine ultrastructure are found in theCompositae, the caveate Helianthoid pattern and the non-caveate Anthemoid pattern. TheHeliantheae, Astereae, Inuleae, Sececioneae, Calenduleae andEupatorieae all have pollen with caveate exines. TheMutisiseae, Vernonieae andCardueae have predominately Anthemoid pollen. TheAnthemideae, Arctoteae andLactuceae have pollen with exines of both patterns. Recent investigations of pollen in theVernonieae suggest that these exine ultrastructures in the family have evolved in response to mechanical stresses on the wall which are caused by changes in volume of the grain as it loses or gains water from its environment.     

Elektronenmikroskopische Untersuchungen zur Frage der verwandtschaftlichen Beziehungen zwischenTheligonum undRubiaceae: Feinbau der Siebelement-Plastiden und Anmerkungen zur Struktur der Pollenexine

Theligonum cynocrambe and 13 species ofRubiaceae contain S-type sieve-element plastids, wide-spread in Dicotyledons. Alignment ofTheligonum toCaryophyllales (Centrospermae), especiallyPhytolaccaceae, is unlikely, because this order is characterized by specific P-type plastids. SEM investigations show the pollen exine ofTheligonum to be microreticulate, with additional supratectate spinules.Asperula and other genera of the tribeRubieae have a tectum perforatum (punctitegillate sexine), also with supratectate verrucae or spinulae.—Thus ultrastructure suggests (but not definitely proves) relationships betweenTheligonum andRubiaceae, while affinities betweenTheligonum andCaryophyllales are excluded.

     

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.