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.

     

THE NATURE OF THREADLIKE STRUCTURES AND OTHER MORPHOLOGICAL CHARACTERS IN JACQUESHUBERIA POLLEN (LEGUMINOSAE: CAESALPINIOIDEAE)

Threadlike structures were observed on many pollen grains of Jacqueshuberia amplifoliola, J. purpurea, and J. quinquangulata of the Leguminosae/Caesalpinioideae. Each end was attached to the highly reticulate polar exine surface of different grains. The consistency of this observation, hence, the absence of free thread ends, is interpreted to mean that they should be considered as either exine bridges or exinial connections rather than viscin threads, although, most likely functionally homologous to viscin threads. Other significant morphological characters were the tricolporate apertures and strikingly lamellate foot layer and endexine.     

The Morphology of Pollen Tetrads and Viscin Threads in Some Tertiary,Rhododendron-Like Ericaceae

The morphology of pollen tetrads and viscin threads is described in fossil Ericaceae pollen from various Eocene/Oligocene/Miocene localities in Europe (Germany, Austria), North America (eastern U.S.A.), and Asia (eastern China). The typical characters of the tetrad configuration, the exine ornamentation and sculpturing, and especially the viscin thread morphology are extremely similar to or even indistinguishable from that in extant members of Rhododendron. All these pollen morphological features strongly suggest that all the investigated material can be assigned to a modern taxon of the Ericaceae: either to Rhododendroideae or even to Rhododendron itself.     

Pollen structure, tetrad cohesion and pollen-connecting threads in Pseuduvaria (Annonaceae)

The structure of the pollen of 42 species of Pseuduvaria (Annonaceae) is described. The pollen is consistently inaperturate, isopolar and radially symmetrical. Four basic patterns of exine sculpturing are identified: rugulate, verrucate, scabrate and psilate. The exine stratification of one representative species, P. macrocarpa , is shown to be entirely ectexinal. The ectexine consists of a discontinuous outer tectal layer, a columellar infratectal layer, and an inner lamellar foliated foot layer; the intine is very thin and fibrillar. The pollen is invariably released as acalymmate tetrads, in which the tectum is absent from the proximal walls. The individual pollen grains within the tetrads are connected by crosswall cohesion, involving both exine and intine; this form of cohesion has not hitherto been reported in the Annonaceae. In addition, pollen grains of neighbouring tetrads are connected in two different ways, viz. short exine connections and non-sporopollenin pollen-connecting threads. Neither of these cohesion mechanisms has previously been reported for the genus. The function of the various forms of cohesion between pollen grains and tetrads in Pseuduvaria is discussed as a mechanism to enhance the efficiency of pollination by enabling the fertilization of multiple ovules following a single pollinator visit.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 69−78.     

Diverse fossil Onagraceae pollen from a Miocene palynoflora of north-east China: early steps in resolving the phytogeographic history of the family

The origin and evolution of angiosperms can be unravelled by using fossil records to determine first occurrences and phytogeographic histories of plant families and genera. Many angiosperm families, for example the Onagraceae, have a poor macrofossil record, but are more common in palynological records. Modern Onagraceae produce pollen clearly distinct from that of other angiosperms. Combined morphological features obtained by use of light and scanning electron microscopy have enabled assignment of fossil Onagraceae pollen to extant genera, and therefore tracing of the origin and past distributions of extant Onagraceae lineages. We studied a Miocene palynoflora from the Daotaiqiao Formation of north-east China. Using the single-grain technique, we examined individual Onagraceae pollen/tetrads using both light and scanning electron microscopy. Fossil Onagraceae pollen is more frequent than macrofossil remains, but is still rare, and usually represented by a single taxon in palynological samples. Remarkably, samples from the Miocene of north-east China contain five different species: two of Circaea, one of Epilobium, and two of Ludwigia. Such a large number of Onagraceae taxa from a single palynoflora is unknown elsewhere. Whereas Ludwigia pollen is known from Cenozoic sediments of the northern hemisphere, the Circaea pollen is the first fossil pollen assignable to this extant genus. This is also the first fossil record of Epilobium from China. Although the young geological age of this sample does not enable consideration of time of origin for the genera encountered, the co-occurrence of Circaea, Epilobium, and Ludwigia in the mid to late-Miocene of East Asia sheds some light on their phytogeographic histories.     

Pollen Morphology of the Periplocaceae from Madagascar

The pollen morphology of eleven (Baroniella Constantin & Galland, Baseonema Schltr. & Rendle, Camptocarpus Decne., Cryptolepis R. Br., Cryptostegia R. Br., Gonocrypta Baill., Harpanema Decne., Ischnolepis Jum. & H. Perrier, Menabea Baill., Pentopetia Decne., and Tanulepis Balf. f.) of the 13 genera of the Periplocaceae from Madagascar have been examined using light-, scanning-, and transmission electron microscopy. All of the genera are characterized by pollen grains arranged in tetrads. The arrangement of the grains may be rhomboidal, decussate or tetragonal. The 4–6 pores present are restricted to the junction area of adjacent grains. Cryptostegia differs from the other genera in that the arrangement of pollen grains is only decussate. In Menabea the tetrads are united into a pollinium. The exine is smooth and consists of a distal stratum (tectum), subtended by a granular stratum consisting of granules of unequal size. Towards the base larger granules are present. In Camptocarpus, Harpanema and Tanulepis the exine is stratified into a distal stratum (tectum), a thin granular stratum and an almost continuous basal stratum (foot layer). The inline is well developed. The pollen grains of tetrads are connected by wall bridges (cross-wall cohesion). The internal walls in Camptocarpus, Harpanema and Tanulepis differ from the other genera in the absence of a tectum. The pollen morphology of the taxa investigated is very similar and of little value for distinguishing the species and genera investigated. The distinctive difference in exine structure between the above mentioned three genera and other genera investigated emphasizes the importance of exine ultrastructure in the Periplocaceae.     

Unique tetrads of <Emphasis Type="Italic">Epilobium luteum</Emphasis> (Onagraceae: Onagreae) pollen from Alaska

Aperture morphology of tetrad pollen of Epilobium luteum (Onagraceae: Epilobieae) from three Alaskan collections is highly variable. The first collection appears to lack apertures altogether and is presumed to consist of immature pollen gains in a genus known to achieve mature size before the apertures become distinctly protruding. A second collection has tetrads with 3- and 4-apertured grains, the apertures in the latter are often irregularly spaced and not in apposition with the apertures of neighboring members. The third collection consists of the more typical 3-apertured members that characterize the majority of Epilobium pollen grains. In all of these collections individual pollen grains (monads) are interspersed among the tetrads. The variations in the number of apertures emphasize the importance of having a comprehensive understanding of the stage of development of the pollen (taxon) examined when describing pollen collections. In the first collection this would mean the recognition that in Onagraceae apertures occur in the later stages of microspore ontogeny. In the latter two collections a thorough background of the literature of the pollen morphology on this largest Onagraceae taxon is useful for the understanding of the significance of a range of aperture numbers on Epilobium pollen grains.     

VISCIN THREADS,POLLINATION EFFICIENCY AND LOW POLLEN-OVULE RATIOS

Some Onagraceae are characterized by quite low pollen-ovule ratios. We suggest they are an evolutionary response to highly efficient pollination, mediated through viscin threads, which hold large numbers of pollen grains together, and by stigmas, which are large relative to the area of the pollinator over which the pollen is spread. The pollen grains of some Leguminosae are joined by “exinal connections” which are chemically and morphologically similar to viscin threads. In the legumes the area of the stigma is small relative to the area of pollen on the pollinator and pollination is facilitated by short connections which hold small numbers of pollen grains together.     

Dissimilar pollen tetrad development inEricaceae andOnagraceae causes family-specific viscin thread configuration

Genetically fixed polarities in pollen tetrad development lead to different, but family-specific arrangements of particular ektexine elements in the two unrelated angiosperm familiesEricaceae andOnagraceae. In the former, the so-called viscin threads are inserted on the distal, in the latter on the proximal pollen face only. Despite their diametrically opposed configurations, the viscin threads of the two families have attained identical functions in pollination ecology by convergent evolution.     

Pollen harvest by solitary bees (Ptilothrix relata,Hym. Apidae,Emphorini) in the Argentine pampas – preliminary results

In order to determine the plants foraged by solitary Ptilothrix relata, the pollen content of food provisions was identified. The main pollen types confirm foraging by these bees on Malvaceae, Asteraceae and Onagraceae. Pollen sources belonging to Cucurbitaceae, Dipsacaceae and Portulacaceae families enrich the harvesting range of P. relata. According to the preliminary results, these bees can be considered more as narrowly polylectic than oligolectic because they collect pollen from a few unrelated species. Among the most abundant pollen collected, two major morphological types were recognized: (1) pollen with long spines (4–25?μm), many single apertures and large size (115–135?μm of diameter); and (2) tricolporate pollen with viscin threads.     

Pollen morphology and ultrastructure of selected species of Magnoliaceae

The pollen morphology and ultrastructure of 20 species, representing eight genera of the Magnoliaceae are described based on observations with light, scanning and transmission electron microscopy. The family represents a homogeneous group from a pollen morphological point of view. The pollen grains are boat-shaped with a single elongate aperture on the distal face. The tectum is usually microperforate, rarely slightly or coarsely rugulose. Columellae are often irregular, but well-developed columellae do occur in some taxa. The endexine is distinct in 14 species, but difficult to discern in the genera Parakmeria, Kmeria and Tsoongiodendron. Within the aperture zone the exine elements are reduced to a thin foot layer. The intine has three layers with many vesicular-fibrillar components and tubular extensions in intine 1. The symmetry of the pollen grains, shape, type of aperture and ultrastructure of the intine show a remarkable uniformity in the family. Nevertheless there is variety in pollen size, ornamentation and the ultrastructure of the exine. The pollen of Magnoliaceae is an example of an early trend of specialization, and supports the view that Magnoliaceae are not one of the earliest lines in the phylogeny of flowering plants.     

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.     

Beschorneria yuccoides and Asimina triloba (L.) Dun: Examples for proximal polar germinating pollen in angiosperms

Beschorneria yuccoides (Agavaceae) microspores are arranged mostly in planar tetrads. Later on, the pollen grains of the tetrad usually fall apart, but sometimes remain loosely connected by ektexine elements. The ektexine consists of a tectum, of short columellae, and of a thin, discontinuous foot layer. An endexine is absent. The bilayered intine is without any additional thickening that would usually indicate an aperture region. From this point of view the pollen grain might be considered as omniaperturate. The pollen ornamentation is reticulate with wide lumina and robust, smooth muri.

The pollen grains show an indistinct sulcus characterised by a loose reticulate ornamentation. The sulcus is not exactly at the distal pole, but shifted towards the equator. No pollen tubes are formed regularly at the sulcus. Instead, pollen tubes are normally formed at the proximal pollen face. The proximal area, indicating a large germination field, is morphologically and functionally clearly an aperture (a germination zone); however, it does not represent a sulcus. The proximal face of all pollen grains appears as ornamented, with some exine lumps.

Asimina triloba (Annonaceae) pollen is shed in permanent planar or decussate tetrads. The distal sides are microreticulate to foveolate, and do not show an aperture; the psilate proximal sides are the germination areas of A. triloba.

The presence of apertures placed at the proximal pole was reported for distinct taxa of several angiosperm families. For Drosera, Dionaea (Droseraceae) and most probably for the diaperturate Cuphea species (Lythraceae) the existence of polar germination areas can be excluded. However, in some Annonaceae taxa with permanent tetrads (Annona cherimola, Asimina triloba) a situation similar to Beschorneria might be present, and indeed a proximal polar pollen tube is formed. Beschorneria yuccoides, Annona cherimola and Asimina triloba are unequivocal examples of angiosperm pollen with an exactly proximal aperture (germination area).     

Aperture variation in the pollen of Nelumbo (Nelumbonaceae)

The evolutionary and developmental origin of tricolpate pollen is of great interest because pollen of this kind defines a major clade of angiosperms (eudicots), a clade that is also well supported by molecular data. We examined evidence that tricolpate and monosulcate pollen types are produced alongside each other in the anthers of Nelumbo flowers, as has previously been reported. Observations of pollen in situ within individual anthers revealed mainly tricolpate pollen produced in tetrahedral tetrads, but also a small percentage of clearly aberrant pollen grains that have a great variety of aperture configurations. Previously published evidence for tetragonal tetrads is not supported, and previously reported monosulcate grains are part of a continuum of variation among the aberrant grains in aperture number, position and form. Other eudicots show similar variability in their pollen apertures. The variation in the pollen of Nelumbo is not exceptional, and may not be more significant than variation seen in the other taxa with regard to the origin of the tricolpate and tricolpate‐derived pollen characteristic of eudicots. Nevertheless further studies of aberrant pollen in Nelumbo and other eudicots, together with comparisons of pollen development in “normal” eudicots and closely related species that show radical, and developmentally fixed, reorganization of apertures and pollen polarity, may be helpful in understanding the processes that controlled the transition from the monosulcate to the tricolpate condition.     

Pollen Tetrads of Hedycarya arborea J. R. et G. Forst. (Monimiaceae)

Hedycarya has pollen in permanent tetrads. H. arborea, the New Zealand species, differs from others studied, in having a cap of more or less imperforate tectum at the distal pole of each grain. This polar region is not an aperture and the pollen tube emerges through a papillose part of the external wall of each grain. Transmission electron microscope studies of immature and mature tetrads reveal a most unusual exine structure. "Radial processes" develop by accumulation of sporopollenin around unit membranes of similar dimensions to the plasmalemma, and extend from just beyond the intine to the tectal region. The entire exine is considered ectexinous. During development, members of a tetrad are interconnected by cytoplasmic channels and the synchronous division into generative and vegetative nuclei within each tetrad is attributed to their presence. The channels become closed by the deposition of intine. Comparisons are made with exine structure in some other members of the woody Ranales and with some other plants with tetrad pollen.     

The Structure of Pollen Wall and Its Relation to Po41en Aggregation in Cymbidium goeringii (Rchb. F.) Rchb. F.

The pollen wall of tetrads located in different positions of a mature pollinium of Cymbidium goeringii was examined with the electron microscope, and the compositions of wall materials were also tested with different histochemical methods. In all tetrads of a pollinium, the pollen wall can be distingished into an exine and an intine, but the exine may be varied greatly according to the tetrad position in a pollenium. The part of the pollen wall (the outer wall) of the external tetrads, lying close, to the tapetum, is composed of two layers, i.e. the exine, and the intine. Theexine consists of tectum, granulate ectexine and endexine, without foot layer. The intine is cellulose in nature. In the outer wall between different groups of: tetrads and in the inner wall within an individual tetrad, the structure of ectexine becomes simple and the deposition of sporopollenin is roduced The degree of reduction of ectexine nicreases from the outer to inner tetrads in several external layers of a pollinium, and even the internal tetrads have a reduced ectexine or lack of it. The present study also demonstrates that the mechanism of pollen aggregation into a pollinium is built on a combined effect of the following features: (1) connected bridges formed' by intine between two pollens within a tetrad, (2) formation of cytoplasmic channels between two pollens within a tetrad, (3) incomplete cell wall formation within a tetrad, (4) little size of tetrads and compact arrangement of mature tetrads and (5) a sticky viscin material surrounded on the outside of a pollinium.     

Studies On the Pollen Morphology of the Genus Iris in China

In this paper, pollen grains of 32 species of the genus Iris in China were examined under light microscope and scanning elrctron microscope. Pollen grains in Iris of China are navicular or subspheroidal. According to the characters of aperture and shape, pollen grains may be divided into four types: (1) Monocolpate (distal): pollen grains navicular or subspheroidal, exine reticulate. (2) Monocolpate-colpoidal: pollen grains subspheroidal, exine pilate. (3) 2-syncolpate: polen grains subspheroidal or navicular, exine reticulate. (4) No aperture: pollen grains subspheroidal; exine verrucate. The evolutional trends of aperture and exine ornamentation are traced and systematic po-sitions of some species are discussed based on characteristics of pollen grains and other organs.     

LATE CRETACEOUS FOSSIL FLOWERS OF ERICALEAN AFFINITY

Fossilized flowers of ericalean affinity are reported from the Turanian (ca. 90 MYBP, million years before present) of New Jersey. The fossils are remarkably well preserved and three-dimensional, and are the oldest known floral remains of Ericales. The series of fossil flower buds, floral fragments, and fruits are not identical to any modern genus of Ericales. The inverted U-shaped anthers with pseudoterminal awns, and the fluted syncarpous ovary of the fossils suggest affinities with basal Ericaceae, probably near extant Enkianthus, a taxon that also shares monadinous pollen with the fossil. Pollen grains were observed clumped on a stigma in one of the fossil flowers. Fossilized acid-resistant strands having characteristics, including similar diameter and sculpture pattern, in common with the muri connect pollen grains and, with scanning electron microscopy, appear continuous with the tectum, supporting the interpretation that they are viscin threads. These are the oldest reported fossilized viscin threads, and the only fossilized viscin threads found in situ in flowers. In modern Ericales and Onagraceae, the presence of viscin threads is associated with highly specific plant-pollinator relationships, raising the possibility that such specific pollinator-plant relationships had developed by the mid-Cretaceous. This is consistent with floral characters in these ericalean fossils, the presence of advanced meliponine bees in slightly younger sediments from the same region, and with the morphology and affinities of other fossil flowers from the same sediments.     

Pollen morphology of some selected plant species from Horton Plains,Sri Lanka

The pollen morphology of four species: Gaultheria leschenaultii (Ericaceae), Psychotria zeylanica (Rubiaceae), Rubus leucocarpus (Rosaceae) and Symplocos bractealis (Symplocaceae) from Horton Plains, Sri Lanka was investigated using LM and SEM. Psychotria zeylanica is endemic to Sri Lanka and the other three taxa are common in Horton Plains. Gaultheria leschenaultii has tetrads with 3-colporate, coarsely granular to granulo-rugulate pollen grains; in the polar region and at the colpus margin the exine is smooth with micro-rugulate structure. Psychotria zeylanica has 3- (or 4-) colporate, reticulate-spinulose to granular pollen grains. Rubus leucocarpus has 3-colporate, striate-perforate pollen grains and Symplocos bractealis has 3- (or 4-) colporate pollen grains with a verrucate-perforate exine. Pollen grains of these four species are very indicative of human impact and palaeoclimate.     

Pollen morphology and its taxonomic significance of the monotropoideae (Ericaceae)

A palynological survey, including LM, SEM and TEM is presented for eight genera, nine species and 76 samples of the Monotropoideae which is composed of 10 genera and 13 species. On the basis of the aperture number and shape, the following six pollen types are recognized: 1) 3-colp(oroid) ate—Allotropa, 2) 2-colpor(oid)ate—Monotropsis andHemitomes, 3) 4-colpor(oid)ate—Pterospora, Sarcodes andPleuricospora, 4) 2- and 3-colpor(oid)ate—Monotropa hypopitys, 5) 3-colporate and-porate —Monotropa uniflora, 6) 3- to 4- porate—Monotropastrum humile. Relationships among taxa within the Monotropoideae are illustrated on the palynological characters including the aperture type, exine sculpture and structure. Both pollen grains with two and four apertures have evolved independently from pollen grains with three apertures, according to the infraspecific variation of aperture numbers and the usual occurrence of three-aperturate pollen grains in the Ericaceae. 3-colp(oroid)ate pollen ofAllotropa is the most primitive and occupies an isolated position in the subfamily, on the other hand 3- to 4-porate pollen ofMonotropastrum humile is most advanced in the subfamily, probably even in the Ericaceae. A reduced sexine inMonotropastrum humile appears to be a specialized pollen character. Infraspecific geographical difference in palynological characters is revealed in two species ofMonotropa for the first time.M. hypopitys is basically characterized by pollen grains with two apertures in the New World vs. three apertures in the Old World.M. uniflora is basically characterized by 3-porate pollen in the New World vs. 3-colporate in the Old World. In both species pollen grains with more primitive characters usually occur in the Old World.