Viscin Threads Research Articles

Pollen aggregation by viscin threads in Rhododendron varies with pollinator.

Pollen grains can be dispersed singly or variously aggregated in groups. Whether the evolution of pollen aggregation is driven by the pollinator remains unexplored. We hypothesize that an extensive pollen aggregation is favored under a scarcity of pollinators. Variation in pollen aggregation by viscin threads in 13 Rhododendron species was measured as it is related to pollen removal in a visit. Visitation rates of functional pollinator groups that vary in their grooming behavior were investigated in each species. Pollen deposited on stigmas in the field was also sampled. Seven Rhododendron species were infrequently pollinated by low-intensity grooming animals, including birds, butterflies and moths. The other six species were more frequently pollinated by bees with a high intensity of pollen grooming. Bird- and Lepidoptera-pollinated species produced longer pollen-connecting threads that connected more pollen grains. Phylogenetically independent contrast analysis of the 13 species showed that pollinator visitation frequency was negatively related to amounts of pollen removal per visit but not to stigmatic pollen loads. The finding of interspecific patterns in pollen removal related to pollinator visitation frequency suggests pollinator-mediated selection on pollen packaging strategies, supporting the hypothesis of floral evolution via pollen export.

Convergent evolution of pollen transport mode in two distantly related bee genera (Hymenoptera: Andrenidae and Melittidae)

AbstractPurposeful transport of pollen represents a key innovation in the evolution of bees from predatory wasps. Most bees transport pollen on specialized hairs on the hind legs or ventral metasoma in one of three ways: moist, dry, or “glazed,” which combines dry and moist transport. The evolutionary pathway among these three transport modes is unclear, though dry transport has been hypothesized to be ancestral. We address this hypothesis using museum specimens and published records of the bee genera Perdita (Andrenidae) and Hesperapis (Melittidae), two distantly related groups whose pollen transport modes appear to have converged. Most species in both genera transport moistened pollen; glazed and dry transport are limited to derived clades of specialists on floral hosts in Asteraceae and Onagraceae, with specialization on Asteraceae associated with more elaborate scopal hairs. The associations between transport mode, host plant, and hair type may be due to the sticky pollenkitt of asteraceous pollen grains and the viscin threads of Onagraceae pollen, which provide alternates to the binding properties of nectar. These findings suggest that the hypothesis that dry transport is ancestral in bees should be reexamined.

Pollen morphology of the tribe Phyllodoceae (Ericoideae, Ericaceae) and its taxonomic significance

Pollen morphology of 13 taxa belonging to 5 genera of the tribe Phyllodoceae (Ericaceae) was examined by means of light and scanning electron microscopy (LM and SEM, respectively), or SEM alone. In Phyllodoceae, 3-colpor(oid)ate, minute to medium, oblate pollen grains are united usually in tetrahedral tetrads. Pollen tetrads are generally characterized by the presence of viscin threads except Elliottia pyroliflora, Epigaea asiatica and Phyllodoce caerulea. The absence of viscin threads might indicate to a secondary loss, since these are present at least in some species within all the genera of Phyllodoceae. The pollen morphological data confirm the infra- and inter-generic relationships as identified by molecular phylogeny of Phyllodoceae and/or vice-versa. Although various palynological characters were found to be taxonomically important at different taxonomic levels, the apocolpial exine sculpture is emerged as one of the most important palynological features of systematic importance. The rugulate apocolpial exine with striate secondary sculpture and a reduced colpus might be apomorphic palynological character states for this tribe and Ericaceae as well. Hitherto, it is the first SEM study of Rhodothamnus pollen.DOI: http://dx.doi.org/10.3329/bjpt.v21i2.21351Bangladesh J. Plant Taxon. 21(2): 129-137, 2014 (December)

Sporoderm and tapetum ontogeny in Juniperus communis (Cupressaceae). Connective structures between tapetum and microspores

The purpose of this work was to study in detail the successive stages of pollen wall development in Juniperus communis and intimate connection between the microspores and nutritive tapetum with TEM. Another goal was to clear up the mechanisms underlying the developmental processes. The key stages observed are: the appearance of the initial outer glycocalyx layer as a row of spherical units at the early tetrad stage, followed by the emergence of the underlying radially oriented string-like units, resulting in the formation of a reticulate layer. After sporopollenin accumulation, spherical units turn into granules. String-like units start from the microspore plasma membrane, pierce the glycocalyx layer, pass between the primexine granules and through callose jacket and come out to the anther loculus, reaching the tapetal cells. Their substructure and size are similar to viscin threads, but not their function and chemical composition. All the structures, observed in sporoderm development, correspond to subsequent mesophases of a micellar system. The latter develops by self-assembly, under genomic control, regulating increasing concentration of glycoprotein surfactants in the periplasmic space. A thick network of filaments, connecting microspores and tapetum, supplies microspores with necessary nutrients and testifies the opinion that, in the process of development, exine is a living, dynamic system, arranged from plasmodesmata-like units.

Some cytological, ecological and evolutionary aspects of pollination

Different types of pollen dispersing units are described. The pollen of angiosperms may be dispersed in monads, tetrads, polyads, massulae or compact pollinia. The monads and tetrads may form larger clumps of pollen with the aid of the following substances: a) pollenkitt, a product of <em>in situ</em> degeneration of the tapetum; b) tryphine, a product of tapetal extra situm degeneration, found only in <em>Cruciferae</em>; c) elastoviscin, a highly viscous product of the degeneration of a limited number of tapetal cells, found only in certain <em>Orchidaceae</em>; d) viscin threads or sporopollenin filaments arising from the pollen exine. The number of pollen grains composing the clumps depends on pollen diameter and the cohesive forces holding the pollen grains together. The various types of pollen dispersing unit are discussed in relation to the type of pollination and number of ovules per ovary.

Pollen-ovule ratios in seven species ofVaccinium(Ericaceae) and stamen structure inVaccinium myrtilloidesandVaccinium vitis-idaea1This article is part of a Special Issue entitled “Pollination biology research in Canada: Perspectives on a mutualism at different scales”.

Pollen-ovule (P/O) ratios were determined for five Vaccinium species (Ericaceae) native to eastern Canada (Nova Scotia: V. angustifolium, V. boreale, V. caespitosum, V. corymbosum, V. uliginosum), one in western Canada (Saskatchewan: V. myrtilloides), and one (V. vitis-idaea) from both provinces. Pollen, released at maturity as tetrads, was converted to total pollen grains per flower to yield P/O ratios ranging from 238 (V. caespitosum) to 2008 (V. vitis-idaea), but 736 for the latter in Saskatchewan. These P/O ratios are indicative of facultative autogamy to facultative xenogamy. Additionally, the structure of mature stamens and pollen tetrads was studied in V. myrtilloides and V. vitis-idaea. Each anther was functionally bilocular; had a single-cell thickness (i.e., epidermis) with regularly occurring papillae; lacked an endothecium; and possessed two distal, hollow tubules each terminating in a pore. Overall pollen grain viability was 76%–97% (V. myrtilloides) and 51%–93% (V. vitis-idaea), with about 20% of tetrads having only 1–3 grains viable, and 12% and 27% of tetrads entirely nonviable in V. myrtilloides and V. vitis-idaea, respectively. Pollen tetrads occasionally were connected by a sticky substance resembling pollenkitt, but viscin threads were absent. One instance of precocious (in situ) germination of tetrads was recorded within anthers of V. myrtilloides.

Pollen with viscin threads from the late cretaceous and Paleocene, Mérida Andes, Western Venezuela

Fossil pollen exhibiting viscin threads are reported from Upper Cretaceous and Paleocene deposits in western Venezuela. The sediments were deposited in a marginal marine environment in an area presently occupied by the northern flank of the Venezuelan Andes. The palynofloras are well preserved and diverse and provide insight into the ancient vegetation of Venezuela. Pollen‐bearing viscin threads from the Upper Cretaceous Mito Juan Formation are assigned to the genus Colombipollis Sarmiento Pérez 1994. The pollen grains are tri(col)porate, normally quite smooth, and most often viewed in an oblate (flattened) state. The viscin threads are on the proximal and distal sides of the pollen and vary in length; they appear to have a simple morphology unlike the complex, rope‐like threads seen on pollen of the Onagraceae and some other angiosperm families. All observations to date are made using a standard light microscope but more morphological detail could be revealed using scanning electron microscopy. The pollen exhibiting viscin threads from the Paleocene Los Cuervos Formation are clearly related to onagraceous types and are assigned to the genus Corsinipollenites Nakomen 1995. The pollen is triporate and the viscin threads originate from the polar regions of the pollen. They resemble pollen reported from Eocene to Miocene deposits in the southern hemisphere (Australia, New Zealand) although are smaller in overall dimensions. The Venezuelan pollen reported in this paper post date the earliest record of viscin threads in the Turonian of the eastern United States. They represent the first reported forms from the South American region. In modern plants the presence of viscin threads has been related to the development of highly specific plant‐pollinator relationships; their presence in the Late Cretaceous and Paleocene in Venezuela may help constrain the timing of the origin of certain insect groups in this area.

FLORAL SYNDROME AND BREEDING SYSTEM OF LOBELIA CHINENSIS, A PERENNIAL HERB WITH STAMEN FUSION

Aims Lobelia chinensis (Campanulaceae), a perennial herbaceous creeper that grows mainly on riverbanks or wet farmland in central and southern China, is characterized by stamen fusion. The anthers of its five stamens are fused tightly into a tube in which the stigma and style are packed, and the filaments are also fused at the upper parts, forming a column-like structure surrounding the pis-til. The objective was to elucidate the adaptive significance of this growth pattern, the floral syndrome and breeding system using anatomical analyses, pollination surveys and bag experiments. Methods Flowers were observed at different developmental stages to determine the growth patterns of each floral part, especially morphologies of sexual organs, dichogamy and numbers of pollen grains and ovules, and their possible changes in spatial position and morphology. Pollination surveys were conducted to find the primary floral visitors. Bag experiments were conducted to determine self-compatibility, autogamy and apomixy. These studies would reveal the breeding system. Important findings Flower diameter was 1cm. The lower parts of perianth were fused into an open tube. The stigma and style were enclosed in the anther tube and the united parts of filaments. The flower was protandrous, and the pollen grains were released into the anther tube and dropped on the stigma surface. Growth of the style sent the stigma out of the anther tube 1 or 2 days after opening and then it became receptive. The main floral visitors were ants, hoverflies and flies. The pollen-ovule ratio (P/O) was 100, suggesting facultative autogamy, but the outcrossing index suggested the breeding system is outcrossing with partial self-compatibility and a pollinator is needed. Bag experiments indicated no apomixy and no spontaneous autogamy, but self-compatibility. The species largely outcrossed but suffered pollen limitation in field. The low P/O in this stamen-fused species might be due to the higher siring probability of individual pollen grains resulting from the accumulation and collective pollination of pollen, a similar mechanism to pollen aggregation such as pollinia, tetrads and viscin threads. The correlation of stamen fusion with perianth tube, dichogamy and low pollen-ovule ratio in this and other anther-fused species suggested that these floral traits probably formed a floral syndrome adaptive for small pollinators.

POLLEN WITH VISCIN THREADS FROM THE LATE CRETACEOUS AND PALEOCENE, MERIDA ANDES, WESTERN VENEZUELA

Abstract Fossil pollen exhibiting viscin threads are reported from Upper Cretaceous and Paleocene deposits in western Venezuela. The sediments were deposited in a marginal marine environment in an area presently occupied by the northern flank of the Venezuelan Andes. The palynofloras are well preserved and diverse and provide insight into the ancient vegetation of Venezuela. Pollen-bearing viscin threads from the Upper Cretaceous Mito Juan Formation are assigned to the genus Colombipollis Sarmiento Perez 1994. The pollen grains are tri(col)porate, normally quite smooth, and most often viewed in an oblate (flattened) state. The viscin threads are on the proximal and distal sides of the pollen and vary in length; they appear to have a simple morphology unlike the complex, ropelike threads seen on pollen of the Onagraceae and some other angiosperm families. All observations to date are made using a standard light microscope but more morphological detail could be revealed using scanning electron microscopy. T...

Pollen morphology of Gaultheria L. and related genera of subfamily Vaccinioideae: Taxonomic and evolutionary significance

The pollen morphology of 86 samples from 84 species of Gaultheria and the closely related genera Chamaedaphne, Craibiodendron, Diplycosia, Eubotrys, Gaylussacia, Leucothoe, Lyonia, Oxydendrum, Pieris, Satyria, and Vaccinium (subfamily Vaccinioideae) was investigated with light and scanning electron microscopy. The tetrahedral tetrads are tri-zonocolporate and range from 22.9 to 51.5 μm in diameter. Exine ornamentation (apocolpia and mesocolpia) of pollen grains is highly variable. Six exine types of apocolpia and five exine types of mesocolpia are distinguished. Viscin threads on the pollen grains of members of Gaultheria are first reported in the study. Colpus length is correlated with both pollen size and colpus maximum width. There is a weak correlation between corolla length and pollen size when the length of the corolla is less than 6 mm. Infrageneric relationships of taxa in Gaultheria are discussed on the basis of pollen data. Pollen morphology is ubiquitously variable at both the species and genus levels. Pollen data support the sister relationships between Satyria and Vaccinium and between Chamaedaphne and Eubotrys of a previously reported molecular phylogenetic analysis. The evolution of pollen characters is traced by character optimization and results discussed. An evolutionary trend of mesocolpium ornamentation within Vaccinioideae from granulate through granulate–rugulate to rugulate is apparent.

Pollen development in Epilobium (Onagraceae): Late microspore stages (a review)

Development of the exine of Epilobium angustifolium L. [= Chamaenerion angustifolium (L.) Holub] prior to microspore mitosis and formation of the intine results in a mature-like pollen grain. Several aspects of this process are particularly analyzed here. They are (1) Development of viscin threads is especially pronounced during vacuolated stages. They are found to be structurally the same as columellae (tufts). (2) In early microspore stages endexine tufts extend outward into the ectexine. Later there is development of a foot layer and a white line between endexine and foot layer. After formation of the foot layer and the adjacent white line there is no further interchange between tufts in the endexine and ectexine. We find that endexine tufts have a radial orientation, at least throughout microspore stages. (3) There are extensive bulges in the endexine that aid in the transport of material through the exine of microspores as indicated by the accumulation of vesicles and globules in the cytoplasm under bulge regions. Light microscopical observations of living microspores showed that bulges extend like eskers for many microns and change positions both laterally and radially with closure of a bulge region. The bulges close up apparently due to a reduction in energy, e.g., experimental temperature reduction, and the endexine returns to its usual solid appearing morphology. The bulges and especially their recovery attest to the great plasticity of the exine. During and after each bulge event there are white lines at either side of each bulge. Our interpretation is that white lines are the result of broken and repaired tufts. Since white lines are resistant to acetolysis and are not contrasted by stains we suggest that they are composed of sporopollenin. (4) The tectum forms during mid- to late microspore stages. Supratectal knobs are formed at least in large part by the core subunits but their surfaces become smooth during this microspore period. After acetolysis and potassium permanganate exposure only 10–15 nm subunits remain in the knobs showing that the smooth surface has been due to secondarily deposited (receptor independent) sporopollenin that is not resistant to the permanganate exposure. Secondarily deposited sporopollenin that coats exines like a spackle or stucco, smoothing and enlarging the exine surface without greatly changing its form, is deposited during the late microspore stages. (5) Evidence is shown for the transfer of a tracer from the loculus through both the ectexine and endexine.

Viscin threads on pollen of Circaea ×intermedia Ehrh. (Circaeeae: Onagraceae)

AbstractViscin threads are here described for the exine of Circaea ×intermedia Ehrh. (C. alpina L. × C. lutetiana L.). Often, the threads bifurcate, taper distally and have branched terminal ends. The smooth surface has lightly sculptured bands and randomly occurring and positioned sporopollenin globules. At the point of attachment with the exine the threads show either a natural transition with exine surface elements or have bulb­like enlargements. Circaea ×intermedia viscin threads present a remarkable contrast to the generally poorly defined viscin threads on Circaea pollen.

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.

Transfer of material through the microspore exine — from the loculus into the cytoplasm

Our results and those we review indicate that the exine has a great capacity for modifications that enable nutrients to pass through from the anther loculus to the microspore cytoplasm. Avenues of passage include strands, some of which are viscin threads, from the tapetum to microspores in, for example, Betula, Fuchsia, and Epilobium. Micro channels in Lopezia, Gaura, and Gelsemium extend through the ectexine, endexine, and intine to the cytoplasm. The bulge regions in Epilobium represent portions of the endexine that become very greatly enlarged, forming conducting channels that transport materials into the microspore cytoplasm. Results with tracers such as colloidal iron and lanthanum have also shown that exines of microspores are permeable across areas lacking obvious channels.Key words: Betula, Epilobium, exine, Fuchsia, Gaura, Gelsemium, Lopezia, microchannels, pollen, tapetum, tufts, viscin threads.

On the pollination of Bejaria resinosa Mutis ex Linné f. (Ericaceae), an ornithophilous Andean páramo shrub

At the Colombian Páramo de Los Dominguez, 3.700 m a.s.l., the role of hummingbirds as pollinators of the ornithophilous Bejaria resinosa was examined. Despite the plants appearance, flower visits by hummingbirds are extremely rare. Obviously, the rather short-billed species are not able to reach the nectar. At the shrubby habitats preferred by the hummingbirds, the available nectar is reduced by nectar-robbing flowerpiercers by some 60%. The low visiting frequency of hummingbirds may be compensated by the high degree of self-compatibility, the long anthesis and the sticky viscin threads that last on the birds for a long time. Additionally, the flowers are very attractive to bumble bees, which show much higher visiting frequencies than hummingbirds and seem to be the plants' most effective pollinators at the site. Hummingbirds, however, may still play an important role for long-distance pollination. The flowering peak in the dry season and the highly concentrated nectar may be judged as preconditions for a shift to bumble bee pollination.

Stamen development in the Ericaceae. I. Anther wall, microsporogenesis, inversion, and appendages

Development of the introrse, tetrasporangiate, and normally dorsifixed and poricidal stamens has been studied at the gross morphological and cellular level in ten species of Ericaceae. Microsporogenesis, followed in four species, is normal, with cytokinesis simultaneous, forming tetrahedral tetrads. The tricolp(or)ate pollen is shed as permanent tetrads with each segment two-celled except in Enkianthus in which pollen grains are three-celled monads. Anther-wall development is similar in all four species initially, but no regular pattern of wall development could be recognized thereafter. The tapetum, of parietal origin, is binucleate, glandular, and mainly uniseriate. Viscin threads occur with the tetrads in the three rhododendroid species. A well-developed endothecium appears only in Enkianthus.Soon after stamen initiation, anthers of nine species invert at the eventual filament-anther junction to become introrse; in Enkianthus inversion occurs close to anthesis. Microsporogenesis starts during early inversion; greater cell elongation on the abaxial side of the young anther completes inversion by the late sporogenous-tissue stage. In Erica and, to a lesser extent Calluna, inversion results from greater abaxial than adaxial increase in cell number and length just above the filament-anther junction. The single vascular strand reflects the degree of inversion. Stamens of six species are appendaged; three have only awns, two only spurs, while one has both. Appendages arise from residual meristems after inversion is completed (or almost so) in all except Enkianthus. Awns develop at what will be the apex at maturity of each anther half. Their length and orientation vary among species. Only in Vaccinium do the awns become hollow (tubules). Spurs, varying in length, shape, and size, arise on the abaxial side from the filament, connective, or thecae.

Phylogenetics of Lopezia (Onagraceae): Evidence from Chloroplast DNA Restriction Sites

Nineteen taxa representing all six sections and 17 of the 22 species of Lopezia have been examined for restriction site mutations of chloroplast DNA using homologous probes from Lopezia miniata subsp. miniata. Restriction sites were analyzed using maximum parsimony, weighted parsimony, maximum likelihood, and neighbor-joining. The results of these analyses support the monophyly of sect. Riesenbachia. Sections Jehlia and Lopezia may, however, be paraphyletic depending on the placement of L. racemosa subsp. racemosa. Section Nannolopezia is clearly derived from within sect. Lopezia and should be reduced to synonymy. A single reduced consensus cladogram is presented that fails to resolve the placement of sect. Riesenbachia and the placement of L. racemosa subsp. racemosa, L. miniata subsp. miniata, and L. concinna within sect. Lopezia. The chloroplast DNA results indicate that stamens, a means of mechanically depositing pollen on visiting pollinators, have evolved only once in the genus. Hummingbird pollination, which is plesiomorphic in the genus, may have been secondarily regained in sect. Jehlia after its loss to fly pollination. Lopezia (Onagraceae: Lopezieae) consists of 22 species and seven subspecies (26 taxa) unequally divided among six morphologically well-marked sections (Plitmann et al. 1973; Raven 1977, 1988). The genus has been variously divided into several segregate genera in the tribe Lopezieae, primarily by affording the hummingbird-pollinated species generic status because of their striking morphological characteristics (e.g., Munz 1961). Morphological and anatomical studies, however, emphasize similarities and shared features that mark members of the Lopezieae as a monophyletic group better considered a single genus (Eyde and Morgan 1973; Plitmann et al. 1973). Lopezia is distinct in the Onagraceae by possessing a four-merous perianth and a two-merous androecium. The genus is mainly restricted to Mexico, but one species ranges to El Salvador, and a second as far south as Costa Rica (Plitmann et al. 1973). Members of the genus have long attracted interest because of their morphological diversity and intricacy of floral structures (e.g., Bolivar 1940; Baehni and Bonner 1948; Endress 1994). Also of interest is the large range of chromosome numbers (n = 7-11, 20, 22) (Plitmann et al. 1975). Lopezia is an excellent subject for detailed molecular studies because of the large body of information available from morphology (Plitmann et al. 1973, 1975), anatomy (Eyde and Morgan 1973; Eyde 1982), cytology (Plitmann et al. 1975), and ecology (Plitmann et al. 1973, 1975). Previous work and a recent cladistic analysis of morphological characters (Hoch et al. 1993a) supplies a wealth of testable phylogenetic hypotheses. A brief description of the sections of Lopezia (Table 1; Plitmann et al. 1973) illustrates the phenotypic diversity of the genus. Section Diplandra, consisting of L. lopezioides, is monotypic, hummingbird-pollinated, and is unique in the genus in possessing two fertile stamens. All other species have a single fertile stamen and a staminode. Section Riesenbachia consists of two morphologically very distinct species: L. semeiandra is a hummingbirdpollinated shrub, and L. riesenbachia is an autogamous annual herb. The two species, however, share the unique character of a floral tube that is internally divided by a septum forming two deep pockets lined with nectariferous tissue. Section Pelozia contains species that have the upper three sepals and upper two petals adnate at their bases. These species are either outcrossing and fly-pollinated or are primarily autogamous. Section Jehlia consists of three species of hummingbird-pollinated shrubs that have floral parts free above the ovary. In L. grandiflora and L. langmaniae, the stamen and staminode are united at their bases around the style. Section Lopezia s. str. is unique within the genus and the family because of its snapping stamens. In these species the fertile stamen is held under tension by the expanded tip of the subtending staminode. When triggered by a visiting insect, the stamens explosively separate, dusting the venter of the insect with a loose aggregation of pollen held together with viscin threads. Most of these species

A new technique of examining pollen‐connecting threads

A new technique of obtaining high resolution structural information on pollen‐connecting threads is outlined. The method employs a transmission X‐ray microscope which has a resolution down to 30 nm, which is greater than a light microscope (∼ 200 nm), but retains the simplicity of sample preparation for light microscopy. The advantage of the method is that there is no requirement for extensive sample preparation. Unlike electron microscopy preparation techniques, in X‐ray microscopy there is no need for dehydration, fixation, staining or coating, and the sample is imaged at ambient pressure. Viscin threads from 9 plant species were investigated, 7 from the family Onagraceae, and two Ericaceae. It was found that in some species, structural detail complemented previously presented scanning electron microscopy images of specimens. However, in a number of species we found contrasting structures, to previous scanning electron microscopy work. It was concluded that by using X‐ray microscopy we were able to both complement existing knowledge, and give new information, that had previously been lost during conventional sample preparation.

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

Abstract 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.

Bumble Bee-Initiated Vibration Release Mechanism of Rhododendron Pollen

Although Rhododendron spp. anthers have apical pores and should be expected to be buzz pollinated, bees do not normally sonicate them to release pollen. Stamens were examined to determine their pollen release mechanism; the filaments were shown to have natural vibration modes that cause pollen to be forcibly ejected by vibration, providing the pollen is mature. The first natural vibration mode of the stamens was found to generate centrifugal force sufficient to throw the pollen toward the apical pore and out of the anther, so that it adheres to the anther tip and hangs in the bee's path to the nectaries. These vibrations may be triggered by bees brushing past the long stamens when foraging for nectar, so that the flowers retain their pollen until insect vectors begin foraging on them. The low frequency vibration modes of the viscin threads, which hold the pollen together, may contribute towards the pollen ejection by low frequency stamen vibration. Vibration transmission of dehiscent anthers containing pollen was good from 50 Hz to 850 Hz, suggesting that sonication should be an efficient method of pollen collection, although this rarely occurs. Vibratory ejection occurs at a constant energy level, rather than the constant force levels found in previous research on Actinidia.