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Brasenia is a monotypic genus sporadically distributed throughout the Americas, Asia, Australia, and Africa. It is one of eight genera that comprise the two families of Nymphaeales, or water lilies: Cabombaceae (Brasenia, Cabomba) and Nymphaeaceae (Victoria, Euryale, Nymphaea, Ondinea, Barclaya, Nuphar). Evidence from a range of studies indicates that Nymphaeales are among the most primitive angiosperms. Despite their phylogenetic utility, pollen developmental characters are not well known in Brasenia. This paper is the first to describe the complete pollen developmental sequence in Brasenia schreberi. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using combined scanning electron, transmission electron, and light microscopy. Both tetragonal and decussate tetrads have been identified in Brasenia, indicating successive microsporogenesis. The exine is tectate-columellate. The tetrad stage proceeds rapidly, and the infratectal columellae are the first exine elements to form. Development of the tectum and the foot layer is initiated later during the tetrad stage, with the tectum forming discontinuously. The endexine lamellae form during the free microspore stage, and their development varies in the apertural and non-apertural regions of the pollen wall. Degradation of the secretory tapetum also occurs during the free microspore stage. Unlike other water lilies, Brasenia is wind-pollinated, and several pollen characters appear to be correlated with this pollination syndrome. The adaptive significance of these characters, in contrast to those of the fly-pollinated genus Cabomba, has been considered. Brasenia does not produce pollenkitt nor develop tectal microchannels as does Cabomba. Instead, the discontinuity of the tectum reduces the amount of sporopollenin in the wall, which may allow for more effective wind dispersal. The importance of reassessing palynological characters in light of new ontogenetic data and the phylogenetic implications of this reevaluation are also discussed. Key words: anther; Brasenia; Cabombaceae; development; Nymphaeales; ontogeny; pollen; ultrastructure.

In small, fragmented populations of self-incompatible plant species, genetic drift and increasingly close relationships between plants may restrict the number of genetically different pollen donors, the availability of compatible mates, and the opportunity for pollen competition and selection. These restrictions may reduce the siring success or increase the probability of inbreeding depression in the offspring. To test if this was the case, we hand-pollinated maternal plants in small and large populations of the rare, endemic plant Cochlearia bavarica (Brassicaceae) with pollen from one, three, or nine donors from the same population or with nine donors from a different population. In one additional population of intermediate size, maternal plants were hand-pollinated with ten donors located at a distance of 1, 10, 100, or 1000 m. We then recorded seed and offspring characters. On average, offspring from small populations were smaller than normal and fewer survived to maturity. Increasing the number of pollen donors had a positive effect on reproductive success in small and large populations, but at the highest pollen diversity this occurred at the expense of slightly reduced offspring fitness. Because the total amount of transferred pollen was held constant, these effects could not be attributed to increasing pollen load. Rather, the increasing pollen diversity may have increased the chances of selecting a particularly 'good' donor for fertilization--an example of a sampling effect of diversity. Pollen from outside a population or from 10-100 m away resulted in higher reproductive success and greater offspring size. Effects of population size and pollination treatments on reproductive success and offspring fitness were additive. Apparently, there is no obvious size threshold above which the potential of inbreeding depression can be ignored in C. bavarica. Key words: Brassicaceae; Cochlearia bavarica; endemic plant species; inbreeding depression; Munich, Germany; pollen competition; pollen diversity; pollination distance; population size; sampling effect.

Meiotic tetrads of Annona glabra and A. montana build up a well-developed proexine (protectum, probaculum, and pronexine) at the proximal side but only a thin pronexine at the distal side during the tetrad stage. The callosic envelope is only partially digested by the end of tetrad stage. The remaining, undigested part is composed of the intersporal mass and thin peripheral layers, and the latter is conjunct with the distal pronexine of the microspore. In this remaining callosic structure celluloses are also present. Later on, due to the continuous slow decomposition of this callose-cellulose structure and microspore expansion, microspores break up the callose-cellulose envelope. Because all the four microspores are bound together by the callose-cellulose structure, they move out of the chamber in rotation. Eventually the thin pronexine is pulled toward the center of the tetrad and the well-developed proexine becomes the distal wall. These descriptions of the partial digestion of callosic envelope, the transformation from a callose-cellulose structure to the binding system of tetrad pollen, and microspore rotation in Annona are unusual in the angiosperms. Key words: Annona; Annonaceae; callosic envelope; microspore rotation; pollen; proexine; tetrad.

The occurrence of clonal growth of distylous Lithospermum caroliniense was investigated in a population in the Nebraska Sandhills, an area where sand dunes have been relatively stable for at least 1500-3000 yr, and compared to a population occurring at the Indiana Dunes, an area of active sand dune formation. Spatial autocorrelation analysis indicated the occurrence of significant clonal propagation of genetically based floral morphs at Arapaho Prairie, but not for the Indiana Dunes. Apparent clonal growth in the Sandhills population had no overall negative effect on pollen deposition or fecundity relative to the Indiana population, although in some large clones the proportion of compatible pollen grains on stigmas was lower. Clonal growth may have occurred in the Sandhills population because of the greater age and stability of the Nebraska Sandhills; infrequent establishment of seedlings permits detection of clonal growth using the spatial pattern of floral morphs. At the Indiana dunes, repeated cycles of dune formation provide conditions favoring establishment of seedlings, and sand dune succession results in disappearance of L. caroliniense before the development of clones. Key words: Boraginaceae; emergence; establishment; fecundity; heterostyly; Lithospermum caroliniense; pollen flow; spatial autocorrelation; self-incompatibility.

I investigated the mating systems and phenotypic variation of two sympatric spring ephemerals, Trillium erectum and T. grandiflorum (Liliaceae), and phenotypic selection acting through female reproductive success for 11 morphological characters in five sympatric populations of the two species. I examined the degree of self-compatibility, pollinator-visitation rates, and pollen limitation of fruit and seed production in both species. Both Trillium species were self-compatible, but outcrossed flowers produced more successful fruits and seeds than self-pollinated flowers. Pollinator-visitation rates to the two species were low compared to other insect-pollinated spring ephemerals. In addition, both T. erectum and T. grandiflorum experienced pollen limitation in fruit and/or seed production; however, levels of fecundity in both species may be influenced by resource availability as well. I found significant phenotypic variation in 11 morphological characters within and among the five study populations. The sizes of all morphological characters were positively correlated. In general, larger T. erectum and T. grandiflorum produced more seeds. Phenotypic selection analysis revealed that direct and indirect selection acted on the size of morphological characters for both species. But there was no detectable selection acting on plant shape. This study reveals that variation in plant size exists within and among populations of both species, and this variation is associated with variance in female reproductive success. Spatial and temporal variation in pollinator and/or resource abundance may play a role in the phenotypic variation exhibited by both Trillium species. Key words: Liliaceae; mating system; phenotypic selection; pollen limitation; self-compatibility; spring ephemeral; Trillium erectum; Trillium grandiflorum.

Many species exhibit reduced siring success of self-relative to outcross-pollen donors. This can be attributed either to postfertilization abortion of selfed ovules or to cryptic self-incompatibility (CSI). CSI is a form of self-incompatibility whereby the advantage to outcross pollen is expressed only following pollinations where there is gametophytic competition between self and outcross pollen. Under the definition of CSI, this differential success is due to the superior prefertilization performance (pollen germination rate and pollen tube growth rate) of outcross pollen relative to self pollen. Although CSI has been demonstrated in several plant species, no studies have assessed among-population variation in the expression of CSI. We conducted a greenhouse study on Clarkia unguiculata (an annual species with a mixed-mating system) to detect CSI, and we compare our observations to previous reports of CSI in C. gracilis and another population of C. unguiculata. In contrast to these previous studies of CSI in Clarkia, we used genetic rather than phenotypic markers to measure the relative performance of selfed vs. outcross pollen. In this study, we measured the intensity of CSI in C. unguiculata from a large population in southern California and we determined whether the magnitude of pollen competition (manipulated by controlling the number of pollen grains deposited on a stigma) influenced the outcome of competition between self and outcross pollen. In contrast to previous investigations of Clarkia, we found no evidence for CSI. The mean number of seeds sired per fruit did not differ between self and outcross pollen following either single-donor or mixed pollinations. In addition, the relative success of selfed vs. outcross pollen was independent of the magnitude of pollen competition. These results suggest that: (1) one of the few nonheterostylous species previously thought to be cryptically self-incompatible is completely self-compatible (at least in the population studied here) or (2) phenotypic markers may be problematic for the detection of CSI. Key words: Clarkia; cryptic self-incompatibility; gametophytic competition; mating system; Onagraceae; outcrossing; pollen performance; selfing; siring success.

Patterns of cytoskeletal organization during distinct polarizations that characterize pollen development in the sedge Carex blanda (Cyperaceae) were studied by correlated methods of immunohistochemistry and confocal and transmission electron microscopy. As is typical of the family Cyperaceae, Carex produces a unique pollen type known as a pseudomonad in which all four microspores of the tetrad are enclosed within the wall of a single pollen grain. Only one member of the tetrad is functional and the other three abort. The pseudomonads are precisely oriented in the locule with the functional microspore in the wide abaxial portion of the wedge-shaped cytoplasm adjacent to the tapetum, and the degenerative microspores are packed tightly in the pointed adaxial portion. A unique sequence of post-meiotic developmental events reflects both intracellular and intercellular polarity. Development includes: (1) random placement of tetrad nuclei in the coenocytic sporocyte after meiosis, (2) interrupted cytokinesis resulting in a tetrad of nuclei that migrates as a unit into the narrow adaxial tip, (3) completion of unequal cytokinesis and centering of the functional nucleus in the wide abaxial portion of the microsporocyte via a radial array of microtubules and microfilaments, (4) unequal mitosis resulting in a small generative cell at the proximal surface of the functional microspore (adjacent to the abortive microspores), and (5) recentering of the vegetative nucleus in the abaxial cytoplasm via a radial cytoskeletal array. Key words: Cyperaceae; cytoskeleton; development; polarity; pollen.

The Nelumbonaceae are a small family of aquatic angiosperms comprising Nelumbo nucifera and Nelumbo lutea. Historically, the genus has been considered to be closely related to Nymphaeales, however new systematic work has allied Nelumbo with lower eudicots, particularly Platanus. In recent years, studies of pollen development have contributed greatly to the understanding of phylogenetic relationships, but little has been known about these events in Nelumbo. In this paper, pollen and anther development are morphologically described for the first time in N. lutea. A comprehensive ontogenetic sequence is documented, including the sporogenous tissue, microspore mother cell, tetrad, free spore, and mature pollen grain stages. The deposition of a microspore mother cell coat and callose wall, the co-occurrence of both tetrahedral and tetragonal tetrads, the formation of a primexine in tetrads, and primexine persistence into the late free spore stage are shown. The majority of exine development occurs during the free spore stage with the deposition of a tectate-columellate ectexine, a lamellate endexine, and an unusual granular layer below and intermixed with the endexine lamellae. A two-layered intine forms rapidly during the earliest mature pollen stage. Major events of anther development documented include the degradation of a secretory-type tapetum during the free spore stage and the rapid formation of U-shaped endothecial thickenings in the mature pollen grain stage. The majority of mature pollen grains are tricolpate, however less common monosulcate and diaperturate grains also develop. Co-occurring aperture types in Nelumbo have been suggested to be an important transition in angiosperm aperture number. However, aperture variability in Nelumbo may be correlated with the lateness of aperture ontogeny in the genus, which occurs in the early free spore stage. This character, as well as other details of pollen and anther ontogeny in Nelumbo, are compared to those of Nymphaeales and Platanus in an effort to provide additional insight into systematic and phylogenetic relationships. Although Nelumbo is similar to both groups in several characters, the ontogenetic sequence of the genus is different in many ways. Key words: anther; development; morphology; Nelumbo; Nelumbonaceae; pollen; ultrastructure.

The subgenus Tridentatae of Artemisia (Asteraceae: Anthemideae) is composed of 11 species of various taxonomic and geographic complexities. It is centered on Artemisia tridentata with its three widespread common subspecies and two more geographically confined ones. Meiotic chromosome counts on pollen mother cells and mitotic chromosome counts on root tips were made on 364 populations (X = 3.1 plants per population). These population counts are approximately 60% of all Tridentatae counts. Some are first records for taxa. The Tridentatae are a polyploid complex (x = 9) with ploidy levels from 2x to 8x, but mostly 2x (48%) and 4x (46%). Polyploidy occurs in nine of the 11 species and in many subspecies as well. Supernumerary or b chromosomes are present only at a low frequency. In the principal species, A. tridentata, 2x plants are larger than 4x ones, which are adapted to drier conditions, probably in consequence of their slower growth rates. Gigas diploidy is a phenomenon shared by some other woody genera, but is in contrast to the gigas polyploid nature of many herbaceous genera. Polyploidy occurs within populations and is essentially autoploid. Hybridization sometimes occurs at taxa interfaces in stable hybrid zones. Stable Tridentatae hybrid zones coupled with the group's inherent propensity for polyploidization has led to the establishment of a geographically and numerically large and successful complex of species. Key words: Artemisia; Asteraceae; cytogeography; hybridization; polyploidy; sagebrush; Seriphidium; Tridentatae.

Pollen of ten species of Panax and six species of Aralia was examined in light microscopy and scanning and transmission electron microscopy. Grains of both genera have similar complex apertures, short columellae, and overlapping tectal sculptures, suggesting a close relationship. Most species of Panax have pollen characterized by striato-reticulate tecta, short columellae, thick foot layers, costa ectocolpi, and lalongate endoapertures. The eastern North American P. trifolius, commonly known as the dwarf ginseng, has a distinctive pollen morphology and exine structure, supporting the hypothesis of its phylogenetically isolated position. Pollen of the eastern Asian P. ginseng (ginseng) can be distinguished from the eastern North American P. quinquefolius (American ginseng) by differences in ultrastructure. The monophyly of the three medicinally important species, P. ginseng, P. notoginseng, and P. quinquefolius, suggested by triterpenoid data, is not supported by pollen data. The results of the pollen study are generally congruent with those from the sequences of nuclear ribosomal DNA. Key words: Aralia; Araliaceae; ginseng; Panax; pollen ultrastructure.

Wind pollination was experimentally demonstrated in Linanthus parviflorus (Polemoniaceae), a predominantly beefly-pollinated, self-incompatible annual. Seed set in plants enclosed in mesh tents that excluded pollinators but allowed airborne pollen flow provided evidence for wind pollination, and the extent of seed set due to wind pollination was compared to that in open-pollinated controls and pollen-supplemented treatments. Additional controls were included to test for possible confounding effects of the mesh tent. Mean seed number in open-pollinated plants was 72.8-81.1% of that in pollen-supplemented plants, while wind pollination alone produced 49.5-52.2%, a smaller but substantial proportion of seed set with pollen supplementation. Further evidence for wind pollination was found in a comparison of sites differing in the extent of wind exposure in two populations of L. parviflorus. Airborne pollen counts were higher in exposed sites than in protected sites, and the difference was marginally significant. Seed set was significantly pollen limited in protected sites, but not in exposed sites. Taken together, the data suggest that wind pollination provides some reproductive assurance in this obligately outcrossing species. Wind pollination is hypothesized to represent an alternative to selfing as an evolutionary solution to the problem of temporal or spatial variation in pollination visitation. Key words: Linanthus parviflorus; Polemoniaceae; pollen limitation; reproductive assurance; self-incompatibility; wind pollination.

The organization of microtubules in germinated pollen of the conifer Picea abies (Norway spruce, Pinaceae) was examined using primarily confocal microscopy. Pollination in conifers differs from angiosperms in the number of mitotic divisions between the microspore and the sperm and in the growth rate of the pollen tube. These differences may be orchestrated by the cytoskeleton, and this study finds that there are important functional differences in microtubule organization within conifer pollen compared to the angiosperm model systems. Pollen from P. abies contains two degenerated prothallial cells, a body cell, a stalk cell, and a vegetative cell. The body cell produces the sperm. In the vegetative cell, microtubules form a continuous network from within the pollen grain, out through the aperture, and down the length of the tube to the elongating tip. Within the grain, this network extends from the pollen grain wall to the body and stalk cell complex. Microtubules within the body and stalk cells form a densely packed array that enmeshes amyloplasts and the nucleus. Microtubule bundles can be traced between the body and stalk cells from the cytoplasm of the body cell to the adjoining cell wall and into the cytoplasm of the stalk cell. Body and stalk cells are connected by plasmodesmata. The organization of microtubules and the presence of plasmodesmata suggest that microtubules form a path for intercellular communication by projecting from the cytoplasm to interconnecting plasmodesmata. Microtubules in the elongating tube form a net axial array that ensheathes the vegetative nucleus. Microtubules are enriched at the elongating tip, where they form an array beneath the plasma membrane that is perpendicular to the direction of tube growth. This enriched region extends back 20 [[micro]meter] from the tip. There is an abrupt transition from a net perpendicular to a net axial organization at the edge of the enriched region. In medial sections, microtubules are present in the core of the elongating tip. The organization of microtubules in the tip differs from that seen in angiosperm pollen tubes. Key words: conifer; cytoskeleton; microtubule; Picea abies; Pinaceae; pollen; pollen tube.

The influence of pollen competitive environment on pollen performance (pollen germination, stigmatic penetration, and pollen tube growth rate), the maturation or abortion of initiated fruit, seed size, and seedling vigor was explored by manipulating the size and diversity of stigmatic pollen loads on Mirabilis jalapa. All aspects of pollen performance significantly increased with the number of pollen grains on a stigma or pollen tubes in a style, but was not influenced by the diversity of pollen donors. Plants tended to mature single-ovulate fruits that came from flowers where pollen load size and diversity were greatest and aborted those where these were lowest. No plants from seeds resulting from pollinations with a single pollen grain survived, but other fitness measures were mostly determined by maternal plant. The data suggest that pollen performance is influenced by pollen competitive environment, and both the genetic diversity of the pollen load and number of competing pollen tubes are important determinants of seed/fruit abortion. Key words: competition; fitness; fruit; fruit abortion; maternal effect; Mirabilis jalapa; Nyctaginaceae; pollen.

Sacci of conifer pollen do not function primarily to increase the efficiency of wind pollination as is widely thought. Rather, they are bladders and cause pollen to float upwards in a liquid drop into the ovules. This observation is seemingly unsupported in the case of oriental spruce (Picea orientalis (L.) Link), which has saccate pollen. Ovulate cones are pendant at the time of pollination, which requires that pollen sink into the ovules. Pollen of oriental spruce floats at first but within 1-2 min sinks into the ovule. As sinking does not occur in saccate pollen of other Pinaceae, a variety of techniques was used to determine anatomical differences leading to this uncharacteristic tendency. Light, scanning electron, and confocal microscopy of the pollen surface yielded no significant appearing difference between pollen of oriental spruce and white spruce. However, transmission electron microscopy of freeze-fixed/freeze-substituted hydrated pollen revealed that the ektexine of oriental spruce pollen sacci is porous compared to that of white spruce. Confocal microscopy allowed examination of pollen hydration dynamics. Water enters pollen at the distal pole between sacci, and resulting rapid expansion of the tube cell forces air out of the saccate space. White spruce pollen remains buoyant because of enclosed air pockets in the saccus ektexine. Evolutionary change in pollen wall anatomy with resultant loss of saccus function is correlated with a change in ovulate strobilus orientation at pollination in oriental spruce. A suite of characters interact in the conifer pollination mechanism, and concerted change in these characters may lead to speciation. Key words: Picea; Pinaceae; pollen; pollination; sacci.

Understanding plant reproduction requires knowledge of genetic contributions through pollen and seeds. Since direct genetic assessments of fitness through pollen are often intractable, reproductive ecologists use components of male fitness such as pollen production and pollen removal as surrogates for paternity. However, we know little of the strength of the relationship between these components and actual paternity. Here, I report on a study undertaken to examine the relationship of pollen production and removal with paternity in Fragaria virginiana, a wild strawberry. A morphological marker was used to track paternity in experimental arrays exposed to native pollinators. Relative pollen production proved to be a poor predictor of relative paternity in most arrays, and over all arrays there was no significant correlation between relative paternity and relative pollen production. In contrast, relative pollen removed correlated significantly and positively with proportion of seeds sired, suggesting that a plant's contribution to the pool of removed pollen is a good predictor of its male reproductive success. Deviations from expected paternity based on relative pollen removal suggest a systematic overestimation of the siring success of plants with low pollen removal. And, in at least one specific case, low pollen removal may be explained by delayed anther dehiscence, which could lower the effectiveness of the removed pollen. Key words: Fragaria virginiana; male fitness; paternity; pollen; pollen removal; reproduction; Rosaceae; siring success; strawberry.

Self-incompatibility (SI) is an effective method for limiting self-fertilization in flowering plant species, but there are circumstances in which an otherwise functional SI system may fail. One of the most intriguing of these is the induction of selfing by mixed loads of self and heterospecific pollen (the mentor effect) because it is likely to occur under natural conditions, such as in hybrid zones. Here we conducted a series of controlled crosses to determine whether mentor effects operate in two SI annual species, Helianthus annuus and H. petiolaris, and whether the failure of SI results in a decrease in the frequency of hybridization between these two species. Of the 1396 achenes examined from pollen mixtures that included varying ratios of self, intraspecific compatible, and interspecific pollen, 71 (5.1%) were selfed. Selfing frequencies were significantly less than expected based on pollen ratios, except when the proportion of intraspecific compatible pollen was low. Hybridization frequencies from these same pollen ratios averaged 41.8% with H. annuus as the maternal species and 13.3% with H. petiolaris as the mother. Analysis of 1404 achenes from pollen mixtures that excluded self pollen resulted in hybridization frequencies for H. annuus (42.2%) and H. petiolaris (18.2%) that do not differ significantly from those including self pollen. Thus, mentor effects do not appear to play an important role in reproductive isolation between these species. On the other hand, even a modest increase in self-fertilization in hybrid populations, such as that due to mentor effects, could enhance the probability of hybrid species establishment. Key words: Asteraceae; Helianthus; hybrid speciation; introgression; mentor effect; pollen competition; reproductive isolation.

Much of the ultraviolet-B radiation (UV-B) research on plants has concentrated on vegetative plant parts, and only a small fraction has dealt with the reproductive system. The present study analyzed pollen grains of 34 taxa germinated and grown under two levels of UV-B radiation (187 and 460 mW/[m.sup.2]) or no UV-B (control group). Visible radiation at 260 [[micro]molar] [multiplied by] [m.sup.-2] [multiplied by] [s.sup.-1] was present in all treatments. Taxa included those with binucleate and trinucleate pollen types. We detected differences among species. A significant reduction in pollen germination occurred in only five species. Pollen tubes of >50% of the species showed significant reduction in length. Trinucleate pollen types were more likely to exhibit tube length reduction than the binucleate types. Proportionately more monocotyledonpus species were sensitive to UV-B treatment than dicotyledonous species, and proportionately more wild species were sensitive than cultivated species and pollen collected from plants growing in the field were somewhat more sensitive than pollen collected from plants grown in the greenhouse. Species in which pollination occurred earlier in the season were more likely to be susceptible to UV-B radiation than those for which anthesis took place later in the season, suggesting a possible adaptation to UV-B radiation. Key words: binucleate; pollen germination; pollen tube growth; trinucleate; UV-B radiation.

Differences among flagellated and nonflagellated sperm in land plants are striking, but close examination reveals similarities in pattern of cytoskeleton and in nuclear structure. The microtubular cytoskeleton of flowering plant sperm consists of microtubule bundles arranged obliquely around the nucleus, terminating in cellular extensions. Microtubules are linked into bundles that branch and rejoin along the axis of the sperm cell, forming a cytoskeleton that determines cell shape but does not actively participate in cell movement. Generative cells and sperm share a pattern of microtubules not found in somatic cells. This pattern is initiated in the generative cell, one division before sperm formation, a situation parallel to spermatogenous cell development in vascular plants with flagellated sperm. Chromatin in flagellated and nonflagellated sperm is condensed by specialized histones. Key words: cytoskeleton; generative cell; microtubules; pollen; sperm (plant).

The morphology, systematics, and ecology of the extinct juglandaceous genus Polyptera are interpreted on the basis of infructescences, fruits, staminate catkins, pollen, and compound leaves from the Paleocene of Wyoming and Montana. The elongate infructescences of Polyptera manningii bear numerous helically arranged sessile fruits. The fruit is a pyramidal nut with a vascularized husk and a multilobed disk-like wing adaptive for wind dispersal. Associated staminate catkins bear numerous helically arranged florets, each with six or more stamens. Pollen from the anthers is triporate and isopolar, with ultrastructure diagnostic of Juglandaceae, but with a pattern of exinous thinning characteristic of the extinct dispersed pollen type Maceopolipollenites anellus (Nichols et Ott) comb. nov. The correlated leaves, Juglandiphyllites glabra (Brown ex Watt) comb. nov., are deciduous, pinnately compound, and have five to seven petiolulate finely serrate leaflets. Systematically, Polyptera shows relationships both with the Hicorieae and the Juglandeae. Occurrences in the Torrejonian and Tiffanian stages of the Paleocene place Polyptera as the oldest unequivocal record of Juglandaceae. Polyptera manningii typically occurs in low-diversity assemblages of 8-15 species, and usually is a dominant or subdominant constituent of floodplain assemblages. It may have been an early-successional colonizer. Key words: fruits; Juglandaceae; leaves; Maceopolipollenites; pollen; paleobotany; Paleocene; Polyptera.

An anatomical survey of anthers of the Poaceae and Cyperaceae and two related families, the Restionaceae and Flagellariaceae, was conducted to determine the taxonomic distribution and possible phylogenetic significance of pollen arrangement. An unusual pollen arrangement was known from a small number of taxa in the Poaceae and Cyperaceae, in which a single, uniseriate cylinder of pollen grains is arranged in the anther locule such that each grain is in contact with the tapetum (termed here 'peripheral' pollen). This contrasts with the prevailing arrangement in other angiosperms in which the locule contains a relatively large number of pollen grains in no special configuration, with many interior grains that never touch the tapetum (termed here 'central' pollen). A total of 48 species in these four families was examined in this study, and observations on pollen arrangement for numerous additional species in these families were gleaned from the literature. We confirm that the peripheral arrangement is predominant in Poaceae and Cyperaceae, although both families also include species with central pollen, whereas only central pollen is found in Restionaceae and Flagellariaceae. In the peripheral arrangement, the pore of the pollen grain or pseudomonad, when observed, is in contact with the tapetum, but it has not been definitively demonstrated that this is always the case. Peripheral pollen in the Poaceae and Cyperaceae is nonhomologous because of the presence of pseudomonads in the latter family. It remains unexplained why peripherally arranged pollen or pseudomonads, with the attendant reduction in the number of pollen grains, should be associated with anemophily in these two families. Key words: anatomy; anther; Centrolepidaceae; Cyperaceae; Juncaceae; Poaceae; pollen; Restionaceae.

The influence of the sequence of maternal tissue development in Betula pendula upon the potential for male gamete selection was investigated, and the timing of the fixed abortion of one of the two ovules was determined. We used scanning electronic microscopy, confocal laser scanning microscopy, and blue light microscopy. The stigmas remain fresh throughout male anthesis, and may also last after its end, depending on ambient temperatures. The presence of germinated pollen does not induce stigmatic necrotization, and grains may arrive at different times. The pollen tube tips remain within the stigma base until the end of female anthesis. The ovules will not develop until after necrotization of the stigmas. The pollen tubes thus have a fair start to the ovules, regardless of their different arrival times and of the original positions of the pollen grains at the stigma surface. Therefore, competition among different microgametophytes is possible, in spite of low pollination intensity. Our results indicate that when the first pollen tube penetrates an ovule, this ovule starts to outgrow the other one, and even if the other is also penetrated, its vascular support soon atrophies and the megagametophyte will shrivel. Fertilization of both ovules was never seen in this study. Key words: abortion; anemophily; Betula; Betulaceae; brood size; confocal laser scanning microscopy; floral longevity; pollen competition; pollen load.

We conducted three experiments to examine the influence of ultraviolet-B radiation (UV-B; 280-320 nm) exposure on reproduction in Brassica rapa (Brassicaceae). Plants were grown in a greenhouse under three biologically effective UV-B levels that simulated either an ambient stratospheric ozone level (control), 16% ('low enhanced'), or 32% ('high enhanced') ozone depletion levels at Morgantown, WV, USA in mid-March. In the first experiment, we examined whether UV-B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by [approximately equal to]50% under both enhanced UV-B levels relative to ambient controls. While plants under high-enhanced UV-B produced over 40% more flowers than plants under the two lower UV-B treatments, whole-plant production of viable pollen was reduced under high-enhanced UV-B to 17% of that of ambient controls. Whole-plant production of viable pollen was reduced under low-enhanced UV-B to 34% of ambient controls. In the second experiment, we collected pollen from plants under the three UV-B levels and examined whether source-plant UV-B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced-UV-B treatments had initially lower germination and viability than pollen from the ambient level. After in vitro exposure to the high-enhanced UV-B levels for 6 h, viability of the pollen from plants grown under ambient UV-B was reduced from 65 to 18%. In contrast, viability of the pollen from plants grown under both enhanced UV-B treatments was reduced to a much lesser extent: only from [approximately equal to]43 to 22%. Thus, ambient source-plant pollen was more sensitive to enhanced UV-B exposure. In the third experiment, we used pollen collected from source plants under the three UV-B levels to fertilize plants growing under ambient-UV-B levels, and assessed subsequent seed production and germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV-B treatments, than from ambient UV-B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV-B exposure of their pollen source. Our findings demonstrate that enhanced UV-B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa. Key words: Brassica rapa; ozone depletion; pollen; seed yield; UV-B radiation.

The intra- and interspecific relations between the size and number of pollen grains were examined for 21 species of papilionaceous legumes to test for the trade-off expected from the subdivision of limited resources. We observed a trade-off between pollen grain diameter and the number of grains produced per flower within 17 of the 21 species examined. For 12 of these species, the observed partial regression coefficient for In (pollen grain diameter) equaled -3, as expected. The remaining five species exhibited more negative partial regression coefficients than expected. Relations between pollen grain size and number that are more negative than expected may result from a decelerating relation between pollen grain size and the resource investment per grain. Flower production significantly influenced pollen production for only eight species so that pollen production seems to be determined primarily on a per flower basis. Interspecifically, pollen size also varied inversely with pollen number for the 21 species even after the effect of phylogenetic relatedness was removed. Pollen size varies relatively little compared to the number of pollen grains produced per flower and therefore may be the primary target of natural selection. The presence of a trade-off in pollen production probably constrains the evolutionary options by which flowering plants can maximize male success.

The correlation between pollen/ovule (P/O) ratio and breeding system has generally been accounted for either on the basis that P/O reflects pollination efficiency, or in terms of the sex allocation theory. The following were assessed for taxa belonging to genus Astragalus subgenus Epiglottis: 1) Degree of correlation between P/O and the breeding system, measured by means of autofertility; 2) The absence or existence of correlation between P/O and pollen grain size; and 3) The ability of various theories to account for the results obtained. Results showed a minimal correlation between P/O and autofertility, and between P/O and pollen grain size in the taxa studied. Analysis of these results in terms of the sex allocation theory enabled this correlation to be explained as a function of the variation existing between taxa with respect to the resources invested in each pollen grain and in each ovule. The predictive capacity of this theory, which has moreover proven valuable in explaining the structural peculiarities of the androecium in these taxa, was also highlighted. The type of self-pollination applicable was also discussed, as was the phenotypic model of selection of self-fertilization considered most plausible for these taxa.

To determine how the capability of pollen is affected by lengthy exposure to field conditions, we obtained pollen samples (from Erythronium grandiflorum) that were fresh (from newly dehisced anthers) or aged, from 6 to 24 hr. Without pretreating the collected grains, we compared their abilities 1) to retain viability, as indicated by the Heslop-Harrison fluorochromatic reaction (FCR) test; 2) to retain germinability, as indicated by pollen-tube growth in vitro on Brewbaker-Kwack medium; 3) to sire seeds when used as a pollen donor on recipient flowers in the field; and 4) to sire seeds competitively when mixed with another donor pollen in competitive pollinations. FCR scores declined drastically with pollen age. Germinability typically declined with age, but less drastically; the correlation between FCR and germinability ranged from strong to nil in different trials. The sharp decline in FCR was probably due to nonlethal, reversible desiccation. Seed-siring success, both in competitive and noncompetitive trials, bore no relation to pollen age or to germinability. Any loss in seed-siring ability in E. grandiflorum pollen over 24 hr is apparently overwhelmed by other sources of variation. The lack of correlation between seed-siring ability and FCR or germinability scores means that those tests should not be used to estimate overall pollen competence in this species. They should be so used in other species only if experiments have demonstrated their greater reliability.

The developmental defects causing cytoplasmic male sterility in Petunia parodii are described in isonuclear fertile, sterile, and fertility-restored plants using both light- and scanning electron microscopy. The aberrant development of the sporogenous tissue and tapetal layer caused by the cytoplasmic male sterile cytoplasm in both Petunia hybrida and P. parodii nuclear backgrounds is similar in onset and progression. The degeneration of the sporogenous tissue and tapetal layer of sterile anthers is first apparent late in meiosis and results in highly abnormal sterile sporogenous tissue by tetrad stage of fertile anthers. The stomium and endothecium do not show major developmental differences between fertile and sterile anthers, but the inner connective tissue of sterile anthers contained calcium crystals not found at high abundance in fertile anthers. Ovoid bodies containing magnesium and phosphorus were seen only in the vascular bundles of fertile anthers. Material prepared for the scanning electron microscope by freeze drying showed better retention of fragile morphological features, while critical-point drying permitted examination of nonvolatile structures, such as cell walls.

Effects of ovular secretions on pollen grains were examined in Pseudotsuga menziesii. The exine is cast off in the micropylar canal. A membranelike structure covers parts of pollen grains and appears to protect them. The outer intine consists of fibrous materials, but it also shows a thicker filamentous appearance in some ovules during pollen elongation. The inner intine is electron-dense. Its fibrous nature is occasionally visible. Dissolution of the outer intine varies in amount and manner in ovules from different trees. The plasma membrane near the pollen wall alternatively appears normal and distorted. These different morphologies of the outer intine and of the plasma membrane are considered to result from secretions from the ovule. The outer intine may contain electron-dense globules that are formed in the tube cell and traverse the inner intine. Pollen tube formation appears to be triggered by a secretion from the ovule. Cross-pollinated grains are less distorted compared with self-pollinated grains.

To determine the effects of soil nitrogen on pollen production, pollen size, and pollen performance, two cultivars of zucchini (Cucurbita pepo) were grown under two nitrogen regimes in an experimental garden. The two cultivars were true breeding for alternative alleles for a one gene trait, ovary color. The soil nitrogen treatment had a significant effect on most measures of reproductive output through the female function. The nitrogen treatment did not affect the number of staminate flowers per plant but did have an effect on the number of pollen grains per staminate flower and the mean pollen grain size. A pollen mixture experiment revealed that pollen produced by plants in the high nitrogen treatment sired significantly more seeds than pollen from low nitrogen plants. Moreover, we found that the high nitrogen pollen sired even a greater percentage of seeds in the region of the fruit (ovary) previously shown to be fertilized by the fastest growing pollen tubes. Thus, the difference in the number of seeds sired by pollen from the two nitrogen treatments is due to differences in pollen performance. We conclude that spatial heterogeneity in soil nitrogen can influence the paternity of seeds in a plant population.

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

Most plants in Mediterranean ecosystems are insect pollinated, with pollen being the main reward to pollinators. The great majority of pollinators (70%) are bees and flies. We measured the energy content of pollen from 40 plant species in these ecosystems that represent abiotic and biotic pollination modes as well as the number of species of their pollinators. Pollen energy content correlates with pollinator diversity. Pollen of wind-pollinated plants contained less energy than that of insect-pollinated plants; there was no difference between insect-pollinated dicots and insect-pollinated monocots. The median date of flowering (from 1 January) estimated for each of the plant species did not vary significantly either with the number of pollinator species or with the energy content of pollen. The reasons for the differing values of pollen energy content are discussed; in particular, its relation to the type of pollen reserves, length of the flower style, and pollination enhancement.

The mean diameter of viable pollen grains is approximately 13 [micro]m greater than the mean diameter of inviable grains in Mimulus guttatus. We show that this difference is large enough to be detected by particle counters and that these machines can be used to obtain a rapid estimate of pollen viability. While requiring a separate calibration, a size-based statistic is also strongly correlated with pollen viability in Collinsia verna. These results suggest that statistics derived from the size distribution of pollen grains may provide an alternative to more labor-intensive methods for estimating pollen viability, particularly in cases where inviability results from inbreeding depression or hybrid failure. Key words: Collinsia; inbreeding; Mimulus; particle counters; pollen viability.

Chromosome numbers were determined for 48 accessions of breadfruit (Artocarpus altilis, A. mariannensis, and A. camansi [Moraceae]) from 16 Pacific Island groups, Indonesia, and the Philippines. Artocarpus camansi and A. mariannensis exhibit counts of 2n = 56; 2n = 56 (diploidy) and 2n = 84 (triploidy) were observed for A. altilis. Most diploid cultivars of A. altilis were seeded, but two cultivars with reduced seed number were observed. Micronesian accessions included putative interspecific hybrids between A. altilis and A. mariannensis. The majority of these accessions were seedless diploids, but triploid putative hybrids were also observed. Pollen stainablility was shown to correlate with the degree of seediness. Key words: Artocarpus altilis; Artocarpus camansi; Artocarpus mariannensis; breadfruit; chromosome number; pollen stainability; Moraceae; Pacific Islands.

Variation in pollen production was measured within five hermaphrodite species of bromegrass (Bromus). Anther length is an excellent predictor of pollen production in this genus ([R.sup.2] = 0.97). Anther length varied considerably within each of the species, both among and within individual plants. Within plants, most of the variation occurred among florets within spikelets; florets in upper spikelet positions were smaller and produced less pollen. In B. inermis, pollen production was decreased by defoliation and increased in shoots that grew on thatching ant (Formica obscuripes) mounds. Whole-shoot pollen yield was determined by spikelet number, number of florets per spikelet, and pollen production per floret. All of these yield components must be considered in attempts to estimate pollen production accurately.

We are studying gene expression in generative cells using isolated generative cells of lily pollen as our experimental material. Our study indicates that the generative cell displays differential gene expression. Certain transcripts show strict cell specific expression, whereas others show preferential expression in generative cells. One of the preferentially expressed cDNA clones from the generative cell shows a significant nucleotide sequence similarity with a human nucleotide excision repair protein. It is of interest to note that the generative cell, the progenitor of two sperm cells, showed a higher level of the transcripts than the vegetative cell. These results suggest that an active DNA repair process responsible for maintaining the genetic integrity of germ line DNA exists in the generative cells of pollen. One cDNA clone, LGC1 showed generative cell specific expression. The level of LGC1 transcripts was very high in generative cells. In sire hybridization results clearly demonstrate the generative cell specific localization of LGC1 transcripts. LGC1 is the first germ line specific gene to be identified in flowering plants. The availability of this gene has provided us with a molecular marker to study the regulation of cell-specific expression in generative cells of pollen.

A 260-cm sediment core from C.C. Pond, Talladega County, Alabama, yields a pollen sequence spanning the last 33,000 years. Pollen deposition reflects changes in local forest composition through the full-glacial interval. The data are consistent with significant displacement of boreal forest during maximum glaciation, as well as the existence of a more temperate interstadial period preceding the full-glacial.

Helianthus annuus and H. petiolaris are two common annual sunflower species that frequently form hybrid swarms in nature. First generation hybrids suffer highly reduced pollen fertility, at least in part due to chromosomal structural rearrangements differentiating the two species. Previous studies in the Rieseberg lab have demonstrated the importance of rearrangements in limiting molecular marker introgression in synthetic hybrid lineages between Helianthus annuus and H. petiolaris. Here we examine patterns of molecular marker introgression in 139 individuals representing three naturally occurring narrow hybrid zones between these two species in eastern Nebraska. Seeds were collected from plants closest to the H. annuus side of the hybrid zone that appeared to be of mixed parentage, based on morphological intermediacy. Seeds were germinated and plants grown to maturity in the greenhouse. Percent pollen viability and a suite of 13 morphological traits that differentiate the parental species were scored in all plants. All individuals were also surveyed for the presence of species-specific mapped molecular markers characteristic of H. petiolaris. Composite graphical genotypes for each of the three hybrid zones were constructed and compared with those from synthetic hybrid lineages. We found that patterns of molecular marker introgression are far more complex in natural populations than in synthetic lineages. Nonetheless, a number of similarities are evident, both in terms of the frequency and genomic location of introgressed markers. In synthetic lineages, introgression was markedly reduced in non-collinear versus collinear linkages. This pattern is far less striking, though still apparent in natural hybrids. Furthermore, particular linkage blocks that resisted introgression in synthetic lineages also do so in natural hybrids. We also found that pollen viability declines in relation to the number of molecular markers introgressed. Although this relationship is predicted with high levels of introgression, we see evidence of decreased pollen viability with as few as three introgressed molecular markers. Hybrid indices were calculated from morphological data and compared to introgressed marker frequencies and pollen viability. Overall, hybrid indices are poorly correlated with both pollen viability and frequency of molecular marker introgression, suggesting that morphological intermediacy is a poor indicator of hybridity in this system.

Oenothera villaricae Dietrich and Oe. picensis Dietrich, complete translocation heterozygotes, are fully interfertile, giving rise to six discrete classes of true-breeding hybrid from a reciprocal cross. Associated with each parent and hybrid is a characteristic abortive nonstaining pollen fraction easily distinguished from fully developed pollen under the light microscope. Significant reciprocal differences in the abortive fraction between different hybrid accessions suggest partial cytoplasmic control of this phenomenon. Pollen abortion is most severe in the [F.sub.1] hybrid generation, and ameliorates with successive generations of hybrid self-fertilization. Analysis of variance suggests a combined nucleocytoplasmic basis for the pollen abortion, with significant effects also noted for selfed hybrid generation. Correlated with the observation of increased pollen abortion in Oenothera hybrids are meiotic observations of broken chromosome rings (chains, univalents), asymmetric anaphase distributions and trinucleate tetrads. To test the hypothesis that the pollen abortion is due to abnormal meiosis, meiotic disjunction frequency was quantified for each parent, [F.sub.1] and [F.sub.9] selfed hybrid accessions. Analysis of variance shows levels of significance comparable to those noted for pollen stainability for effects of nucleus, cytoplasm and selfed hybrid generation on disjunction frequency. A high degree of correlation ([r.sup.2] = 0.984) is noted between disjunction frequency and pollen stainability. We conclude that the abortive pollen grains are indeed the products of nondisjunctional meiotic events, which themselves are consequences of genome-plastome incompatibility.

Populations of Clarkia tembloriensis have been documented to vary with respect to pollen tube growth rates. Previous investigations have demonstrated that pollen grows more rapidly in an outcrossing population (Idria-2) than in a selling population (Cantua Creek-2). In the present study a series of pollinations were made between plants from the two populations and their hybrids to determine whether differences in pollen tube growth rates and numbers of tubes in the style were due to pollen effects, pistil effects, or both. Reciprocal crosses of all combinations were made. For one experiment flowers were pollinated and their stigma and style samples were removed at 2 hour intervals for 12 hours and at 24 hours post pollination. Fluorescence microscopy was used to determine the number of pollen tubes at the stigma/style junction, 1 cm below the stigma, and at the base of the style. In this experiment Idria-2 had significantly more pollen tubes in all positions of the style than Cantua Creek-2 styles and this trend was maintained through time (3-way ANOVA). Pollen tube numbers in hybrids were intermediate to, and significantly different from Idria-2 and Cantua Creek-2. The source of pollen used did not have an effect on pollen tube numbers in the style except at the stigma/style junction (3-way ANOVA). In a second experiment pollen tube growth rates were determined by measuring the time taken for pollen tubes to germinate, grow the length of the style, and emerge from the cut ends of detached styles. In crosses of pollen and style from the same population, hybrid pollen grew slightly but not significantly faster (0.2902 cm/h) than Idria-2 pollen (0.2804 cm/h) which grew significantly faster than Cantua Creek-2 pollen (0.2378 cm/h). In crosses of pollen and styles from different populations, all pollen ,types tended to grow faster through hybrid styles (0.2845 cm/h), slightly slower through Idria-2 sales (0.2739 cm/h) and significantly slower through Cantua Creek-2 styles (0.2181 cm/h). However. there were slight and significant effects due to pollen type. Our study suggests that the pistil of Clarkia tembloriensis has the strongest influence on pollen tube numbers and growth through the style. However, pollen source also has a small but significant effect on pollen tube growth rates and numbers especially in the upper part of the style.

The extent of pollen limitation in natural populations is still problematic for many herbaceous plants. Burd (1994) demonstrated from survey data that pollen limitation was found in 62% of plant species; however, none of the studies in his survey investigated pollen limitation in a large sample of populations from different habitat types. Trillium grandiflorum is a bee-pollinated, herbaceous perennial common in a variety of habitats in southern Ontario. Controlled hand-pollinations demonstrated that the species is self-incompatible. Comparisons of seed set from open and supplemental hand-pollinations in 18 populations from large woodlands, small woodlands and disturbed sites (roadsides and hedgerows) were undertaken to evaluate the intensity of pollen limitation among populations and habitat types. There was significant variation in the intensity of pollen limitation among populations. However, the extent of pollen limitation was not significantly associated with habitat type, presumably because generalist pollinators that visit T. grandiflorum occur in all three habitat types. Evidence was obtained that T. grandiflorum plants occurring at low density were more susceptible to pollen limitation than those occurring at high density.

SUPERMAN (FLO-10) is a floral mutant in Arabidopsis thaliana that normally produces female sterile flowers. This phenotypic aberration results from a combination of increased stamen number and reduced or abnormal carpels that are non-functional. The flowers of SUPERMAN contain two lateral and four median stamens, as seen in wild-type plants; however, they also contain several additional stamens. The amount of pollen produced from each of the three types of anthers of SUPERMAN and the viability of this pollen was examined and compared to wild-type (cv. Columbia) to determine the consequences of this mutation on male fertility. Both the lateral and the median anthers of SUPERMAN and wild-type plants produced similar amounts of pollen per anther and demonstrated no significant difference in viability. Per anther, the additionals of SUPERMAN produced significantly less pollen than those of the laterals and medians. Furthermore, the pollen produced from these additional anthers was significantly less viable. Currently we are testing the fertility of pollen produced from the various anther types and the germination of the resulting seed.