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Sedges of the genus Carex L. are mostly wind-pollinated, self-compatible, monoecious herbs in which female flowers are receptive before pollen is released (protogyny). One hypothesis was examined for the adaptive importance of the monoecious sexual system, namely that having two flower types permits flexibility in allocation of resources to male and female reproductive functions in the face of changing environmental conditions. Fourteen greenhouse experiments were conducted on a total of six Carex species in which I manipulated one or more of three environmental variables: nutrients, water, and light. These variables had small effects on the proportion of female flowers, though the effects were significant for at least one variable in 8 of the 14 experiments. High nutrient and light levels were generally associated with increased femaleness, while high water levels were associated with increased maleness. Increased female allocation in response to enhanced resource availability has been demonstrated in many species and is thought to reflect the greater potential fitness gains via the female function than via the male function when resources are plentiful. The opposite effects of water are anomalous and contrary to results in the literature. Because the effects of environmental manipulations on sex allocation are modest, the evolution and maintenance of monoecy in sedges probably reflect advantages other than flexible sex expression, at least in part. Selection for avoidance of self-pollination may have been particularly important in favoring the combination of protogyny and monoecy in Carex.

We investigated the sexual system, floral biology, sporogenesis, and gametogenesis of Tetradium daniellii (Bennett) Hartley, which possessed distinct male and monoecious sexual phenotypes. Male individuals produced almost exclusively male flowers. In the years 2000–2002, the protandrous monoecious individuals had male and female flowering phases completely separated in time within and between the plants, while in 2003 and 2004, slight overlap of male and female functions occurred. During early floral development, all flowers were hermaphroditic. Dimorphic unisexual flowers resulted from later selective abortion of the gynoecium or androecium. Male flowers and their pollen in the two sexual morphs did not significantly differ in structure. Pollen grains from male flowers on both male and monoecious plants were fertile and could germinate on the stigmas of female flowers on monoecious plants. Maturation of female gametophytes in monoecious plants coincided with the release of the three‐celled pollen in the ...

We obtained information on dichogamy and other aspects of the biology of over 4200 species of angiosperms from several hundred published and unpublished sources. We used this information to describe patterns of occurrence of dichogamy and to test specific hypotheses relating dichogamy to other characteristics of plants or their environments. Protandry was more common than protogyny at the intrafloral level, but the reverse was true at the interfloral level. Patterns of dichogamy varied significantly among major taxa, with protogyny more common among monocotyledons and primitive dicotyledons, and protandry expecially common in the Asteridae. Arctic species tended to be less dichogamous and more protogynous than temperate and tropical species. Aquatic and alpine species were especially protogynous. Patterns of dichogamy varied among sexual systems, with gynomonoecious and gynodioecious species especially protandrous, and monoecious species highly protogynous. Autogamous and self-compatible species were disproportionately protogynous. Flowers of intraflorally dichogamous species were slightly larger than those of adichogamous species, owing to the presence of many autogamous species in the latter group. Species with interfloral protogyny bore much smaller flowers than did species with interfloral protandry. Early-blooming species in north-temperate and polar regions were disproportionately protogynous. Sexual structures that abscised, shriveled or moved after completion of their function tended to be presented first, and those that facilitated the other sexual function were presented second. A negative association existed between type of intrafloral and interfloral dichogamy in diclinous species. Most animal-pollinated flowers were protandrous, except beetle-pollinated and refuge and trap blossoms. Wind pollination was markedly associated with protogyny. Vertical inflorescences visited by upwardly-moving vectors were protandrous. Our results suggest that three primary factors may be involved in promoting dichogamy: selection for avoidance of pollen-pistil interference, selection for avoidance of self-fertilization, and selection for synchrony of pollen discharge and stigma receptivity in the different flower types of diclinous species. In contrast to many earlier workers we reject the thesis that avoidance of self-fertilization is the universal or even the most important force in the evolution of most forms of dichogamy. We attribute the prevalence of intrafloral protandry to selection for avoiding interference between pollen export and pollen receipt. Intrafloral protogyny was associated with imprecise pollen transfer, where other means of avoiding pollen-pistil interference (e.g., herkogamy) are likely to be of limited value. The prevalence of interfloral protogyny seems to reflect the smaller size of unisexual flowers than bisexual flowers, the absence of intrafloral pollen-pistil interference in diclinous species, and selection for synchrony of pollen discharge from one flower type with stigma receptivity in the other.

We investigated paternal effects on seedling performance in relation to patterns of fruit abortion in Campsis radicans using a series of 14 experiments with various combinations of pollen donors and recipients. In eight experiments significant paternal effects on three or four dependent performance variables were detected by multivariate ANCOVA. These effects were independent of seed weight, which was controlled in the experimental design and in the statistical analysis. In six of the eight experiments with significant results the favored pollen recipient sired the more vigorous offspring. In general, therefore, patterns of fruit abortion with respect to outcross donor enhance progeny fitness. Maternal discrimination is reduced, however, when pollen from several outcross donors is deposited on the same stigma, a likely occurrence in nature. Discriminntion among pollen from different outcross donors is also less important than the ability to discriminate between self- and cross-pollen because the latter di...

I examined effects of pollination intensity on fruit, seed and seedling characteristics in Campsis radicans, and joint effects of pollen donor and pollination intensity on fruit production. Large pollen loads were more likely to initiate fruit production than small pollen loads, and the former fruits contained more seeds and a greater total seed mass. No further increases in seed number or mass occurred for pollen loads above 4,000 grains. The weight of individual seeds was unaffected by pollen load. Effects of pollen donor were generally larger than effects of pollen load, and fruit production from small loads of pollen from one donor were sometimes equal to fruit production from larger pollen loads from another donor. The ratio of pollen grains deposited to resultant seeds increased with pollen load, and several explanations are proposed. Seeds from heavy pollinations emerged better than seeds from light pollinations, but did not differ in speed of germination or in the performance of seedlings up to 126 days. The emergence differences are probably due to differing intensities of pollen tube competition.

We examined spatial and temporal patterns of pollen receipt, pollen removal and maternal reproductive success in Geranium maculatum. These three variables differed among sites and sampling dates. Sites with the densest tree canopy had the lowest pollen receipt and fruit production. Differences between years were accompanied by different climatic conditions and may have been caused by different levels of pollinator activity. Within blooming seasons, fruit production and pollen receipt declined at all sites with date, accompanying canopy closure. Pollen removal varied with date, but not in parallel with pollen receipt, indicating that pollen receipt and deposition were influenced, in part, by different factors. Of three weather variables examined, ambient temperature was significantly positively correlated with rate of floral development, but not consistently with pollen receipt, pollen removal or fruit production. Pollen receipt was inversely related to cloud cover, presumably due to effects of the latter on pollinator activity. The earliest-blooming plants had the highest reproductive success.

I examined the relative success of pollen donors in siring viable seeds following mixed (two-donor) hand pollinations of the woody climber, Campsis radicans. Paternity was assigned using an electrophoretic marker. When pollen from two donor plants was mixed and applied to stigmas, the numbers of viable seeds sired by the two donors were generally unequal. The relative success of the two donors in a mixture was correlated with the relative success of the two donors when their pollen was applied in single-donor loads to different stigmas on the same plant. However, performances of two donors were generally more equal when their pollen was applied in mixed loads than when applied in singledonor loads. While single-donor pollinations are often used to assess donor performance in studies of plant paternity, the above results suggest caution in extrapolating from success in single-donor pollinations to success in nature, where mixed pollinations are probably the rule. When ratios of pollen from two donors in a mixture changed, the success of the two donors changed roughly in proportion. Thus there is no basis for challenging the frequent assumption in gender allocation models that changes in pollen production produce proportional changes in male success. Relative performance of pollens in a mixture was unaffected by the total number of pollen grains deposited. Several lines of evidence suggest that postzygotic phenomena (e.g., seed abortion) are important in determining the success of different pollen donors in Campsis.

The tendency in Campsis radicans for fruits to be produced from certain crosses was consistent between years. This increases the likelihood that observed patterns are genetically based. Early in a season, plants were more selective about which donor they accepted pollen from, as measured by the parentage of mature fruits. Field experiments indicated that this increase in selectivity was related not to date, but to the number of prior pollinations and developing fruits in an inflorescence, and therefore presumably to the availability of resources. These differences in fruiting indicate a non-male component to selectivity (since pollen source was controlled). That the acceptability of pollen from particular donors depends on the resource status of the inflorescence means that the distinction between pollen limitation and resource limitation of fruit production is not a sharp one. A model is proposed to account for patterns of fruit production with respect to paternity in Campsis radicans.

Campsis radicans is almost completely self-sterile following pollinations of pure self-pollen, but when self- and cross-pollen are present in similar amounts, ca. one in six viable seeds is sired by self-pollen. The amount of selfing depends in part on the identity of the cross-pollen donor in the pollen mixture. Selfed seeds are smaller on average than outcrossed seeds. Seed sizes in fruits from mixed (self + cross) pollinations are more variable than those from cross-pollinations, with a larger number of small, often inviable seeds. Most of these small seeds are presumed to be products of selfing. Selfed seeds do not occur in particular positions along the length of the fruit. After 6-7 wk seedling growth, outcrossed seedlings were taller and heavier than selfed seedlings. Selection for the avoidance of selfing is likely to have been important in the evolution of protandry in Campsis and perhaps in the large "overproduction" of flowers, which permits selective fruit abortion. We discuss the possible rol...

The effect of self pollen on the success of cross pollinations was studied in Campsis radicans (L.) Seem., a species previously considered self-sterile. The application of self pollen to stigmas in combination with or preceding the application of cross pollen markedly reduced the likelihood of fruit production. This effect was not due to the dilution of cross pollen nor to physical blocking of the stigmatic surface, since mixtures of talc and cross pollen were as effective as pure cross pollen in causing fruit production. Pollen tubes produced by self pollen grew at rates similar to those from cross pollen, and penetrated the ovary. The interference caused by self pollen appears to take place in the ovary, although it cannot be stated to what degree it is prezygotic or postzygotic. The marked protandry occurring in C. radicans is presumed to be due at least partly to selection for avoidance of these negative effects on female reproductive success. Up to 33% of viable seeds from mixed self + cross pollinations were sired by self pollen. The term "cryptic self-fertility" is coined to describe this phenomenon where pollination with loads of pure self pollen rarely or never yields fruit, but pollination using mixtures of self and cross pollen yields fruit containing considerable numbers of selfed seed. Estimates of selfing frequency that are based on pollination using loads of purely self pollen will be in error for species possessing cryptic self-fertility. SEVERAL REPRODUCTIVE traits of seed plants cause a separation of male and female sexual functions. Such traits include dichogamy, herkogamy, and breeding systems such as monoecy and dioecy. One possible advantage of such traits is that they reduce inbreeding, a reduction that would be favored in species wherein inbreeding depression yields offspring of low fitness. However, dichogamy, herkogamy and monoecy all occur in plant species having physiological self-incompatibility (Lloyd and Webb, 1986), indicating that prevention of inbreeding depression is not a complete explanation of these traits. An alternative explanation is that such traits are beneficial in reducing mutual interference of male and female reproductive functions, caused by self-pollination. Self-pollination could decrease female success even in a self' Received for publication 19 May 1987; revision accepted 4 November 1987. We thank C. Barnes, P. L. Angermeier, M. N. Melampy, and L. Wolfe for assistance with field work, and M. F. Willson for accommodations and lab space in Illinois. S. I. Guttman kindly allowed the use of his laboratory and provided advice concerning electrophoresis. G. J. Anderson, C. T. Philbrick and M. L. Ledbetter graciously allowed use of fluorescence microscopes and/or provided advice on the visualization of pollen tubes. S. C. H. Barrett, K. S. Bawa, R. N. Bowman, B. B. Casper, D. G. Lloyd and an anonymous reviewer kindly commented on earlier versions of this paper. This research was supported by the National Science Foundation (DEB 8206465) and by a Batchelor (Ford) summer fellowship to RIB. incompatible plant if self pollen on the stigma interferes with the deposition or performance of compatible pollen (discussion in Shore and Barrett, 1984). Self pollination would also decrease male success ofa self-incompatible plant because self-deposited pollen is unavailable for transport to other, compatible plants. The potential importance of selection for avoidance of pollen-stigma interference in the evolution of monoecy and dioecy has been noted by Bawa and Opler (1975), Zapata and Arroyo (1978), Bawa (1980) and Bawa and Beach (1981). Others, including Yeo (1975) and Lloyd and Yates (1982), have suggested that reduction of pollen-stigma interference could be important in the evolution of heterostyly. Most recently Lloyd and Webb (1986) and Webb and Lloyd (1 986) have argued for a central role of pollen-stigma interference in the evolution of dichogamy and herkogamy. Despite this growing theoretical literature, empirical studies of pollen-stigma interference are few. Crowe (1971) demonstrated interference of incompatible pollen with the performance of compatible pollen in Borago officinalis. Flowers receiving compatible pollen on one stigmatic lobe and incompatible on the other produced fewer than 5% as many seeds as flowers receiving compatible pollen on both lobes. In the distylous species Turnera ulmifolia, however, Shore and Barrett (1984) found a deleterious effect of self pollen on the performance of cross pollen only when a large (five

Trumpet creeper is self-incompatible and bears long, tubular, orange flowers from June to September. Flowering peaks rapidly, then declines and continues at low levels for several weeks. The initial burst of flowering may attract pollinators that return even during subsequent reduced flowering. Most flowers open before noon and nectar production totals 110 ,l of 26% sucrose equivalents per flower, an exceptionally high production for a temperate zone plant. Production ceases within 20-30 hr of flower opening, but corollas persist for several days and may serve to attract pollinators. Effective pollination reduces the period of stigma receptivity and speeds closing of stigma lobes. Only 1-9Wo of flowers produced mature fruits at four sites in Illinois and Missouri. Roughly 400 pollen grains had to be deposited on a receptive stigma to cause fruit development beyond an initial period of high abortion. At two sites, 17% and 89% of stigmas received over 400 pollen grains. Assuming 50% of deposited grains were from the same plant, fruit production at one site was clearly pollinator limited, that at the second site may have been. Ruby-throated Hummingbirds (Archilochus colubris) deposited ten times as much pollen per stigma per visit as honeybees (Apis mellifera) and bumblebees (Bombus spp.). Fruit set was highest where rubythroat visitation was most frequent. Trumpet creeper appears primarily adapted for hummingbird pollination, but can also be adequately pollinated by honeybees and bumblebees. This is one of the first attempts to relate pollen-depositing capabilities of pollinators of any plant to pollen requirements for fruit production. Several characteristics suggest that trumpet creeper may be adapted to pollination at low densities (often called traplining) in its presumed original, woodland, habitat.

The effect of paternity on the likelihood of fruit production was determined by making 2,486 hand pollinations among 11 stems (9 individuals) of Campsis radicans. All individuals matured significantly different numbers of fruit from different pollen donors. Individuals that were favored as donors by one recipient were not necessarily those favored by other recipients. Selectivity increased slightly as more pollinations were made, and pollination by nonfavored donors was more successful in producing fruit during the first half of the pollination sequence than in the latter half. The pollen donors favored by particular recipients were usually those whose pollinations resulted in fruit with relatively many and large seeds. Fruit abortion seems to have been more important in donor selectivity than prezygotic phenomena, although parallels with multifactorial gametophytic incompatibility are evident. I suggest this selectivity by a pollen recipient with respect to pollen donors is an integrated response increas...

I examined the effects of pollen loads containing pollen from one, three and five donors on fruit production and fruit quality in Campsis radicans. Number of pollen donors had no significant effect on % fruit production, seed number, seed weight or seed germination. In singledonor pollinations the identity of the donor did have a strong effect on the above parameters. Furthermore, the best single donor sired fruits with more seeds and heavier seeds than any mixture containing this donor. This pattern indicates interference of pollens or preemption of some ovules by the inferior pollen. In Campsis, therefore, the number of pollen donors contributing to a pollen load is less important than the identity of these donors in determining fruit production and fruit quality. Seeds from fruits resulting from mixed pollination were slightly more variable than seeds from fruits resulting from single-donor pollinations.