Robert William Cruden

Pollen grains: Why so many?

My objective is the examination of selective forces that affect pollen number. Relationships among other floral traits of animalpollinated plants, including pollen size, stigma area and depth, and the pollen-bearing area of the pollinator may affect pollen number and also provide a model to examine how change in one trait may elicit change in other traits. The model provides a conceptual framework for appreciating intra- and inter-specific differences in these traits. An equivalent model is presented for wind-pollinated plants. For these plants the distance between putative mates may be the most important factor affecting pollen number. I briefly consider how many pollen grains must reach a stigma to assure fruit set. I use pollen-ovule ratios (P/Os) to examine how breeding system, sexual system, pollen vector, and dispersal unit influence pollen grain number. I also compare the P/Os of plants with primary and secondary pollen presentation and those that provide only pollen as a reward with those that provide nectar as part or all of the reward. There is a substantial decrease in P/O from xenogamy to facultative xenogamy to autogamy. Relative to homoecious species the P/Os of species with most other sexual systems are higher. This suggests that there is a cost associated with changes in sexual system. The P/Os of wind-pollinated plants are substantially higher than those of animal-pollinated plants, and the available data suggest there is little difference in the pollination efficiency of the various animal vectors. The P/Os of plants whose pollen is dispersed in tetrads, polyads, or pollinia are substantially lower than those of species whose pollen is dispersed as monads. There was no difference in the P/Os of plants with primary and secondary pollen presentation. The P/Os of plants that provide only pollen as a reward were higher than those that provide nectar as a reward. All of these conclusions merit additional testing as they are based on samples that are relatively small and/or systematically biased.

Pollinators in High-Elevation Ecosystems: Relative Effectiveness of Birds and Bees

During the rainy season bird-flowered plants at high elevations are more effectively pollinated than closely related bee-flowered plants. With good flight conditions the effectiveness of birds and bees is essentially equal. Thus, the higher incidence of bird flowers at higher elevations is attributable in part to the competitive advantage gained through greater reproductive success.

Patterns of biomass allocation to male and female functions in plants with different mating systems

SummaryUsing dry weight biomass we examined the patterns of investment in male and female functions (prezygotic cost) in plants with different mating systems. All the flower parts of both xenogamous and facultatively xenogamous species were heavier, i.e., larger, than those of facultatively autogamous species. Likewise, the dry weights of all the flower parts of xenogamous species exceeded those of facultatively xenogamous species. On a relative basis, xenogamous species invested less in calyces and more in corollas compared to species with the other mating systems. Facultatively autogamous species invested relatively more in pistils. Xenogamous species invested relatively more in stamens than do facultatively autogamous species. The ratios of dry weight stamens to dry weight pistils were equivalent in xenogamous and facultatively xenogamous species.The available data from xenogamous species suggest a pattern of resource allocation that is independent of sexual system (perfect-flowered, monoecious, or dioecious) and pollen vector. The cost of mating (prezygotic cost) was male biased and frequently exceeded by parental investment (postzygotic cost). These results are not consistent with models that predict equal allocation of resources to male and female sexual function but are consistent with those that predict unequal allocation of resources to those functions in outbreeding hermaphroditic angiosperms. Two additional lines of evidence are inconsistent with the expectations of sex allocation theory. First, resource allocation to sexual function was not equal in wind-pollinated species. Second, relative amounts of the resources allocated to male vis-à-vis female function did not decrease between xenogamy and facultative xenogamy i.e., with an increase in the selfing rate.

Temporal Dioecism: Systematic Breadth, Associated Traits, and Temporal Patterns

Temporal dioecism, the temporal alternation of pistillate and staminate functions within a plant, is a relatively common mating system among flowering plants. It occurs in at least 37 angiosperm families and is common in Apiaceae, Araceae, Arecaceae, Juglandaceae, Sparganiaceae, Typhaceae, and hermaphroditic species in Aceraceae and Lauraceae. It is more common in the primitive subclasses, e.g., Magnoliidae, Hamamelidae, and Alismatidae, and less common in the more advanced subclasses, e.g., Asteridae and Liliidae. Because it occurs in more families than does heterostyly (37 vs. 24) and relatively few genera have been studied in those families whose mating systems are characterized by temporal dioecism, I suggest that it is more common than heterostyly. Temporal dioecism is strongly associated with monoecism, self-compatibility, many simultaneously receptive stigmas, and less efficient pollen vectors, e.g., wind, beetles, and flies. In such species, temporal dioecism should increase pollinator efficiency, minimize geitonogamy, and facilitate xenogamy. Because it occurs primarily in self-compatible species and minimizes self-fertilization, it may be an evolutionary alternative to self-incompatibility. There are numerous temporal phenotypes that include many of the possible combinations of protogyny and/or protandry, sequential and/or synchronized flowering, and single or multiple alternations of sexual phases. The simplest phenotype, which results from the sequential opening of the flowers within an inflorescence in monoecious species, is uncommon. The relative lengths of the pistillate and staminate phases result in sex-phase ratios at the population level. Four general associations involving sex-phase ratios, lengths of the two phases, and pollen vectors are evident: (1) wind pollination and relatively long sexual phases with slightly pistillate- or staminate-biased sex-phase ratios; (2) nonspecialized insect vectors and strongly staminate-biased ratios; (3) specialized vectors (beetles, moths, and wasps), short phases, and unbiased ratios; and (4) generalist insect vectors, short and repeated sexual phases, and unbiased ratios. The evolution of facultatively autogamous and facultatively xenogamous species from temporally dioecious relatives is associated with changes in the temporal phenotype that produce an overlap of sexual function, either within and/or between inflorescences.

Correlations Among Stigma Depth, Style Length, and Pollen Grain Size: Do They Reflect Function or Phylogeny?

There is a positive correlation between stigma depth-our approximation of the distance a pollen tube has to grow to reach exogenous resources in the transmission tissue-and pollen grain size, but not between style length and pollen grain size among six species in Umbelliferae, eight species in Cruciferae, six species in Solanum, and 10 unrelated species whose pollen grains contain starch. The lack of a correlation between pollen volume and style length in comparisons of both related and unrelated species is inconsistent with the hypothesis that pollen grains must contain sufficient resources to sustain the growth of a pollen tube to an ovule. Positive correlations among style length, pollen grain size, and stigma depth in Polygonum suggest that correlations between style length and pollen grain volume among related species may reflect a phyletic rather than a functional relationship.

Oriole pollination ofErythrina breviflora (Leguminosae): Evidence for a polytypic view of ornithophily

Erythrina breviflora is visited by large numbers of passerine birds of which orioles (Icterus: Icteridae) are the primary pollinators. The flowers produce large quantities of nectar but they are rarely visited by hummingbirds. Inflorescence and floral morphology, and low levels of sucrose in the nectar probably explain the rarity of foraging hummingbirds. A comparison of Old WorldErythrina and their pollinators with New World species pollinated by orioles and hummingbirds suggests that parallel evolution has occurred. When the comparison is expanded to include other species pollinated by orioles, it is clear that various New WorldIcteridae, Thraupidae, etc. are ecological equivalents of Old WorldOriolidae, Pycnonotidae, Sturnidae, etc. and that flowers pollinated by these birds have similar characteristics.

The pollination biology and breeding system of Monarda fistulosa (Labiatae)

SummarySuccessful cross-pollination of Monarda fistulosa is the result of a complex interaction among flower opening, the pollen-bearing areas of the pollinators and/or their behavior, and the maturation of the stigmas. The flowers open continuously from 0800–2000 h providing a temporally predictable rich source of nectar and pollen. Recently opened flowers may reduce the ability of bees to discriminate between resource rich and poor patches and encourage systematic foraging within patches. The continuous opening of flowers coupled with protandry also results in some flowers of most capitula being in the staminate and others in the pistillate phase. Autogamy is highly unlikely due to strong protandry and the spatial separation of anthers and stigmas. Geitonogamy, at least that mediated by Bombus is unlikely because the pollen is spread over a relatively large area of the wings, which reduces the likelihood of a stigma contacting just deposited pollen. Because pollen is transferred from the much smaller coxal area of Anthophora and other bees that mistake the stigmas of early pistillate phase flowers for stamens some geitonogamy seems inevitable. However, the delayed receptivity of young stigmas to self-pollen decreases the likelihood of self-pollen germinating on such stigmas. Older stigmas are equally receptive to self- and cross-pollen and the number of pollen grains germinating and pollen tubes reaching the base of the style increases with flower age.

Pollen grain size, stigma depth, and style length: the relationships revisited

I examine data and review information in the literature to test hypotheses proposed by Delpino and Darwin to explain the source of nutrients utilized by pollen tubes. In 1867, Delpino, in his discussion of distyly, suggested that the positive relationship between pollen grain size and style length was based on the pollen grains containing sufficient nutrients to sustain the growth of their pollen tubes through their respective styles. Darwin (The different forms of flowers on plants of the same species, 2nd edn. J. Murray, London, 1884) rejected Delpino’s suggestion based on his examination of distylous species whose morphs produced pollen grains whose sizes were not proportionate to the lengths of their respective styles. Darwin then proposed that pollen tubes first grow autotrophically, i.e., through the stigma, then heterotrophically in the style. This should result in a positive relationship between pollen grain size and stigma depth, if pollen tubes grow autotrophically through the stigma. I examined 15 species in Fabaceae and 20 species in Proteaceae to test the two hypotheses. Pollen grain size was correlated with stigma depth among the Fabaceae, i.e., consistent with Darwin’s hypothesis, and was not correlated with style length in either family, i.e., inconsistent with Delpino’s proposal. Comparisons of related species, in general, were consistent with Darwin’s hypothesis. In addition, information in the literature provided no evidence that pollen tubes obtain resources on or in the stigma, i.e., pollen tube growth from the stigmatic surface to the style was autotrophic. In contrast, pollen tubes obtain an array of resources from the transmission tissue, thus there is little reason for pollen grains to contain those resources. In addition, I suggest that positive correlations between pollen grain size and style/pistil length may be a result of both being correlated with stigma depth.

The systematics of Rigidella (Iridaceae)

Rigidella and the four species contained therein are defined primarily on characters related to, and a direct consequence of, adaptation to hummingbird pollination. The genus is probably derived from either the “multiflora” complex inTigridia subgenusHydrotaenia or is an evolutionary line parallel toTigridia. Illustrations, distribution maps, and a key to the species are included. The rediscovery ofRigidella flammea Lindl. is reported, and a new species,R. inusitata, is described.

Sessilanthera, a new genus of American iridaceae

A new genus is proposed for plants heretofore included in Nemastylis latifolia Weatherby. Sessilanthera is related to Tigridia, Rigidella, and Fosteria in tribe Tigrideae.