Pollen grains: why so many?
My objective is the examination of selective forces that affect pollen number. Relationships among other floral traits of animal-pollinated plants, including pollen size, stigma area and depth, and the pollen-bearing area of the pollinator may affect poll
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Abstract. 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. 1 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 PIOs 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 PIO from xenogamy to facultative xenogamy to autogamy. Relative to homoecious species the PIOs of species with most other sexual systems are higher. This suggests that there is a cost associated with changes in sexual system. The PIOs 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 PIOs of plants whose pollen is dispersed in tetrads, polyads, or poilinia are substantially lower than those of species whose
pollen is dispersed as monads. There was no difference in the PlOs of plants with primary and secondary pollen presentation. The PIOs 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 . Key words: Animal-pollination, breeding systems, duration stigma receptivity , pollen grain number, pollen grain size, pollen packaging units , pollenovule ratios, secondary pollen presentation, sexual systems, stigma area and depth, wind-pollination.
In a series of papers my colleagues and I proposed that pollen-ovule ratios (P/Os) reflected pollination efficiency (= total source efficiency of Inouye et al. 1994), i.e. the likelihood of a pollen grain reaching a stigma . That conclusion was based on the differences in P/Os among species with different breeding systems (Cruden 1977, Cruden and Lyon 1989) and differences between species whose pollen is dispersed as monads vis-a-vis those whose pollen is dispersed in polyads or pollinia (Cruden 1977, Cruden 1997, also see Cruden and Jensen 1979). Subsequently, we showed that stigma area relative to the pollen bearing area of the pollinator was negatively correlated with P/O (Cruden and Miller-Ward 1981). In essence, we provided functional expla
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