THE REPRODUCTIVE BIOLOGY OF ASCLEPIAS TUBEROSA: I. FLOWER NUMBER, ARRANGEMENT, AND FRUIT‐SET

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Abstract
SUMMARY: An analysis of the inflorescence of the herbaceous perennial milkweed Asclepias tuberosa was carried out by sampling populations in North Carolina and Virginia in 1975 and 1976. Reproductive characters are strongly associated within a given level of comparison (e.g. stems per plant with flowers per plant) but show relative independence between levels (e.g. stems per plant with umbels per branch). Plant size is negatively correlated with fruit production on a proportional basis (fruit‐set) but positively related to the absolute numbers of fruits produced. The invocation of a ‘pollen donor strategy’ to explain the occurrence of large inflorescence displays in Asclepias is unnecessary, as these large plants have a larger female contribution to the next generation, irrespective of their male contribution. Self‐pollination is proposed as a process determining the upper limit of inflorescence size in Asclepias. Experimental pollinations suggest that early competition among ovaries within umbels is very important in determining levels of fruit‐set. Competition among ovaries in different umbels, time of pollination, and location of umbels also exert an effect. Inflorescence architecture likely represents a series of evolutionary compromises between maximization of pollinator attractiveness on the one hand and maximization of fruit maturation on the other.

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