Effects of Variation in Floral Morphology on Pollination Mechanisms in Asclepias tuberosa L., Butterflyweed (Asclepiadaceae)

Abstract

Field observations revealed the presence of fruit-producing (fruiting) and non-fruit-producing (non-fruiting) plants in two south central Oklahoma populations of Asclepias tuberosa L. Comparative measurements of floral characteristics between fruiting and non-fruiting plants indicated that fruiting plants have significantly larger alar fissure widths and a greater percentage of intact pollinaria. The smaller alar fissure width on flowers of non-fruiting plants apparently reduces the probability of successful pollinia insertions in these plants. Greater numbers of observed pollinia insertions into fruiting-plant fissures appear to be the result of the increased likelihood that pollinia, which are significantly narrower than the fissures of fruiting plants, will be inserted into fruiting-plant fissures than those of non-fruiting plants. Apparently non-fruiting plants act primarily as pollinia donors while fruiting plants act primarily as pollinia recipients. These characteristics of butterflyweed tend to promote a dimorphic, allogamous type of pollination.