EFFECTS OF VARIATION IN FLORAL MORPHOLOGY ON POLLINATION MECHANISMS IN ASCLEPIAS TUBEROSA L., BUTTERFLYWEED (ASCLEPIADACEAE)

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.