The pollination of Paspalum dilatatum was studied in south‐central Oklahoma during the summer of 1979. Pollen was liberated between 0700–0900 hr except on humid mornings (RH ≤ 80%), when there was a delay of 2–3 hr and a reduction in the total air‐borne pollen concentration. A rapid decrease in air‐borne pollen concentration with distance from the source results from: 1) individual pollen grains larger (50–70 μ in diam) than typical wind‐pollinated plants, and 2) some pollen dispersed as clusters of grains. Several floral characters of P. dilatatum led to a hypothesis that this perennial grass may be entomophilous as well as anemophilous. Not only were the pollen grains larger than most other wind‐pollinated taxa but the species produced fewer than 2,500 pollen grains per ovule. Three additional observations corroborate this view: 1) solitary bees (Halictidae) actively collect pollen during the morning, 2) the number of pollen grains per stigma was significantly (P < 0.001) greater on racemes exposed to both wind and bees than on racemes exposed only to wind, and 3) the combination of bees and wind as pollinators significantly (P < 0.001) increased seed set compared to wind alone.

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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.