POLLINATION SYSTEMS IN PASPALUM DILATATUM POIR. (POACEAE): AN EXAMPLE OF INSECT POLLINATION IN A TEMPERATE GRASS

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.