ANTHECOLOGY OF SCHRANKIA-NUTTALLII (MIMOSACEAE) ON THE TALLGRASS PRAIRIE

Schrankia nuttalii flowers through late spring on the tallgrass prairie. Although each stem produces an average of 26 capitate inflorescences only 12% of those inflorescences will open each day to disperse and receive polyads. Each inflorescence may live up to 48 hours but anthers abscise by late afternoon on the first day and the filaments change color and lose their scent. The 78-93 florets comprising each inflorescence open synchronously before dawn or during early morning hours. First day inflorescences of S. nuttallii are herkogamous and fragrant. They are nectarless. Bombyliid flies and male bees are infrequent floral foragers so the major pollinators include female bees representing five families;Anthophoridae, Apidae, Colletidae, Halictidae, and Megachilidae. All foraging insects ignore second day inflorescences although stigmas are still receptive. Although 97% of all bees collected on S. nuttallii carry Schrankia polyads in their scopae or corbiculae 59% also carry the pollen/pollinaria of one or more coblooming angiosperms. At least 98% of all bees carrying mixed pollen loads incorporate the pollen/pollinaria of one or more nectariferous taxa (e.g. Asclepias spp., Asteraceae, Convolvulaceae, Delphinium spec., etc.). Species of halictid bees are more likely to carry pure loads of S. nuttallii polyads (70%) than bees of the four remaining families. Due to the nectarless florets and high degree of polylectic foraging bee-pollination in S. nuttallii converges more closely with the pollination systems of some Australian Acacia spp. than with most other xeric/tropical genera of mimosoids studied in the western hemisphere. © 1990 Springer-Verlag.