SEED BUOYANCY AND VIABILITY OF THE WETLAND MILKWEED ASCLEPIAS-PERENNIS AND AN UPLAND MILKWEED, ASCLEPIAS-EXALTATA

Asclepias perennis, a milkweed restricted to wetlands in the southeast ern United States, possesses a suite of unusual characters that repres ent apparent adaptations for water dispersal of seeds: the fruits are oriented downward at maturity and contain seeds with an expanded seed coat that lack the usual coma of milkweeds. Using concurrent field and greenhouse seed flotation experiments, we compared seeds of A. perenn is with seeds of a typical wind-dispersed milkweed, A. exaltata. Seeds of A. perennis floated significantly longer in both greenhouse and fi eld experiments, with 15% and 35% still floating after 5 months; almos t all seeds of A. exaltata sank within 2 weeks. A second greenhouse ex periment conducted under three different soil-water regimes revealed s ignificant differences in percent germination between species (A. pere nnis = 85.3 +/- 0.1%: mean +/- standard error; A. exaltata = 6.2 +/- 6 .2%), and, to a lesser degree, among soil-water treatments within spec ies. Seed germination tests revealed no significant change in viabilit y for either species in any of the experiments conducted. Milkweed see ds can remain viable in water for long periods of time, but the remark able ability of A. perennis to remain floating for more than 6 months may provide a ''floating seed bank'' that contributes to high levels o f gene flow among populations of this floodplain species.