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 southeastern United States, possesses a suite of unusual characters that represent 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. perennis with seeds of a typical wind-dispersed milkweed, A. exaltata. Seeds of A. perennis floated significantly longer in both greenhouse and field experiments, with 15% and 35% still floating after 5 months; almost all seeds of A. exaltata sank within 2 weeks. A second greenhouse experiment conducted under three different soil-water regimes revealed significant differences in percent germination between species (A. perennis = 85.3 +/- 0.1%: mean +/- standard error; A. exaltata = 6.2 +/- 6.2%), and, to a lesser degree, among soil-water treatments within species. Seed germination tests revealed no significant change in viability for either species in any of the experiments conducted. Milkweed seeds can remain viable in water for long periods of time, but the remarkable ability of A. perennis to remain floating for more than 6 months may provide a ''floating seed bank'' that contributes to high levels of gene flow among populations of this floodplain species.