PHYSICAL SITE CHARACTERISTICS LIMIT POLLINATION AND FRUIT-SET IN THE DIOECIOUS HYDROPHILOUS SPECIES, VALLISNERIA-AMERICANA

Experimental and observational studies of the submersed, freshwater ma crophyte Vallisneria americana Michx. revealed that depth, wind and wa ve exposure, and current velocity may all influence fruit set. In this dioecious species, long-pedunculate female flowers are pollinated by free-floating male flowers at the water surface. Average fruit set in the natural populations studied varied from zero to 97% of the flowers observed. With increasing water depth in New York and Pennsylvania la kes, female plants continued to flower, though these flowers were unab le to reach the surface, and consequently, did not set fruit. Fruit se t was also lower in relatively open sites exposed to wind and waves, p resumably because male flowers do not remain in the vicinity of female flowers long enough for effective pollination. This was particularly striking at a site with low male flower densities, but fruit set was i ncreased to 100% at that site by confining the floating male flowers w ithin a field enclosure. In a river, fruit set was negatively correlat ed with surface current velocity, and was reduced to zero in current v elocities greater than 0.30 m . s(-1). Fruit set in V. americana appea rs to be restricted or precluded by physical environmental conditions in a variety of sites.