Dispersal of Amazonian trees: Hydrochory in Swartzia polyphylla

Hydrochory was investigated in the seeds of the Amazonian floodplain tree, Swartzia polyphylla, in which pods open on the tree to release one large se ed. Seeds collected from beach drift along the Rio Negro showed a high perc entage of floaters (82%). Yet most seeds sank following collection from: ad ult trees (89%), unflooded ground under adults (96%), and flooded ground un der adults (86%). The specific gravity of the seeds was near that of water, 1.04 +/- 0.03 for sinkers and 0.98 +/- 0.02 for floaters. The ability to f loat was correlated directly with the volume of the air pocket between the two cotyledons, which varied from 5.6 to 20.5 percent of the total seed vol ume. In a long-term floatation test lasting 81 days, 45 percent of the seed s never floated, 33 percent always floated, and 22 percent first sank for o ne week and then floated for at least one month. Seeds that never floated e ventually rotted, but not until days 63-73. Seeds that were floating at day 81, regardless of how long they had been floating, were placed on moistene d tilter paper for 18 days during which time 36 percent germinated, 45 perc ent rotted, and 19 percent did neither but remained viable. These results s uggest that S. polyphylla achieves dimorphism in flotation of its seeds, so me sinking and some floating, by producing seeds of continuous variation in specific gravity around a mean close to 1.00. Seeds that float can be disp ersed long distances along river margins, while those that sink may be move d only marginally from the parent tree.