The influence of hygroscopic movement on seed dispersal in Daucus carota (Apiaceae)

The influence of hygroscopic movement on seed dispersal in Daucus carota was examined. When relative humidity increases, umbels containing mature fruits close; when relative humidity drops, umbels open. Tests of the effectiveness of smalland large-angled umbels on dispersing seeds under various conditions demonstrate that umbels responding greatly to relative humidity (i.e. opening wide) lose seeds more quickly than do umbels responding little, and do not disperse them as far. As relative humidity increases, number of dispersing seeds drops to near zero. Response to changes in relative humidity within an umbel progressively increases from late August when dispersal begins. Umbels that slowly increase their response retain some seeds that may disperse over snow in winter. Individual variation in response to relative humidity is high among plants beginning dispersal at the same time. This variability is probably maintained by the variable consequences of dispersing seeds at different times.