Effects of Parent Plant Density on the Production of Achene Types in the Annual Hypochoeris Glabra

Abstract

The annual plant H. glabra produces 2 distinctly different achene types, beaked and unbeaked. These types are produced simultaneously on the same receptacle but the number of each type and the proportions vary. At low parent-plant density a greater proportion of the beaked achene type is produced than at high densities due to changes in receptacle size. Plant weight and scape height are reduced at high parent-plant density. The biggest difference between the 2 achene types appears to be in dispersal potential. The beaked achene, falling through air, has a lower terminal velocity and is more likely to be dispersed further by wind than the heavier unbeaked achene. The structure of the unbeaked achene helps it to adhere to animals. Achenes produced by the shorter, high density, plants are probably dispersed to a smaller distance than are those produced on taller, low density, plants. In these conditions it would be advantageous to have a higher proportion of the animal-dispersed, unbeaked achenes.