Abstract
Inter- and intra-specific variations in seed dispersal in the genus Picris (Compositae) are reported and discussed in relation to mathematical theories of evolutionarily stable dispersal “strategies”. Two main trends are found: (1) seed dispersability is generally reduced in semi-arid and arid regions (compared with more mesic regions) by achene dimorphism in which marginal achenes are larger, lack a pappus, and are retained on the mother plant in unified involucral bracts; (2) dispersability is higher in arid than in semiarid regions, due to an increased frequency of wind-dispersal achenes in species restricted to desert washes. The latter trend is interpreted, on the basis of the mathematical theories, as an adaptation to the greater crowding and spatial variability of water availability in desert washes.
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