Heterosperman pinnatum is an annual composite that produces heteromorphic achenes, with the proportion of achene morphs varying geographically. Field, greenhouse, and laboratory experiments were used to examine the ecological consequences of the heteromorphism. Achenes were classified into three types based on morphology. Central, intermediate, and peripheral morphs, named for the positions occupied within the fruiting head, range in shape from long, thin—beaked achene with barbed awns (central) to short wide achenes without beaks or awns (peripheral). Considerable within— and between—population phenotypic variation in achene and head characteristics was documented. Field experiments using artificial dispersal agents showed that central achenes are the most likely to adhere to animals and that the proportion of a population's achenes presented for dispersal that adhere depends on the proportion of central achenes produced in the population and on the proportion of achenes with adhesive awns. Once achenes had reached the ground, mean and median secondary dispersal distances were short (@<20 cm). Key words: Compositae; Heterosperma; Mexico; reproductive biology; seed bank; seed dispersal; seed dormancy; seed heteromorphism. Schlagworte: Compositae; Heterosperma; Mexiko; Reproduktionsbiologie; Samenbank; Samenausbreitung; Samenruhe; Seed Heteromorphismus.
