POPULATION-DYNAMIC CONSEQUENCES AND EVOLUTION OF SEED TRAITS OF HETEROSPERMA-PINNATUM (ASTERACEAE)

We summarize research on variation in achene morphology of Heterosperm a pinnatum. Each flowering head has a broad range of achene morphologi es. There is no between-year seed bank. There are no ecologically rele vant differences in achene size, seedling size, or seedling growth rat es of the different morphs. Achenes located centrally in the heads los e dormancy earlier during the period between autumn achene production and the onset (May-July) of the rainy season. This results in relative ly more early emergence of central achenes. Awned central achenes also have greater adhesive dispersibility. Emergence, survival, and fecund ity were measured for achenes sown into natural populations in central Mexico. When harsh conditions result in few seeds surviving to reprod uce, the advantage tends to go to the more ''conservative'' peripheral achenes. Greenhouse experiments show a strong genetic component to th e determination of achene proportions within and among populations and a variety of genetic correlations. Populations with a high proportion of awned achenes tend to be found in vegetation types where they are likely to be ephemeral. A high proportion of central achenes tend to b e found in sites with little pre-rainy season precipitation. These pat terns are interpreted in terms of selection for dispersal and the with in-year timing of germination.