ACHENE DIMORPHISM AND AMONG-POPULATION VARIATION IN CREPIS-SANCTA (ASTERACEAE)

Crepis sancta is an annual composite that colonizes abandoned agricult ural land and shows achene dimorphism. The achenes on the periphery of the capitulum are heavy (0.27 mg) and are without a pappus, whereas t hose in the center are light (0.10 mg), more numerous, and bear a papp us. Several studies of other species of Asteraceae have shown that the se morphological differentiations are accompanied by biological differ ences, but few studies have tested whether these differences are const ant between populations. We compared the germination, growth, phenolog y, and reproductive effort of plants derived from these two types of a chenes from three populations that had been established for 3, 13, and 36 yr. The peripheral achenes from the young population germinated fa stest and gave rise to the most vigorous individuals. The differences in the two other populations were slight or nonexistent. By comparing the morphs between populations, pregermination dormancy and plant vigo r were found to be different among populations for the central achenes , whereas the characteristics of the peripheral achenes remained almos t unchanged. It is often hypothesized that achene dimorphism is due to developmental constraints. Our results indicate that these constraint s may be regulated by environmental conditions.