GERMINATION DIMORPHISM IN HETEROTHECA-SUBAXILLARIS VAR SUBAXILLARIS

Achenes of H. subaxillaris var. subaxillaris (Compositae) exhibit germination dimorphism. Freshly-matured disc achenes are nondormant and will germinate in light and darkness over a wide range of temperatures, including those that simulate habitat temperatures at the time of dispersal in autumn. Ray achenes are dormant at dispersal and must after-ripen before they can germinate. After-ripening of ray achenes is inhibited by low and promoted by high temperatures; therefore, germination is delayed for at least a year after dispersal. Low temperatures in the habitat during winter prevent after-ripening, so achenes do not germinate in spring because they are dormant. High summer temperatures promote after-ripening, and nondormant achenes germinate in autumn, if they are exposed to light. Germination dimorphism in H. subaxillaris var. subaxillaris helps spread germination of a seed crop through time and thus may be of survival value to the species in a particular habitat.