SEED DORMANCY IN ISANTHUS BRACHIATUS (LABIATAE)

Seed dormancy and its ecological aspects were investigated in Isanthus brachiatus, a summer annual plant of limestone outcrops in southeastern United States. Freshly matured seeds are dormant and exhibit physiological polymorphism with respect to the conditions necessary to overcome dormancy. Fifteen to thirty-five percent of the seeds in a seed crop require only one stratification treatment and germinate the first spring following their dispersal in autumn. The remainder of the seeds require two, three, or more stratification treatments and thus do not germinate until after two, three, or more overwintering periods in the field. In those seeds that require more than one stratification treatment to overcome dormancy, the stratification periods must be separated by a “rest” period, which in nature corresponds to summer. The ecological significance of this type of seed dormancy mechanism in I. brachiatus is discussed in relation to adaptation to its habitat.