In the winter of 1959 population samples of Diamorpha cymosa Nutt. were transplanted from numerous granite and sandstone outcrops to a simulated outcrop in an experimental garden on the Emory University campus. Population samples from a centrally located outcrop were reciprocally transplanted to each outcrop visited. Phenological observations were recorded during the following 3 years, and population samples of the second seeding generation (1961) were subjected to analyses of comparative physiological tolerances and cytology. Population phenology varies in a northeast to southwest clinal pattern and coincides with moisture and temperature gradients. Population physiological tolerances to light, temperature, and moisture stresses also vary in a clinal pattern, but in a different direction. Marginal populations are most tolerant, and centrally located populations are least tolerant to these stresses. The two discordant patterns of clinal variability indicate that Diamorpha is undergoing ecotypic differenti...