The overall purpose of this research is to examine two opposing views on changes in physiology of closely-related plant taxa that evolved in response to changes in climatic and other environmental factors following geographical breakup of the "Arcto-Tertiary" forest. The first view, advocated by Daniel I. Axelrod, suggests that although modern taxa may have slight to moderate morphological differences compared to Tertiary ancestors, their ecological tolerances and physiological requirements essentially have remained unchanged. The contrasting view, put forth by Jack A. Wolfe, is that taxa diverged in physiological and ecological requirements as lineages were exposed to climatic changes through geologic time. These two hypotheses are being evaluated via a comparative study of seed dormancy and germination characteristics of three closely-related Aristolochia species: the California endemic A. californica, the Appalachian endemic A. macrophylla, and the southeastern U.S. species A. tomentosa. The three species are the North American members of a monophyletic group (subg. Siphisia) that became geographically separated and diverged, presumably in the late Tertiary. Seeds of all three species have linear underdeveloped- and physiologically dormant embryos, thus morphophysiological dormancy (MPD). Seeds of A. californica require warm (e.g., 30/15C) stratification for dormancy-break and a low (e.g., 15/6C) temperature regime for nondormant seeds to germinate. In contrast, seeds of A. tomentosa and A. macrophylla require cold (5C) stratification for dormancy-break, but nondormant seeds will germinate over a wide range of temperature regimes (i.e., 15/6-35/20C). These preliminary results seem to indicate that seeds of A. tomentosa and A. macrophylla have nondeep simple MPD and those of A. californica nondeep complex MPD. In any case, seed dormancy-breaking and germination requirements of the Mediterranean-climate species, A. californica, differ considerably from those of the temperate rainy-climate species, A. tomentosa and A. macrophylla, thus supporting Wolfe's view on physiological changes in a lineage.

Key words: Aristolochia subg. Siphisia, monophyletic group, morphophysiological dormancy, physiological changes in lineages, seed germination, Tertiary Period