Leaf bases of A. tuberosa interior are broadly cordate or lobed at the base, whereas those of terminalis are cuneate or tapered at the base. The most deeply cordate leaves are concentrated in the central Ozarks of Missouri, with a progressive lessening of the cordate shape and increase in frequency of the cuneate shape towards the periphery of the spp. range. Leaf bases were measured by giving an angular value (called <B) of more than 90[degree] to a cordate leaf base, less than 90[degree] to a cuneate base, and 90[degree] to a truncate base. By assigning the<B values to appropriate circles on a map concentric from an arbitrary center in the Ozarks, the centrifugal decrease of <B was demonstrated quantitatively. When the artificial populations from the concentric circles were divided to supply opposed series of concentric arcs from the point of origin in the Ozarks to the n.-e. and s.-w., resp., the 2 curves of <B values were nearly identical. The interpretation is that terminalis is an ancient, ecologically tolerant population and interior a recent genetic innovation which may be supplanting the former subsp. by introgressive hybridization and some selective superiority of heterozygous progenies. In a trip from the central Ozarks to about 100 miles north of Minneapolis 30 colonies of A. tuberosa were found, averaging about 25 plants. The total number of flowers in interior was usually significantly greater (due to more cymes per stem) than in terminalis. Hence, interior should produce more pollen over a longer time. When the relationship between cyme no. and<B was plotted for the means of the 30 colonies, an exponential curve resulted, with the cordate type of leaf base associated with the greater cyme no. and the cuneate type with the smaller cyme no.'' The Missouri colonies showed high variability both for cyme no. and <B, whereas the Minnesota colonies had relatively low variability. When individual plants of 3 colonies were plotted for cyme no. and<B, the colonies from Minnesota and Missouri had distinctive and relatively uncorrelated distrs., whereas the Iows colony had greater variability and higher correlation, indicative of heterozygosity for cyme no. and leaf shape and hence of intro-gressive hybridization. The process of hybridization under selection pressure is termed sabinism.