A 'common garden' experiment established that plants representing two previously described complementary five-locus enzyme genotypes differ genetically with respect to four of five quantitative characters studied. The 'xeric' genotype was found to be highly plastic in its phenotypic response to changes in environment, especially with respect to number of seeds produced under differing moisture levels. This was interpreted as an adaptation to large annual fluctuations in environment, especially in soil moisture levels, which occur in xeric sites. The 'mesic' genotype is a rapid growing, early maturing and profusely tillering type. Its aggressive vegetative growth is evidently an adaptation to the high levels of intra- and interspecific competition it encounters in the dense plant communities where it predominates.