Rates of photosynthesis were measured in each of six replicated genotypes for each of two Polygonum species at five light and seven temperature levels. The early successional species, Polygonum pensylvanicum L., exhibited higher rates of photosynthesis and a more sun-tolerant behavior compared to its later successional relative, Polygonum virginianum L., results that are consistent with previously observed successional trends. Quantitative comparisons of individual genotype plasticity and between-genotype variation indicate that, in general, plasticity contributes more to population flexibility in photosynthetic response of these species than does between-genotype variation. However, the relative contribution of between-genotype variation was found to vary depending on the environmental variable and species studied. Between-genotype variation assumed greater importance in the temperature response than in the light response. Similarly between-genotype variation was more important in P. virginianum than in P. pensylvanicum supporting the notion that later successional species are less plastic. These results establish that significant levels of photosynthetic variation occur even within populations.