The variation in carpel number in Ranunculus flammula was analyzed. Both genetic and environmental components of variance were observed and discussed. Significant correlations between length of growing season and carpel number were observed. It was concluded that the genetic component of the variation results from selection toward some "optimum" carpel number consistent with the amount of food reserve that can be accumulated by the plant for endosperm tissue in the environment in which this selection occurs. The plasticity of individuals for carpel number allows for annual fluctuations in environment. An analogy is made with clutch size regulation in birds.