Stability, or homeostasis, is that property of the organism buffering it against small random fluctuations of the environment and accidents of development. A means of measuring, within the individual, the stability of expression of one foliar and two floral characters is given. Data are presented from a set of diallel crosses among five varieties of Nicotiana rustica, derived from different populations and grown in two successive seasons. This allows stability to be examined over a number of genotypes in the parent and F1 generations, in two differing environments. The leaf characters show some change from node to node, i.e. manifesting a gradual differentiation, as well as random fluctuations which are taken as evidence of instability. Like stability, this differentiation rate varies from genotype to genotype and shows genetic control. Some attempt is made to find to what extent the different phenomena are related.