The recent discovery by Navashin (3), that the chromosomal mutation rate in Crepis was influenced by aging of the seed, has been corroborated by observations on the mutation rate of corn plants grown from seed of various ages.A very high chromosomal mutation rate in barley was induced by heat treatments of seed under various conditions of humidity. The most common type of aberration resulting from these treatments appeared to be fracture of the chromosomes either at the attachment constriction, the secondary constriction or the point of attachment of the trabants. The reattachment of fragments to other chromosomes was observed in two instances.Considerable importance is attached to the discovery that a large proportion of the mutant cells are eliminated during the growth of the plant. The principle that only the fittest survive seems equally true of cells as of individuals and groups of plants or animals.