Measurements of the epidermal cells in Lolium temulentum and assessments of cell number indicate that differences in blade width are due mostly to variations in cell number, whereas changes in blade and sheath length result mainly from differences in cell length. A marked increase in length of the flag-leaf aheath, however, was related to an increase in cell number. Considerable changes in the relative proportions of leaf blade and sheath were observed in the flag leaf, and the leaves immediately preceding it, associated with inflorescence initiation. As one consequence of this the area of the flag leaf, the largest on the shoot, is virtually constant under different environmental condi-tions. It is suggested that this aspect of correlated development is related to the nutrition of the panicle, since in annual grasses such as the cereals the flag leaf may be responsible for producing up to 30 per cent. of the starch in the grain.