Changes in leaf size and in the number and size of the leaf cells have been followed throughout the growing season and during drought and defoliation. The duration of the meristematic phase of primordium growth is the major factor affecting leaf size in all cases, and a hypothesis is developed to account for the changes. A new index of shoot growth reflecting the rate of cell production is used, together with the leaf initiation rate, to distinguish changes in growth rate at the time of runner production and inflorescence initiation.