Leaf development is described quantitatively, with particular reference to rates and durations of cell division and cell expansion in the various tissues of the lamina and to development of the vascular system. From these data the following three hypotheses are suggested and are discussed in relation to the diversity in leaf structure between species and between tissues: (1) Relative rate of leaf expansion is highest in the young primordium and then tends to decline until maturity. (2) Relative rate of cell division is also highest in the young primordium and then declines; but it does not necessarily decline at the same rate as that of leaf expansion and declines at different rates in different tissues. (3) Cell size depends on the proportional rates of leaf expansion and cell division.