Figure 1.

Growth zone of a grass leaf. Growth is limited to the basal part of the growing leaf, the growth zone, which is enclosed by the sheaths of expanded leaves. Meristematic cells in the division zone elongate and divide simultaneously (a→a′). The progeny of the initial cell at the base of the meristem goes through a certain number of division cycles, thus determining the number of cells per meristematic file. Upon entering the elongation-only zone (b→b′), cells elongate without further divisions until they reach their final length at the distal end of the growth zone (c→c′).