The Relationship Between the Distribution of Periclinal Cell Divisions in the Shoot Apex and Leaf Initiation

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Abstract
Periclinal cell divisions in vegetative shoot apices of Pisum and Silene were recorded from serial thin sections by mapping all the periclinal cell walls formed less than one cell cycle previously. The distribution of periclinal divisions in the apical domes corresponded to the distributions subsequently occurring in the apices when the young leaf primordia were forming. In Pisum, periclinal divisions were almost entirely absent from the I1 region of the apical dome for half a plastochron just after the formation of a leaf primordium and appeared, simultaneously over the whole of the next potential leaf site, about half a plastochron before the primordium formed. In Silene periclinal divisions seemed to always present in the apical dome at the potential leaf sites and also round the sides of the dome where the ensheathing leaf bases were to form. Periclinal divisions therefore anticipated the formation of leaf primordia by occuring, in Pisum about one cell cycle and in Silene two or more cell cycles, before the change in the direction of growth or deformation of the surface associated with primordial initiation.