The movement of carbon-particle markers on the surface of a cultured pea apex resembled that previously found for the tomato apex. In the pea the primordium originated lower down on the side of the apical dome than in the tomato, and its general direction of growth was more upright. The results accord well with existing data on the rates and directions of cell division in the pea apex, and show that the primordium is formed by increased cell division on the flank of the apex in a growth centre (or region) analagous to that found in the tomato apex. Because of the distichous phyllotaxis of the pea it appears that in longitudinal section two such growth centres at different stages are visible, whereas in the tomato, which has spiral leaf arrangement, only one is apparent. It is concluded that, while a change in direction of division inevitably occurs in the primordium as it begins to bulge outwards away from the centre of the apex, its initiation can be traced to a local increase in the rate of division some 2 plastochrons before the bulge is well formed.