| Abstract: | The movement of carbon-particle markers on the surface of acultured pea apex resembled that previously found for the tomatoapex. In the pea the primordium originated lower down on theside of the apical dome than in the tomato, and its generaldirection of growth was more upright. The results accord wellwith existing data on the rates and directions of cell divisionin the pea apex, and show that the primordium is formed by increasedcell division on the flank of the apex in a growth centre (orregion) analagous to that found in the tomato apex. Becauseof the distichous phyllotaxis of the pea it appears that inlongitudinal section two such growth centres at different stagesare visible, whereas in the tomato, which has spiral leaf arrangement,only one is apparent. It is concluded that, while a change indirection of division inevitably occurs in the primordium asit begins to bulge outwards away from the centre of the apex,its initiation can be traced to a local increase in the rateof division some 2 plastochrons before the bulge is well formed. |
