Cell Division and Expansion and Resultant Tissue 3

Abstract

By following the movement of carbon particle markers on the exposed surface of a cultured tomato apex it has been shown that a leaf primordium is formed by growth on the flank of the apex raising the tissue upwards and outwards to form the leaf buttress. The whole of the apical surface is in an active state of cell division and expansion except in the axillary region above the primordium. The data provide direct estimates of the rates of division in the outer layer of cells. The distribution of blocked metaphase figures following treatment with colchicine, shows that in the early stages of primordium formation cell divisions are concentrated in what appears to bo a ‘growth centre’ in the corpus to one side of the apical dome. As the bulge of the primordium develops, the growth centre spreads out and splits into two parts continuing the growth of the dome (proximal side) and the primordium (distal side). Between these two regions of active division there arises a small pocket of cells in the axil, whose rate of division rapidly declines. Cuts made in the apical surface in the early stages of primordium formation immediately gape widely, apparently as a result of pressure exerted on the outer layers from within by divisions in the corpus. Once the upper surface of the primordium becomes raised above the dome, the axillary cells seem to become compressed between the two zones of active division. In the axil at this stage (a) cuts do not gape but close up after exuding cell sap and (b) the carbon particle markers move slightly together.