FLEXIBILITY IN ONTOGENY AS SHOWN BY THE CONTRIBUTION OF THE SHOOT APICAL LAYERS TO LEAVES OF PERICLINAL CHIMERAS

The development of leaves on apically stable, periclinal chimeras was studied in a number of dicot genera. The mutant cell layers of the shoot apex and the tissues derived from them were as active developmentally as the normal layers. Ontogeny was the same in these chimeras as in nonchimeras, and growth of their leaves can be outlined as follows. Formation of the buttress, the axis, and the lamina of simple dicot leaves were independent events. In each the first growth included derivatives of the apical layers, usually three in number, found in the apex of the shoot and the lateral buds. Most cell divisions in the outer layers (L-I and L-II) were anticlinal relative to the new structures. Therefore, in the proximal regions of the buttress, axis (petiole and midrib), and lamina, the derivative cells of L-I and L-II were usually present in single layers. The rest of the internal tissue was from L-III. As formation of the axis and the lamina proceeded, derivatives of L-II replaced L-III internally in the distal and marginal regions leaving cells of L-III behind. Both the determinate growth of leaves and the pattern of cell divisions at and near the leading edges of growth meant that no cells in the leaf were comparable to the initial cells of the shoot apex. As the lamina extended, there were extensive intercalary cell divisions, both anticlinal and periclinal, so that in any given region of a leaf the layers of internal cells were from either L-II or L-III. At any point along the axis, L-III participated or did not participate in laminar extension. At any given stage in laminar growth either of two sister cells in any internal layer divided either a few times or extensively. The extreme variability in direction and frequency of cell division during leaf development was under an overriding genetic control, which resulted in the normal or typical size, shape and thickness of leaves.