| Abstract: | The shoot apex of Triticum aestivum cv. Ramona 50 was investigated histologically to describe cell lineages and events during leaf initiation. During histogenesis three periclinal divisions occurred in the first apical layer, with one or two divisions in the second apical layer. This sequence of cell divisions initially occurred in one region and spread laterally in both directions to encircle the meristem. Cells of the third apical layer were not involved in leaf histogenesis. Initially, young leaf primordia were produced from daughter cells of periclinal divisions in the two outer apical layers. Nuclear contents of protein, histone, and RNA in the shoot apex were evaluated as ratios to DNA by means of semiquantitative histochemistry. Daughter cells of periclinal divisions in the outer apical layer which produced the leaf primordia had higher histone/DNA ratios than cells of the remaining meristem. However, protein/DNA and RNA/DNA ratios were similar in both regions. Leaf initial cells had a higher 3H-thymidine labeling index, a higher RNA synthesis rate, and smaller nuclear volumes than cells of the residual apical meristem. |
