Developmental studies of cell differentiation in the epidermis of monocotyledons: I. Allium, rhoeo, and commelina

Abstract

Various characteristics make the epidermis of certain monocotyledonous plants, particularly the cereal grasses, favorable for studies of certain phases of developmental morphology and genetics. In the Commelinaceae, Araceae, Gramineae, and other families, development of the stomatal complex involves the following epigenetic sequence of events. In certain localized rows or areas, the cytoplasm of the protodermal cells becomes polarized, so that it is denser and contains more inclusions at the distal end of the cell. The nucleus then divides by a mitotic figure oriented across the cytoplasmic gradient. The chromosomes of this figure are similar at the two poles until early telophase, when the proximal nucleus becomes larger and more weakly staining than the distal one, both with the Feulgen technique and with acetocarmine. The distal nucleus eventually divides to form the two guard cells, but before doing so it induces division of two or more of the surrounding epidermal cells. Since in these cells there is also a cytoplasmic gradient, with a denser cytoplasm adjacent to the guard mother cell, these induced divisions are asymmetrical, with the nucleus in the center of the epidermal cell becoming larger and more weakly staining than that near the guard mother cell. In this way, subsidiary cells are formed. In the Commelinaceae, the subsidiary cells lateral to the guard mother cell are always formed before those proximal and distal to it, and a particular epidermal cell adjoining a guard mother cell always contributes a lateral subsidiary before a proximal or distal one. In Allium and other Liliaceae differentiation of the guard mother cell and guard cells is similar, but no subsidiary cells are formed. Some possible clues to the explanation of this sequence are offered, and pertinent experiments are suggested.