Plants of the genotype A Bz1 Bz2 C2 R-r B/b Pl/pl have been used extensively for clonal analysis because x-ray induced sectors resulting from the loss of B or Pl can be observed in every vegetative organ of the plant. Vegetative tissue carrying B and Pl is purple; tissue lacking B is green and tissue lacking Pl is sun-red. Until recently the pigmentation in B Pl plants was thought to be restricted to the epidermis (except in the sheath), implying that data from clonal analyses were relevant only to the behavior of this tissue layer. It now appears that this is not the case. Freehand sections of irradiated and unirradiated B/b Pl/pl plants demonstrate the presence of anthocyanin not only in the epidermis of the culm, but in internal tissue as well. All the cells in the peripheral millimeter of the culm are intensely pigmented; throughout the rest of the culm pigment is usually restricted to vascular bundles. The pigmentation of vascular bundles is most intense within about 1 cm of the node and becomes progressively weaker--and may disappear in some cases--toward the base of the internode.

Because of the intense pigmentation of subepidermal cells, b and pl sectors are virtually invisible when they are restricted to the epidermis. Only sectors in the subepidermal tissue of the culm are distinct enough to be readily observable. Although sectors induced at a dry seed stage frequently encompass both the epidermis and subepidermal tissue, these tissues are not necessarily clonally related because sectors restricted to one or the other layer also occur. Thus, the pattern of cell division in the epidermis of the shoot meristem is somewhat variable. Sometimes epidermal cells divide anticlinally, and produce only a single layer of cells; however, they can also divide periclinally and contribute to internal tissue.

R. S. Poethig

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