FINE STRUCTURE STUDIES OF PHLEUM ROOT MERISTEM CELLS. II. MITOTIC ASYMMETRY AND CELLULAR DIFFERENTIATION

Abstract

Avers, Charlotte J. (Douglass Coll., Rutgers—The State U., New Brunswick, N.J.) Fine structure studies of Phleum root meristem cells. II. Mitotic asymmetry and cellular differentiation. Amer. Jour. Bot. 50(2): 140–148. Illus. 1963.—An electron microscopical study showed that ultrastructural differences distinguished the cell dividing symmetrically from that undergoing asymmetrical division in timothy grass epidermis. The spindle orientation led to cytokinesis which produced either equal‐ or unequal‐sized sister cells, but the mitotic apparatus itself varied in the mitoses. In asymmetrical cells, the basal pole showed more extensive endoplasmic reticulum (ER) polarizations, which intruded into the spindle area during metaphase and anaphase. Such ER polarity was not obvious in symmetrical mitosis or in the apical end of asymmetrically dividing cells. The mitotic sequence is described photographically. Foci of ER were observed as early as prophase in the polar region, and it is suggested that there is a resemblance to astral ray foci seen in prophase of animal cell mitosis. Cell plate formation could be detected in anaphase by accumulations of vesicles and ER fragments along the spindle equator. Phragmosomes apparently were not involved in cell plate formation in Phleum, unlike Allium, cytokinesis. The mitotic asymmetry is discussed as a consequence of an intracellular gradient separate from the intercellular gradient of differentiation along the entire length of growing root tip epidermis.