Regulation of anisotropic cell expansion in higher plants.
Evidence is presented to show that a given changc in cell form or size may generally be brought about by a variety of patterns of local surface distortion and cxpansion. Structural and chemical features of the cell which are important in morphogenesis may thus be cxpccted to relate not to form per se but to the kinctics of surface behavior which establish form. These kinetics cvaluatc both the rate at which local regions of cell surface expand and the directed character (anisotropy) of this expansion. These variables have been studied in modcl systems and, through marking experiments, in growing cells of various shapes in Phycomyces, Clypeaster, and particularly Nitdla. In thc latter plant, prominent "giant internodes" display a well defined longitudinal anisotropic expansion devoid of sizeable gradients in expansion rate. These ceils have their origin, however, in apical cells which have a pronounced gradicnt in area expansion rate (maximal at the tip). The great part of the expansion in the apical cell is apparently isotropic (equal in all directions), but the basal region often shows predominant expansion laterally. This transverse stretching in the apical cell could align cell wall tcxturc and possibly fibrous cytoplasmic constituents, such as microtubules, into configurations significant in later morphogcnctic stages, including the elongation of the internodes. I N T R O D U C T I O N When cells change their configuration or size, there are generally a number of ways in which the change could take place when the behavior of the various regions of the cell surface is considered. In other words, knowledge of the change in outline of the cell usually does not specify the kind of local changes that bring it about. In cells which are figures of revolution, zones or bands constitute convenient subdivisions of the cell surface. The development of form can be resolved into the local changes taking place in these zones. Description of the exact nature of the local changes reveals whether there is variation in the rate of area expansion along the cell surface and also whether there is local variation in the directed character of surface expansion (whether extension is uniform in all directions). Prior to presenting a quantitative treatment, selected patterns of local changes that can bring on changes in size or form in ceils will be discussed qualitatively. The patterns are chosen on the basis of their simplicity, their pertinence to common cell shapes, and their relation to actual expansion patterns for which information is available. In Fig. I, various figures of revolution are divided into zones to be followed through growth. In part I-A of Fig. 1 a cylinder and a sphere are