NONESSENTIALITY OF CONCURRENT CELL DIVISIONS FOR DEGREE OF POLARIZATION OF LEAF GROWTH. II. EVIDENCE FROM UNTREATED PLANTS AND FROM CHEMICALLY INDUCED CHANGES OF THE DEGREE OF POLARIZATION

Abstract

Haber, Alan H., and D. E. Foard. (Oak Ridge Natl. Lab., Oak Ridge, Tenn.) Nonessentiality of concurrent cell divisions for degree of polarization of leaf growth. II. Evidence from untreated plants and from chemically induced changes of the degree of polarization. Amer. Jour. Bot. 50(9): 937–944. Illus. 1963.—Tobacco leaves grow with a constant degree of polarization (i.e., ratio of rate of increase in length per mm length to rate of increase in width per mm width). During this growth there is decreasing mitotic activity. Data from the literature, including Sinnott's classic studies with cucurbit fruits, provide additional specific examples of growth of (1) constant degree of polarization during which (2) mitotic activity falls. The generalization that cell division plays no role in maintaining a constant degree of polarization is suggested by the widespread occurrence of these 2 features of growth in determinate organs. These considerations are consistent with our earlier finding that the degree of polarization of growth of the first foliage leaf of wheat is the same in seedlings normally growing with oriented cell divisions and in gamma‐plantlets, which are seedlings growing without cell division owing to gamma‐irradiation of the grain before sowing. The present work shows that the degree of polarization can be significantly increased by treatment with gibberellic acid and decreased by colchicine, even though it is unaffected by radiation‐induced mitotic inhibition. These chemically induced changes in the degree of polarization are, moreover, the same in unirradiated and in gamma‐plantlet leaves. We conclude that cell division is essential neither for maintaining the degree of polarization nor for changing the degree of polarization. These considerations lead to 3 biological conclusions, each of which is in harmony with simple geometric considerations: (1) cell divisions do not directly contribute to or cause growth; (2) cell division plays an essential and immediate role in influencing cell forms, but plays a secondary and much less important role in influencing organ form; and (3) there is a fallacy in the usual and accepted manner of interpreting changes in organ size as being due to changes in cell size and changes in cell number.