Sclereid Distribution in the Leaves of Pseudotsuga Under Natural and Experimental Conditions

Abstract

Al-talib, Khalil H., and John G. Torrey. (U. California, Berkeley.) Sclereid distribution in the leaves of Pseudotsuga under natural and experimental conditions. Amer. Jour. Bot. 48(1): 71–79. Illus. 1961.—A study of the distribution of sclereids in cleared leaves taken from 1-, 2-, and 4-year-old shoots of an adult tree of Pseudotsuga menziesii (Mirb.) Franco showed a repeated pattern of sclereid distribution along the shoot axis with many sclereids in the basal leaves grading into few or no sclereids in the terminal leaves of each year's growth. Attempts were made to influence sclereid distribution by bud defoliation of attached branches with and without auxin treatment and by testing the effects of growth-regulating substances on sclereid formation in leaves of excised buds of Pseudotsuga cultured in vitro. Whereas removal of the basal ¾ of the leaves at the time of bud unfolding had no effect on bud, leaf or sclereid development, removal of the leaves of the upper half or complete defoliation led to premature expansion of next year's terminal bud with leaves developing in part from presumptive bud-scale primordia. Indoleacetic acid at 0.5% in lanolin paste applied to the defoliated region prevented this premature bud expansion. Defoliation of the basal half did not affect sclereid formation in the terminal leaves. Sclereid development in leaves of prematurely expanded buds on defoliated branches was normal except in the few cases where bud expansion occurred in the presence of low-auxin concentrations. Then, sclereid development was inhibited. Sclereid formation in leaves of excised buds grown in nutrient culture was generally much less frequent than in intact branches, and auxin treatment still further reduced the frequency of sclereids. It was concluded that sclereid initiation and differentiation in the intact plant may well be under the control of hormonal factors in the plant, one of which may be auxin.