SHOOT GROWTH CHARACTERISTICS OF HETEROPHYLLOUS WOODY PLANTS

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Abstract
Observations were made on relationships between contents of winter buds and subsequent shoot growth characteristics of Betula papyrifera, B. alleghaniensis, Populus tremuloides, and P. grandidentata. In both species of Betula, winter buds expected to produce long shoots contained both embryonic leaves and leaf primordia. Both early and late leaves were produced by growing shoots. In contrast, in both species of Populus all leaves produced on the observed shoots appeared at bud break and matured rapidly. Evidence for a variety of patterns and degrees of dependency of expanding shoot systems on reserve foods and currently produced carbohydrates is presented. Shoot elongation of species with shoots fully preformed in the winter bud utilizes large amounts of reserve carbohydrates. In growth of long shoots of heterophyllous species considerable current photosynthate appears to be used. In Betula the contribution of early and late leaves to shoot growth differed markedly. Covering of early leaves before mid-June inhibited shoot growth. The presence of normally growing early leaves was essential for normal shoot development and survival.