Lateral branch vascularization: its circularity and its relation to anisophylly

Abstract

The vasculature of elongating lateral branches of Populus deltoides Bartr. ex Marsh. was examined to determine how vascular continuity was attained around the entire branch circumference. In a previous study it was found that a pair of original bud traces (A, A′) gave rise to three pairs of bud traces in sequence (a, a′; b, b′; g, g′) that vascularized the axillary bud; the original bud traces then continued upward in the main stem axis. In this study we demonstrated that the lower, abaxial part of the branch cylinder was vascularized by derivatives of the first pair of bud traces (a, a′), the lateral parts primarily by derivatives of the second pair of bud traces (b, b′), and the upper, adaxial part by derivatives of the third pair of bud traces (g, g′). Thus, the organizational pattern for branch vascularization was established during the earliest stages of axillary-bud initiation. Leaves on all lateral branches were anisophyllous; the condition was related to the position of leaves in the phyllotactic array and to their vascularization. The smallest leaves always occurred on the upper branch side and their central traces were diverted upward in the main stem vascular cylinder. The largest leaves were usually on the lower stem side and their central traces were diverted downward. Some first-formed leaves were falcate, and the lateral traces serving the suppressed sides of their laminae were also found to be diverted upward in the main stem axis. It was suggested that both the small anisophyllous and the falcate leaves might result from a lower nutritional status because of their upward-directed leaf traces.