Seasonal Development of the Secondary Phloem in Populus tremuloides

Abstract

Judging from numerous collections made over a 1-year period (April, 1962-Aug., 1963), the seasonal cycle of phloem development in Populus is as follows. Cambial activity (division) begins in late March or early April, giving rise primarily to assemblages of sieve elements, companion cells, and parenchyma cells. By mid-May about half of the new phloem increment has been produced. By the end of July or early Aug. sieve element differentiation is completed. Cessation of function begins in late Sept. or early Oct. with formation of definitive callose in sieve elements on the outer margin of the conducting phloem. By late Nov. all sieve elements are devoid of contents and their companion cells collapsed. Phloem differentiation precedes xylem differentiation, which begins in mid-May, by approximately a month and a half; xylem and phloem differentiation ceases almost simultaneously.