SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN ACER NEGUNDO

Functional sieve elements are present year-round in the secondary phloem of the trunk of Acer negundo L., the box elder tree. Judging from numerous collections made between May, 1962, and May, 1964, the seasonal cycle of phloem development is as follows: cambial activity and new phloem differentiation begin in late March or early April; xylem differentiation begins about a month later and is completed in most trees in late August. At the time of cessation of cambial activity most of the relatively wide sieve elements of the current season's increment are mature. However, numerous groups of narrow, immature sieve elements and companion cells located on the outer margin of the cambial zone do not reach maturity until fall and winter. By the time of cambial reactivation in spring, most, if not all, of these narrow elements are mature. Some of the sieve elements which reach maturity either shortly after cessation of cambial activity or during dormancy become non-functional within 6 weeks after resumption of cambial activity in spring, while others remain functional until mid-August. For the phloem increment of a given year, cessation of function begins in September with the accumulation of definitive callose on the sieve plates of the first-formed sieve elements and spreads to all but the last-formed ones by the end of December.