Structure and Seasonal Development of the Secondary Phloem in the Pinaceae

Abstract

Functional sieve cells are present for the entire year in the secondary phloem of Larix laricina (Du Roi) K. Koch, Abies balsamea (L) Mill, and Picea mariana (Mill.) BSP. With regard to a given year's growth increment, all but the last-formed sieve cells (one to four layers) cease functioning in the same season in which they are derived from the cambium. The last-formed sieve cells overwinter and remain functional throughout the next growing season. Toward the end of April, undifferentiated cells in the outer margin of the cambial zone begin to differentiate into sieve cells. About a week later, cambial activity (cell division) commences All early phloem is produced before new xylem differentiation begins, that is, by mid-May in A. balsamea and by the end of May in L. laricina and P. mariana Most sieve cells are differentiated by early September Cessation of function begins by the end of September with the formation of definitive callose on the overwintering sieve cells and continues to sieve cells of the current season's early phloem. By early December, all but the last-formed sieve cells (i e., those which will overwinter in a functional state) are devoid of contents. Phloem differentiation precedes xylem differentiation by approximately 1 month. Xylem and phloem differentiation and cambial activity cease at approximately the same time in early September. Resin cells that are characteristic of the nonconducting phloem in A. balsamea consist of walls and protoplasts with intact nuclei.