Seasonal Changes in the Phloem of Ephedra californica Wats.

Abstract

Mature sieve cells were present year-round in the secondary phloem of Ephedra californica Wats In a given year's growth increment, all but the last-formed sieve cells (three to four layers) died the same season in which they were produced. The last-formed sieve cells overwintered and remained functional throughout the first part of the new growing season. Cambial expansion and differentiation of xylem commenced simultaneously in early February. Phloem differentiation followed xylem differentiation by ca. 3-4 wk. Cessation of function began in mid-March with formation of definitive callose on the overwintering sieve cells and continued to the sieve cells of the current season's early phloem. Cambial activity and xylem and phloem differentiation ceased simultaneously in late May. Xylem and phloem differentiation resumed in late September, apparently in response to unseasonal rains in summer and fall, and ended in late December, when all but the last-formed sieve cells that overwintered had become nonfunctional.