THE SEASONAL CYCLE OF PHLOEM DEVELOPMENT IN JUNIPERUS CALIFORNICA

In Juniperus californica, all sieve cells of the previous season's phloem growth increment overwinter in a mature state. Initiation of cambial activity begins in early March and, by the end of March, the oldest sieve cells that overwintered lose their contents and die. By mid-April, even the youngest sieve cells of the previous season's growth increment have lost their contents. The period of greatest cambial activity begins in the last half of April and continues through May. With the slowing of cambial activity in June, callose begins to collect on the sieve areas of the first-formed sieve cells of the new increment. By July, the cambium and phloem are in a dormant state. Initiation of phloem production precedes that of the xylem by about 1 month. Production of new xylem and phloem ceases simultaneously in July.