Structure and development of sieve cells in the secondary phloem of Larix decidua Mill. as related to function

Abstract

Only one or two layers of sieve cells of the previous year's phloem in lateral branches of Larix decidua persist as fully mature cells. Immature sieve cells or cambial derivatives that have not completed differentiation may also over-winter. Periclinal cell divisions of the vascular cambium were first observed by mid-April. During the short period of greatest cambium activity (mid-April to mid-May), the early phloem is laid down. Late phloem is formed over a much longer period, from mid-May to late September. Microautoradiography revealed that only mature sieve cells of the early phloem are involved in translocation of 14C assimilates in June. The fine structure of actively translocating sieve cells is described. The impact of structure on long-distance transport of assimilates is discussed.