These are most concentrated on their end walls . This favors vertical translocation of carbohydrates but some lateral transport also occurs via lateral sieve pores. Sieve pores are lined with the carbohydrate Callose . Callose reversibly closes sieve pores during periods of stress. Sieve cells produce an enzyme which can hydrolyze callose so that the pores can reopen.
Sieve cells provide little structural support .
Phloem may contain sclerenchyma fibers but these protect the phloem from being crushed and do not lend rigidity to the stem. Phloem is crushed by the accumulation of Secondary Xylem which increases the stems' diameter. However, the Vascular Cambium replaces dead cells like a dynamic equilibrium. The Epidermis does not resume meristematic activity. This produces cells towards the outside in radial files .