Significance of Cell Divisions in Differentiating Secondary Phloem

Among 91 species in 77 genera of 44 families of dicotyledons surveyed in the present study 35 species showed evidence that various anticlinal divisions occur in the secondary phloem mother cells that produce sieve elements. These divisions range in orientation from transverse through oblique to vertical. The derivatives of these divisions consist of two or more sieve elements with their companion cells or of combinations of sieve elements, companion cells, and parenchyma cells. Frequently the divisions result in sieve elements shorter than their mother cells. The occurrence of these anticlinal divisions has a bearing upon the problem of phylogenetic and physiologic specialization of sieve elements.