Fig. 16.1-1. Transverse section of pine (Pinus). This is a low magnification view for orientation. Wood (secondary xylem) is at the bottom of the micrograph (the inner part of the tree trunk), the cambial zone runs across the center, and the secondary phloem is all the tissue above the cambium (to the outside of the cambium). The fusiform initials that produce rows of tracheids in the secondary xylem also produce sieve cells in the secondary phloem (the cambium is bidirectional), and likewise, ray initials produce rays in phloem as well as xylem. The cells with darkly stained contents in the phloem are tannin cells. Although sieve cells are abundant here, they are virtually impossible to identify in transverse section – the sieve areas cannot be seen in this micrograph. Albuminous cells occur in the rays; they are not sister cells to the sieve cells and are not even produced by the fusiform initials (see pages 343 and 344 in Plant Anatomy (Mauseth)).
Much of the outer part of the phloem rows are undulate: as the sieve cells stop functioning and collapse, axial tissue contracts into this irregular pattern. Because most ray cells do not collapse, rays retain their size and consequently are forced into folds by the collapsing sieve cells.