Fine structure of callus phloem in Pinus pinea.

The fine structure of phloem formed spontaneously in callus grown on leaf explants of Pinus pinea is very similar to that found in the stem. On the other hand very little callose is formed on callus phloem pores, and their structure is thus easier to investigate. No trace of fibrils typical of the lumen of angiosperm phloem has been found at any stage of phloem development. The usefulness of callus phloem in studies on phloem structure and function is discussed briefly.