The Differentiation of Contact Cells and Isolation Cells in the Xylem Ray Parenchyma of Populus maximowiczii

A histochemical analysis was made of the differentiation of contact cells and isolation cells in the xylem ray parenchyma of Populus maximowiczii. The contact cells formed secondary walls at approximately the same time as adjoining vessel elements. The lignification of the cell walls of contact cells and vessel elements began earlier than that of wood fibres and isolation cells. Thus, the formation of the secondary wall, including lignification, of the contact cells might occur at the same time as that of the vessel elements to which they are directly connected. By contrast, the isolation cells began to form secondary walls later than the vessel elements and wood fibres in the vicinity of the isolation cells. After the deposition of the secondary wall, a protective layer was formed in contact cells but no isotropic layer was observed in isolation cells. The results suggest the importance of vessel elements in the determination of the differentiation of adjoining ray parenchyma cells.