Secondary phloem inPinus radiataD. Don

Abstract

Light and electron microscopy have been used to describe the structure of axial and ray parenchyma cells in the phloem of Pinus radiata D. Don. Axial cells are found containing tannins, crystals and tannins, or containing none of these. Two types of cell occur in the rays: the albuminous cells which form the upper and lower margins of the rays, and the ray parenchyma cells. The rays are continuous through the cambium into the xylem. The accumulation of tannins in the axial parenchyma cells has been studied. The tannins first appear in the cell vacuoles and later in the cytoplasm. The tonoplast frequently appears to be highly active during tannin accumulation. Of the cell organelles, the nucleus alone remains free of tannin and may persist in cells well beyond the region of active phloem. Albuminous cells have plasmodesmatal connections with the cytoplasm of sieve cells. Thus, through their similar connections with ray parenchyma, they must form an important link in the translocation system.