CELLULAR CHANGES OF TRACHEIDS AND RAY PARENCHYMA CELLS FROM CAMBIUM TO HEARTWOOD IN CUNNINGHAMIA LANCEOLATA

Cellular changes of the cell wall and protoplasm in tracheids and ray parenchyma cells during heartwood formation were investigated in a 26-year-old Cunninghamia lanceolata at micro- and ultrastructure levels. Cell morphological and ultrastructural features changed significantly from the cambium to inner heartwood. In the cambial zone, the thickness of fusiform and ray cell walls were thin and their radial walls were much thicker than tangential walls. Fusiform cells were highly vacuolated with the protoplasm confined to the periphery of cell lumen. At the time of wood cell differentiation, differentiating xylem mother cells began to lose the protoplasm. Concomitantly, their walls thickened and showed characteristically distinct wall layers. In comparison, ray cell walls appeared as typical polylamellate structure and had thinner walls than the tracheids. Tracheids completed their differentiation and left hollow dead tracheary elements to decline into sapwood, while the xylem ray cells remained alive. The ray cells contained cell protoplasm, although the amount, shape and size altered when shifting towards intermediate wood. Subsequently, the ray cells disintegrated their protoplasm, including the nuclei, organelles and reserve materials, which marked the formation of heartwood.