Ultrastructural Observations of Parenchyma and Sclereids in Douglas-Fir (Pseudotsuga Menziesh (MIRB.) FRANCO) Bark

Pitting that occurs in sclereids and parenchyma cells of Douglas-fir bark was examined, along with differences in structure of cell walls in parenchyma from inner and outer bark. Axial parenchyma is connected by pit areas that appear as a slightly thinner region in the adjacent walls where plasmodesmata are found. Sclereid-to-parenchyma pitting consists of a long pit canal through the sclereid wall about 1 μm in diameter with little or no evidence of a pit chamber. The pit membrane is the original double cell wall of the parenchyma as found in parenchyma-to-parenchyma pitting. Pitting between two sclereids exhibits similar long pit canals through the selereid walls, but a pit chamber is present with a thick membrane likewise consisting of the double cell wall of the parenchyma. During sclereid formation the secondary wall of the sclereid appears to be deposited by apposition on the original parenchyma cell wall. Parenchyma cells in inner bark have small wrinkles and folds in the cell wall, whereas enlarged parenchyma in outer bark have very smooth walls