ULTRASTRUCTURE OF PITS IN PINUS-BANKSIANA LAMB

The cross-sectional view of pitting between various cell types in Pinu s banksiana Lamb. was studied at the ultrastructural level. Cell types in Pinus banksiana include longitudinal tracheids, ray tracheids, ray parenchyma cells, ''buffer cells'' and epithelial cells. Two common c haracteristic features of bordered pit-pairs between longitudinal trac heids are an initial pit border and a thickened torus at the center of the pit membrane. The shape and size of the pit border and torus of b ordered pit-pairs between two compression wood cells, and between the last-formed latewood longitudinal tracheid and first-formed earlywood longitudinal tracheid were different from those in the earlywood and l atewood longitudinal tracheids. The pit border on the ray tracheid sid e varied in size and shape due to wall dentation. No initial pit borde r was found on the pit border of the ray tracheid side. The shape of b ordered pit-pairs between two ray tracheids varied considerably due to irregularity of the dentate cell wall. The size of bordered pit-pairs in longitudinal tracheids was between 16 mu m to 20 mu m, which was t wice the diameter of bordered pit-pairs in ray tracheids. Bordered pit -pairs at the end wall of two ray tracheids appeared to be the smalles t at 5 mu m. Pit aspiration occurred in the bordered pit-pairs with or without a torus. In the heartwood zone, some half-borders pit-pairs b etween tracheary and ray parenchyma cells showed an additional seconda ry wall on the ray parenchyma cell side. Plasmodesmata were found in t he half-bordered pit-pairs as well in the simple pit-pairs. Blind pits were observed between a ray tracheid and a longitudinal tracheid. Bor dered pit-pairs between two ''buffer cells'' were also observed. The p ossible functions of buffer cells were discussed.