Spruce bordered pits

Fig. 1-7. Radial section of a spruce ray (above) and radial and tangential section of a pine ray (below), (a) Longitudial tracheids. (b) Rows of ray tracheids (small bordered pits), (c) Rows of ray parenchyma, (d) Pits in the cross fields leading from ray parenchyma to longitudial tracheids. (e) A bordered pit pair between two tracheids. (f) A bordered pit pair between a longitudial and a ray tracheid (llvessalo-Pfaffli, 1967).

The pit chambers are reduced to what ontogenetically is called the initial pit chamber, and the pit membranes are dissolved or detached. Fengel (15) reports pit membranes in fossil spruce to be quite resistant. However, in tissues where all secondary cell walls have been degraded we have not seen any pit membranes in bordered pits. 

FIGURE 40.4 Wood is formed by cell division in the cambium zone, hence the radial cell lines appear clearly in this figure, in spite of the large variation of radial diameter between earlywood and latewood. Some bordered pits allowing sap flow from one tra-cheid to the other can also be observed on radial cell walls. (ESEM Photograph Norway spruce (Picea abies), LERMAB-ENGREF, Nancy, France.)... 

In softwoods, woods formed by cone-bearing trees (e.g., fir, pine, and spruce) with naked seeds, the xylan contains mainly tracheids (90%). Tracheids are considerably elongated cells (around 40 pm in diameter and between 2 and 8 nun in length), which ensure both sap flow, by means of numerous bordered pits situated on the radial cell walls, and mechanical strength. In softwoods, the earlywood is characterized by cells with large radial diameters and thin walls, and hence relatively large cavities. Latewood cells have a much smaller radial diameter and thicker walls, which result in much smaller cavities (Figure 40.4). In addition, some softwoods have resin canals. Parenchyma cells surround these canals and actively secret resin into the canals, and ultimately into the heartwood. 

In softwoods, woods formed by cone-bearing trees (e.g., fir, pine, and spruce) with naked seeds, the xylem contains mainly tracheids (90%). Tracheids are considerably elongated cells (around 40 pm in diameter and between 2 and 8 mm in length), which ensure both sap flow, by means of numerous bordered pits situated... 

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