20 



WOOD AND FOREST. 



Fig. 10. Annual King-s in 

 Bark (cork). 



edge, Fig. 11. After they have grown somewhat radially, partition 



walls form across them in the longitudinal, tangential direction, so 



that in place of one initial cell, there are two daughter cells radially 



disposed. Each of these small cells grows and re-divides, as in 



Fig. 12. Finally the inner- 

 most cell ceases to divide, and 

 uses its protoplasm to become 



jfl X A thick and hard wood. In like 



manner the outermost cambium 

 cell becomes bast, while the cells 

 between them continue to grow 

 and divide, and so the process 

 goes on. In nearly all stems, 



there is much more abundant formation of wood than of bast cells. 



In other words, more cambium cells turn to wood than to bast. 



In the spring when there is comparatively little light and heat, 



when the roots and leaves are inactive and feeble, and when the 



bark, split by winter, 



does not bind very 



tightly, the inner cam- 

 bium cells produce ra- 

 dially wide wood cells 



with relatively thin 



walls. These constitute 



the spring wood. But 



in summer the jacket 



of bark binds tightly, 



there is plenty of heat 



and light, and the 



leaves and roots are 



very active, so that the 



cambium cells produce 



thicker walled cells, 



called summer wood. 



During the winter the 



trees rest, and no 



development takes place until spring, when the large thin-walled cells 



are formed again, making a sharp contrast with those formed at the 



end of the previous season. 



Fig-. 11. Diagram Showing Grain of Spruce 



Highly Magnified. PR, pith rays; BP, bordered 



pits; Sp W, spring wood; SW, summer wood; 



CC, overlapping of chisel shaped ends. 



