18 



WOOD AND FOREST. 



as spruce and bass. Indeed, some species never form a distinct heart- 

 wood, birch (Betula alba) being an example. 



In a living tree, sap-wood and heart-wood perform primarily 

 quite different functions. The sap-wood carries the water from the 

 roots to the leaves, stores away starch at least in winter, and in other 

 ways assists the life of the tree. The proportional amount of sap- 

 wood varies greatly, often, as in long-leaf pine, constituting 40 per 

 cent, of the stem. 



As the sap-wood grows older, its cells become choked so that the 



sap can no longer flow 

 thru them. It loses its 

 protoplasm and starch 

 and becomes heart- 

 wood, in which all 

 cells are dead and 

 serve only the me- 

 chanical function of 

 holding up the great 

 weight of the tree and 

 in resisting wind pres- 

 sures. This is the rea- 

 son why a tree may 

 become decayed and 

 hollow and yet be alive 

 and bear fruit. In a 

 tree that is actually 

 dead the sap-wood rots 

 first. 



Chemical s u b - 

 stances infiltrate into 

 the cell walls of heart- 

 wood and hence it has a darker color than the sap-wood. Persimmon 

 turns black, walnut purplish brown, sumac yellow, oak light brown, 

 tulip and poplar yellowish, redwood and cedar brownish red. Many 

 woods, as mahogany and oak, darken under exposure, which shows 

 that the substances producing the color are oxidizable and unstable. 

 Wood dyes are obtained by boiling and distilling such woods as su- 

 mach, logwood, red sanders, and fustic. Many woods also acquire 

 distinct odors, as camphor, sandalwood, cedar, cypress, pine and 

 mahogany, indicating the -presence of oil. 



Fig-. 8. Section of Doug-las Fir, Showing- Annual 



Ring-s and Knots at Center of Trunk. American 



Museum of Natural History, N. T. 



