I2 EXOGENOUS TREES. 



the tree, it becomes denser, its canals are filled with gums or 

 tannin, color changes, and the result is heartwood (Duramen). 

 This change goes forward rapidly in some trees, such as locusts, 

 so that their sections appear to be almost wholly heartwood; 

 other species require longer time, and sapwood then predomi- 

 nates. 



Professor von Schrenk believes that sapwood changes to 

 heartwood suddenly; that the change does not take place in 

 one ring every year, but that it frequently skips many years, so 

 that eight, ten or even more rings may change from sapwood to 

 heartwood in one year. He also calls attention to the fact that 

 one side of the tree may change before the other, and that part 

 of a ring may be heartwood while the rest remains sapwood.* 



Heartwood gives stability to the tree, but is not needed in 

 its physiological processes. It is tougher, heavier, stronger, 

 and more valued in construction. Sapwood is vitally essen- 

 tial to the life of the tree, but is lighter, weaker, less durable, 

 and less valued in construction. Sapwood is pliable, and the 

 sapwoods of several trees are valued for this reason. 



Wood-making varies as it takes place in the springtime 

 and in the summer. Consequent differences in the densities 

 of the deposits serve to mark the limits of the yearly rings. 

 Some species, as the oaks and hickories, show pores through- 

 out their spring woods which thus contrast with denser summer 

 growths. Others, as Southern pines, change sharply, and 

 their spring and summer growths appear as solid bands. In 

 even climates, where seasons are not pronounced, growth is 

 more regular and layers correspondingly less definite, f 



The cellular structure of wood is principally vertical, a fact 

 that explains the ease with which wood is split up and down. 

 Beside the vertical, there are horizontal cells, that cross the 

 tree, strengthen and bind the vertical cells, and assist in the 

 life-processes of the tree. These horizontal cells form what 

 are known as medullary or pith-rays and appear as simple 



* U. S. Bureau Plant Industry, Bulletin No. 14, p. 15. 

 t Circular No. 16, U. S. Forestry Division. 



