8 THE PRINCIPAL SPECIES OF IVOOD. 



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.* 



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 

 lines or glistening plates according to the way in which the 

 wood is cut. Woods differ in the size and number of these rays, 

 which are by no means always visible to the eye (see plate 3).. 



Woods are easy or difficult to work in proportion as their 

 fibres are arranged in a simple or a complicated manner. This 

 is shown in the figures on plate 4. A knife pressed upon 



Fig. 3. 



the oak must crush or cut into the fibres themselves, whereas 

 with the pine it finds some natural passage between the cells. 

 Wood may be cut so as to develop cross-sections {C, Fig. 3), 

 radial sections {R), or tangential sections (?"). The respect- 



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



