14 



A PRIMER OF FORESTRY. 



then either used at oiice in growth or stored away until 

 the proper moment arrives. This is the general rule, 

 but it is believed that in some cases food taken up by 



the roots can be used 

 without first being 

 digested in the leaves. 



THE COMPOSITION 

 OF WOOD. 



Wood is made up 

 chiefly of carbon, oxy- 

 gen, and hydrogen. 



FIG. 10. Bark of the Western Hemlock. 

 Washington Forest Reserve. 



When perfectly dry, about half its 

 weight is carbon, and half oxygen 

 and hydrogen, in almost the same 

 proportion as in water. It contains 

 also about 1 part in 100, by weight, 

 of earthy constituents, and nitrogen 

 to the same amount. When wood is 

 burned, all these materials disappear 

 into the air except the earthy con- 

 stituents. Now the nitrogen and 

 water taken up by the roots were 

 originally in the air before they 

 reached the ground. It is true, there- 

 fore, that when wood is burned those 

 parts of it which came from the air 

 go back into it in the form of gas, 

 came from the soil remain behind in 



FIG. 11. Wood and bark 

 of the Western Yellow- 

 Pine. The cut is per 

 pendicular, and tlie 

 specimen stands as it 

 did in the tree. The 

 picture shows the di- 

 vision of the bark into 

 scales by the succes- 

 sive layers of cork 

 cambium. The true 

 cambium is between 

 wood and bark. 



while those which 

 the form of ashes. 



