THE CARE OF STREET-TREES 105 



laginous substance called the cambium layer. The cam- 

 bium is under every portion of the bark, which covers the 

 tree completely from the tip of the deepest root to the top of 

 the highest twig. Through the sapwood the soluble inor- 

 ganic materials drawn from the soil by the roots ascend to 

 the leaves, and are there elaborated in connection with the 

 materials taken from the air into organized compounds. 

 This elaborated food material descends through the cambium 

 layer to every part of the plant to build up its tissues. All 

 the tissue arising from the inner side of the cambium ring 

 goes to form the wood, while that produced on the outside 

 goes to make up the bark. 



The cambium is the life of the tree. If the limb of a tree 

 is removed, a new one may develop near its place. Trees 

 live for years with the trunks hollow, but if they are girdled 

 by the cutting away of a ring of bark, there is interrupted the 

 tissue through which the descending food material is con- 

 ducted from the leaves, and the roots are starved and the 

 tree dies. 



Owing to the climatic variations during the growing 

 season, the cambium tissue is not uniformly active. Dur- 

 ing the spring, the period of energetic growth, wood of a 

 coarser texture is deposited than later in the season, when 

 it is more closely grained. Through the contrast in the 

 structure of the early and the late wood, the limits between 

 successive annual rings become sharply defined and serve 

 as a means of computing the age of a tree. 



Essentials for Normal Growth. — The food of trees comes 

 from two sources — the air and the soil. The tree can trans- 

 form the raw materials into wood tissue only under the 

 proper conditions of soil, water, light, air, and climate. 

 Water serves the double purpose of keeping in solution the 



