Additional J\il>crs. ' ■ 315 



dioxide of the atmosphere. The most active a.ccnts in the transformation 

 of carbon are in the cells of the green leaves. Air containing carbon- 

 dioxide enters throngh the same pores from which water vapor escapes 

 and, under the influence of light, the green coloring matter of the leaf 

 takes it up and forms more complex carbon compounds from it, such as 

 starch, sugars, gums, etc. 



These considerations show us two important rctjuircnients for the 

 growth of the tree, viz., leaves and light. Without ihi- former the tree 

 would be unable to obtain sufficient carbon-dioxide or to keep a stream 

 of water passing upward through the body of the tree. Without the 

 latter the process of food-formation in the leaves cimld not go on. 



What practical bearing have these facts? They teach us that the leaf- 

 destroying fungi and insects should be kept in check ; that trees should 

 be planted far enough apart to allow the access of light ; and that by 

 proper pruning light and air should be admitted to the interior of the 

 tree top. 



These newly formed food substances do not remain in the leaf, but 

 are transferred to the regions where growth is going on, or to special 

 organs for storage. The paths through which this food material is trans- 

 ported lie in that part of the trunk known as the bast, or ''inside bark." 



When a tree trunk is girdled the downward flow is stopped at the 

 place where the soft "inside bark" is removed. The tree does not imme- 

 diately die, but in time, as a result of depriving the roots of their food, it 

 does die. 



Growth. — The growth of the tree does not occur alike in all parts, 

 but is limited to certain tissues, viz., the so-called meristematic tissues. 



The growing tissue of the trunk is confined to a few layers of deli- 

 cate cells, between the bast and the wood, known to botanists as the 

 cambiums. The increase in diameter is due to the division and growth of 

 the cells in this layer. The food they need is in part furnished by the 

 dissolved minerals absorbed by the roots and in part by organic sub- 

 stances manufactured in the green parts of the tree. These substances 

 are absorbed by the cells of the cambium layer and used in the formation 

 of new cells and cell products. When first formed, the cambium cells 

 have thin walls, but as they grow older they become thickened by the ad- 

 dition of successive layers of cellulose. Later the walls undergo "chemical 

 changes and become infiltrated with various minerals and gums. They 

 are no longer cambium cells, but are wood cells. 



The character of the wood depends to a large extent updn the amount 

 and kind of substances with which the walls are infiltrated. All the tissue 



