THE STRUCTURE AND GROWTH OF STEMS 179 



The outer bark of a tree may be cut through without 

 seriously injuring the tree. It is usually able to heal 

 wounds of that kind by the growth of tissue from all sides 

 of the wound; wounds appear to stimulate to division 

 adjacent cells which otherwise would never divide. Some- 

 times, however, especially if the wounded tree is not 

 vigorous, decay will start at such wounds and gradually 

 cause the death of the entire tree. The bark of a tree pro- 

 tects it from decay somewhat as our skin protects us from 

 disease. When our skin is broken, we do what we can to 

 prevent the wound from becoming infected. Similarly, 

 shade trees often have their wounds treated to prevent or 

 to remove disease and decay. Boring insects are one 

 cause of the decay of wood. Probably more deaths of trees 

 are caused, however, by insects like the gypsy moth which 

 eat the leaves. 



To cut through the inner bark of a tree and completely 

 around the trunk will cause its death. To cut through to 

 the heartwood of trees is a method often used to deaden a 

 forest where land is to be cleared; this process is called 

 girdling. Girdling stops the descent of food into the roots 

 and most of the ascent of sap to the leaves. Sometimes 

 there appears just" above the girdle a considerable swelling 

 of the trunk. This swelling is due to the accumulation of 

 food which has been interrupted in its downward move- 

 ment. This accumulation of food induces an abnormal 

 growth, hence the swelling. Sometimes in young plants, 

 just above a girdle, roots also will appear ; the plant seems 

 to be trying to save its life by developing an entirely new 

 root system. 



If you have been where there are birch trees, you have 

 noticed their papery, white bark. You have been in- 



