STRETCHING OF THE BARK 



77 



inch in diameter, yet the bark at the top is 

 smooth and intact. At one time, the shoot was 

 not more than one -eighth of an inch in diameter 

 at this point. The reader may figure out how 



69. Piece of bark from an 

 old elm trunk. 



70. A dead branch and the mass 

 of healing tissue at its base. 



much this bark has expanded by the combined 

 action of intercalary growth and stretching. 



The lower part of the limb shows that the 

 outer layers of bark (which are long since dead, 

 and act only as protective tissue) have reached 

 the limit of their expanding capacity and have 

 begun to split. The reader will now be inter- 

 ested in the bark upon the body of an old elm 

 tree (Fig. 69); and he should be able to suggest 

 one reason why stems remain terete or cylin- 



