GROWTH 



cause of the long clean boles ; the wood itself is 

 of greater density and strength. This may be 

 explained as follows : 



In the annual rings of trees two zones of wood 

 may be distinguished. These are the spring 

 wood, which is formed in the first part of the 

 growing season, and is composed of large thin- 

 walled wood cells, and the autumn wood, formed 

 in the latter part, and made up of thicker- walled 

 cells radially compressed. The spring wood is 

 consequently the weaker, lighter, and softer of 

 the two, since it is made up of cells which bear 

 this character. The chief function of the spring 

 wood is to conduct water from the roots to the 

 crown. The autumn wood, while to a certain 

 extent it also serves as a water-channel, has for 

 its chief function to strengthen the stem. The 

 larger the crown of a tree, and therefore the 

 larger the evaporating surface in the leaves, 

 the greater will be the demand for water, and^ 

 consequently the greater the need for water- 

 conducting organs. On the other hand, when 

 trees grow in crowded groups, the crowns are 

 smaller, the need for water-conducting organs 

 is less, and the proportion of autumn wood is 

 larger in the stem. The timber of the whole 

 tree is therefore denser, stronger, and far less 

 apt to rot. 



This view is fully confirmed by our observa- 



