10 THE WHITE PINE 



Upon trees fifty years old the average is 13 per 

 cent. Upon very young trees the bark is thin 

 and tender, and easily injured, especially by 

 fire. 



When a Pine is first cut there is little or no 

 difference in the color of the sap- and the heart- 

 wood. But upon exposure the heart- wood be- 

 comes reddish-brown, because of the oxidation 

 of foreign matter stored in the walls of the 

 wood cells. The separation between the heart- 

 and the sap-wood is, however, clearly distin- 

 guishable on a White Pine stump, on account of 

 the large amount of pitch which oozes from 

 the sap-wood almost immediately after the tree 

 is felled. 



The number of annual rings in the sap-wood 

 increases with the age of the tree. The follow- 

 ing table shows the average for the trees mea- 

 sured in Pennsylvania : 



It has been said that trees in the open have a 

 broader sap-wood at the same age than those 



