GROWTH 11 



in dense forests. That it is absolutely wider in 

 such cases is usually true. But the diameter 

 also is apt to be greater, and the proportion 

 between its width and that of the sap-wood 

 is not greatly changed. In general it is true 

 that the percentage of the whole width in- 

 cluded in the sap-wood decreases with the 

 increase in diameter. The following table 

 gives the average values for trees over 100 

 years old: 



The decrease from 15.1 per cent, at ten inches 

 diameter to 4.8 per cent, at forty inches is strik- 

 ingly regular. 



Still more interesting and far more important 

 is the relation of the sap-wood to the total vol- 

 ume of wood in the stem. We find that, with 

 increase in the diameter of a tree, the percen- 

 tage of sap-wood falls off. The following table 

 gives the average values obtained for trees over 

 100 years old : 



