154 ECONOMICS OF FORESTRY. 



species ; then follows the even rate of the adult virile, 

 or young-timber period, during which maturity and 

 frequent seed production absorb part of the energy 

 until the maximum height is reached, and in the 

 senile or old-timber stage height growth stops alto- 

 gether. The virile stage is of most uneven length, 

 and here the " law of the lever " asserts itself often : 

 those which grow most rapidly in their youth, 

 as a rule, cease soonest to exert themselves, while 

 the slow growers are persistent and finally over- 

 tower the rapid ones. 



The diameter growth proceeds slowly until a fully 

 formed crown and root system can elaborate the 

 material to be deposited along the bole in annual 

 layers. As these conditions improve during the 

 adolescent period, so does the rate of diameter 

 growth increase and the maximum rate does not 

 occur until the fortieth to eightieth year, then very 

 evenly declining into late life ; but the area of a 

 cross-section taken in any part of the bole, usually 

 breast high, increases a considerable time after the 

 diameter rate has begun to sink, as mathematical 

 reasoning requires, the* deposit each year being 

 made on a larger periphery. 



Of greatest economic interest is the form devel- 

 opment of the bole, which depends upon the man- 

 ner in which the wood is deposited over the 

 previous year's deposits. In well-fed trees, with 

 fully developed crowns, standing in the open, so 

 much food is elaborated that the lower portions 



