TREES. 



93 



These may be taken as the measurement of trees at their full 

 growth, and fit for felling. The soil being of the same quality, the 

 dimensions of trees depend especially upon their age ; individual 

 trees of the same species, however, occasionally acquire extraor- 

 dinary dimensions. 



Every one must have noticed the rapidity with which young trees 

 grow ; but is the growth the same for every period of the existence 

 of trees, or do they attain a certain determinate size like animals, 

 and then cease from further increase ? We have found that in 

 those climates where vegetation is suspended for a portion of the 

 year, the increase in the diameter of trees takes place periodically 

 by the addition of a concentric layer of woody tissue ; so that it is 

 possible to determine the age of a dicotyledonous tree by the number 

 of its concentric rings, counted at the bottom of the trunk. With a 

 view to ascertain the amount of increase in the woody layers at dif- 

 ferent periods of vegetable life, De Candolle measured their thick- 

 ness, and found that if the annual increase presented a certain re- 

 gularity, it was still very far from being absolute even in the case 

 of a single species. The oak especially offered striking anomalies ; 

 thus a trunk which had grown slowly in diameter was found to have 

 increased more rapidly as it got older. He found young trees ol 

 the same species, the growth of which, very slow at first, by and 

 Dy became accelerated, and then fell off in a third period of their 

 existence. From the whole of his observations, De Candolle 900- 



