LETTER VII. 



65 



mer especially, when in full leaf), from its greater size 

 and breadth, enhancing the power of the wind, — and, 

 on the other, from the more extensive decay of the 

 heart-wood, making it less able to withstand the agency 

 of the wind. 



16. Moreover, the nourishment existing in the soil 

 comes often to be exhausted, and even that supplied 

 by the atmosphere to be rendered unavailable. Of 

 this, plantations furnish us with continual examples. 

 The forester or the wood-merchant may tell us that 

 certain sorts of trees become ripe " after seventy, 

 eighty, or ninety years, and after that do no more 

 good. They thus speak of trees growing together in 

 a wood — of Firs, for example. So far, or in a certain 

 sense, they speak truly. And the explanation of the 

 thing is, that the trees so circumstanced have by that 

 time extracted from the soil all the nutriment it can 

 yield them, and particularly the saline matters (ashes) 

 that are essential to their growth. And you must 

 yourselves have often noticed (in plantations especially 

 that are densely crowded, and have not been duly 

 thinned), that such trees are wholly, or at least almost 

 quite destitute of branches, — the side plants, that is 

 to say, having either been stunted in their growth, or 

 having altogether died out. And this happens, because, 

 although the air (from the carbonic acid of which they 

 derive a large part of their nourishment), can gain 

 access to them, the light and the heat of the sun, with- 



E 



