164 



ROTATION OF CROPS. 



with their sap and wood, we find that they differ essentially from 

 each other, both in their composition and characters. 



True wood yields only one-fourth to two per cent, of ashes, 

 while the bark of the oak, fir, willow, and beech, gives 6, 10, to 

 15 times more. The ashes of wood and of the bark have a very 

 different composition. The inorganic ingredients of the bark 

 are obviously inorganic substances expelled by the living or- 

 ganism. 



The same reasoning holds good in the case of the organic sub- 

 stances as it does in the case of the bark. The bark of the cork- 

 tree contains nearly half its weight of fats, or of fatty substances, 

 which we also find present, although in smaller proportion, in the 

 bark of firs and pines. The solid material (insoluble in alcohol 

 or ether) of these barks is entirely different from woody fibre. 

 The barks of firs and pines are completely soluble in potash leys, 

 forming a liquid of a dark brown color, which yields, on the ad- 

 dition of an acid, a precipitate strongly resembling the substance 

 called humic acid. But wood is not attacked by potash ley. 



These barks are in so far true excrements, that they arise from 

 living plants, and play no further part in their vital functions ; 

 they may even be removed from them, without thereby endanger- 

 ing their existence. It is known that certain trees throw off an- 

 nually their barks : this circumstance, viewed in its proper light, 

 shows that, during the formation of certain products formed by the 

 vital processes, materials arise which are incapable of experien- 

 cing a further change. 



^co-oo 



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