EXCREMENTS OF PLANTS. 117 



red to, which have shown that plants have the power 

 of returning to the soil all substances unnecessary 

 to their existence ; and the conclusion to which all 

 the foregoing facts lead us, when their real value and 

 bearing are apprehended, is that the alkaline bases 

 existing in the ashes of plants must be necessary to 

 their growth, since if this were not the case they 

 would not be retained. 



The perfect development of a plant, according to 

 this view, is dependent on the presence of alkalies 

 or alkaline earths ; for when these substances are 

 totally wanting its growth will be arrested, and when 

 they are only deficient it must be impeded. 



In order to apply these remarks, let us compare 

 two kinds of trees, the wood of which contains une- 

 qual quantities of alkaline bases, and we shall find 

 that one of these grows luxuriantly in several soils 

 upon which the others are scarcely able to vegetate. 

 For example, 10,000 parts of oak-wood yield 250 

 parts of ashes, the same quantity of fir-wood only 

 83, of linden-wood 500, of rye 440, and of the herb 

 of the potato-plant 1500 parts.* 



Firs and pines find a sufficient quantity of alkalies 

 in granitic and barren sandy soils in which oaks will 

 not grow ; and wheat thrives in soils favorable for 

 the linden-tree, because the bases which are neces- 

 sary to bring it to complete maturity, exist there in 

 sufficient quantity. The accuracy of these conclu- 

 sions, so highly important to agriculture and to the 

 cultivation of forests, can be proved by the most 

 evident facts. 



All kinds of grasses, the Equisetacece, for exam- 

 ple, contain in the outer parts of their leaves and 

 stalk a large quantity of silicic acid and potash in 

 the form of acid silicate of potash. The proportion 

 of this salt does not vary perceptibly in the soil of 

 corn-fields, because it is again conveyed to them as 

 manure in the form of putrefying straw. But this is 



• Berthier, Annates de Chimie et de Physique^ t. xxx. p. 248. 



