On the Application of Manures. 



257 



So also does the juice of the fresh tobacco leaf^ and that which 

 exudes from the vine, when an incision is made into it during the 

 period that the sap is flowing. 



Its occurrence in short in the juices of plants seems not less 

 certain than its existence in the atmosphere, and there can be 

 little doubt that it is the decomposition of this compound which 

 mainly supplies the nitrogen present in the constitution of organic 

 bodies. 



It may be inferred, however, from some experiments made by 

 Boussingault, that a great difference exists between plants in their 

 power of assimilating nitrogen, and to this difference that chemist 

 is disposed to attribute the advantage of alternately growing what 

 are called fallow-crops, for the purpose of refreshing the soil. 



'•'During germination," he remarks, ''^the quantity of azote 

 which seeds contain appears to be on the increase, but there is 

 this curious difference between different kinds, that whilst those 

 of leguminous plants, sown in pure earth and moistened with 

 nothing but distilled water, obtained an increase of nitrogen 

 which the atmosphere alone could have afforded, those of barley 

 and other cerealia remained in that respect stationary, unless 

 manure were afforded." 



Boussingault also shows, in a subsequent memoir, that peas, 

 clover, and other legumes, absorb azote, even when planted in a 

 soil that contains no decomposing animal or vegetable matter, but 

 that the cerealia, although, if so placed, they may grow, do not 

 appear to secrete this principle. 



Boussingault, however, does not go so far as to maintain that 

 the latter in no stage of their existence are capable of discharging 

 this function, but only that the plant must have already arrived 

 at a higher state of vigour, in order to derive its supply from 

 such a source. 



It is on the same principle, that although the animal in general 

 obtains its food from the various organic bodies on which he 

 subsists, yet that in an early stage of existence, before his organs 

 are fitted for undergoing the labour of assimilating such materials, 

 nature has provided him in his mother's milk with aliment al- 

 ready almost elaborated. 



It is thus, too, that in the seed the embryo is surrounded with 

 a mass of albumen, from which it derives its support, until its 

 roots become sufficiently vigorous to extract nourishment from the 

 ground. 



Hence it becomes in most cases necessary that crops cultivated 

 as articles of food should have access to vegetable or animal 

 manure from which they may derive their azote ; but as this supply 

 would soon be exhausted, were it not at the same time regenerated 

 from the atmosphere, we see the advantage of intercalating a green 



