/4 GENERAL TONCLUSIONS. 



is capable of entering into the circulation of living plants — and of yield- 

 ing to them, in whole or in part, the nitrogen they contain. 



But here, again, as in die case of ammonia, we are at fault in regard 

 to the quantity of nitrogen which plants in a state of nature actually 

 derive from nitric acid or the nitrates. The compounds of this acid with 

 potash, soda, lime, and magnesia (the nitrates of th'jse substances), are 

 all very soluble in water. The quantity of this fluid, therefore, which 

 enters by tlie roots of plants, could easily convey into their circulation 

 far more of these nitrates than would be alone sufficient to supply the 

 whole of the nitrogen they require — for the formation of all their parts 

 and products. But so it might of ammonia or its salts, as has already 

 been shown. I shall hereafter lay before you certain considerations 

 which may probably lead us to approximate conclusions in regard to 

 the relative influence exercised by these two compounds on the general 

 vegetation of the globe. 



Conclusions. — Respecting tte form in which nitrogen enters into the 

 circulation of plants, we have therefore, I think, fairly arrived at these 

 deductions: 



1°. That the nitrogen of the atmosphere may, to a^mall extent, enter 

 directly into the living vegetable either in the form of gas or in solution 

 in water, but that supposing nitrogen to be in this way appropriated* by 

 the plant, the quantity so taken up could form only a small quantity of 

 that w^hich vegetables actually contain. 



2°. That ammonia is capable of entering into plants in very large 

 quantity, and of yielding nitrogen to them, and that in European agri- 

 culture, which employs fermenting animal manure as an important 

 means of promoting vegetable growth, it does appear to yield to cultiva- 

 ted plants a considerable portion of the nitrogen they contain. 



3°. That nitric acid, in like manner, is capable of entering into and 

 giving up its nitrogen to plants; and that where this acid is employed as 

 an instrument of culture, the crops obtained owe part of their nitrogen 

 to the quantity of this compound which has been applied to the grow- 

 ing plants. The same inference may fairly be drawn in regard to the 

 effect of nitric acid — when, in the form of nitrates, it exists or is pro- 

 duced naturally in the soil. 



4°. That other compound bodies, such as are contained in urine, or are 

 produced during the decay of animal matter, niay also enter into the 

 circulation of plants, and yield nitrogen to promote their growth. 



On the whole, however, there seem strong reasons for believing that 

 plants are mainly dependent on ammonia and nitric acid for the nitro- 

 gen they contain ; and that they obtain it most readily, and with least 

 labour, so to speak, from these compounds, — though nature has kindly 

 fitted them for deriving a stinted supply from other sources, when these 

 substances are not present in sufficient abundance. 



How far each of these compounds is employed by nature, as an in- 

 Btrutnent in promoting the general vegetation of the globe, will be con- 

 sidered in a subsequent lecture. 



• Lieblg and others say that plants are incapable of appropriating or assimilating the nitro 

 gen which enters into their circulation in the simple state. We shall const ler this ques- 

 tion hereafter. v 



