FIRST REPLY. 147 



for respiration, which contains no nitrogen, in the form of 

 fat. But we do not arrive at the conclusion of our inquiries 

 here ; for now the question comes : What do plants live 

 upon? 



The reply to this question comprehends the subject of 

 the most lively discussion that has occupied science in recent 

 times ; it includes the theory of the most important mode 

 of applying his industry that Man has invented, namely 

 Agriculture. The correct solution of this question had 

 already been in part found by vegetable physiologists and 

 chemists, in the middle of the last century, and has been 

 subsequently undergoing more minute development by 

 particular individuals, but was first asserted with so much 

 liveliness and clearness in the present times by Liebig, that 

 an active and universal strife has been stirred up, which 

 will end in the general recognition of the true basis, and its 

 introduction as new-found letters into the ABC of 

 science. 



In the first place we must ask, what is the plant com- 

 posed of? Disregarding for the moment, as we did in 

 regard to the animals, the inorganic constituents, the earths 

 and salts, the answer has been already given in the fore- 

 going account of the two classes of substances. The body 

 of the plant is made up of constituents which contain no 

 nitrogen, namely of cellulose and vegetable jelly, which 

 have altogether similar composition with the other matters, 

 sugar, gum, and starch, and are only different from 

 the various fatty and waxy substances, in that the latter 

 contain a smaller proportion of oxygen. But besides these, 

 the plant requires nitrogenous matters ; not indeed to form 

 part of its frame, but to give rise to those chemical processes, 

 through which the transformation of the nutrient matter 

 which has been taken up, is effected. The inquiry into 



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