OBJECTIONS TO LIEBIg's THEORY. 149 



mould, and with it, the fertility, proves conclusively, that plants 

 derive their carbon from other sources than carbonic acid. 

 " A seed germinates in a soil in which no vegetable matter 

 exists; it sprouts vigorously, increases then slowly, grows 

 languidly at the expense of the air ; and the plant dies stinted 

 or immature."* 



6. Finally, it appears upon a general view of the subject, 

 that although carbonic acid is absorbed by the leaves and 

 roots of plants, and is a source of a large quantity of their 

 carbon, yet other substances, rich in carbon, must enter 

 the roots of plants, must be decomposed, and their carbon 

 assimilated. 



The fact, that the atmosphere contains carbon in sufficient 

 quantities to supply the whole vegetation of the globe with it, 

 does not prove it the only source. The fact, that the atmos- 

 phere is not, in time, filled with this acid, does not show that 

 plants must derive all their carbon from it, or else its purity 

 would be destroyed. There is a vast ocean of water which 

 is constantly absorbing carbonic acid. Growing vegetables are 

 acknowledged by all, to decompose a large quantity of it, and 

 thus to contribute to the purity of the atmosphere. And this 

 process is truly for a sublime purpose ; but yet, the necessity 

 of deriving carbon from the humus of the soil, is not, on this 

 account, wholly dispensed with. 



This theory, then, must not be received in the absolute 

 sense, but only as showing, in a strong light, the principal 

 source of the carbon of plants ; while the nature of humus is 

 such, according to the theory itself, as to furnish abundant 

 ground for the opinion, that a part of the carbon is derived 

 from that source. Although it may be true, that the whole of 

 the carbon was originally derived from the atmosphere, it is 

 not true that any single crop derives the whole, directly, from 

 this source. ,.;,< . - 



* Johnson's. Lectures. 



13 



