GROWTH BY THE BOOT. 19 



life: but, as in nature it is universally mixed with 

 various other substances, it conveys to the roots the 

 organisable matters that are required; and it fur- 

 nishes, by its decomposition, a considerable supply 

 of the oxygen consumed in the formation of carbonic 

 acid, and all the hydrogen that is incorporated in the 

 tissue of plants.* It has been proved, experimen- 

 tally, that plants cannot long exist upon pure water ; 

 but if they are so circumstanced as to be able to 

 obtain and decompose carbonic acid, they will grow 

 in the absence of other matters. It is only, however, 



* [Tha natural food of plants consists of water holding in solution 

 a quantity of carbonic acid and ammonia : tlie two former, viz. car- 

 bonic acid and water, are absolutely indispensable to the support of 

 vegetable life ; the latter is equally essential to complete develope- 

 ment and for the formation of particular organs or products. In 

 assimilating their food, vegetables extract hydrogen from the water, 

 and carbon from the carbonic acid; the oxygen of both being 

 restored (either wholly or in part, according to the kind of product 

 which is formed,) to the atmosphere from which both were derived. 

 The statement in the text, that water " furnishes, by its decomposi- 

 tion, a considerable supply of the oxygen consumed in the formation 

 of carbonic acid," is very ambiguous, if not altogether incorrect ; 

 since plants do not form, but decompose or consume carbonic acid ; 

 and they do not consume oxygen, except very partially, and in the 

 formation of some of their products ; for, even in the formation of 

 those products which contain most oxygen (such as the vegetable 

 acids) from their universal food, a portion of oxygen is liberated. 

 Water is not only an essential portion of the food of plants, inas- 

 much as it furnishes all the hydrogen they consume ; but also 

 the vehicle by which the other elements they require, nitrt^en, and 

 a certain (but variable and more less essential) quantity of saline or 

 earthy matters, are conveyed into their system. — ^We shall have 

 occasion to return to the consideration of this subject in a note upon 

 Chap. XX. G.] 



