136 THE ART OF CULTURE. 



the diffused light of day, as well as in the direct 

 solar rays ; in short, all actions of a similar kind 

 proceed in the same way in diffused light as well 

 as in the solar light, the only difference consisting 

 in the time in which they are effected. It cannot 

 be otherwise in plants, for the mode of their nutri- 

 ment is the same in all, and their component 

 substances afford proof, that their food has suffered 

 absolutely the same change, whether they grow in 

 the sunshine or in the shade. 



All the carbonic acid therefore which we supply 

 to a plant will undergo a transformation, provided 

 its quantity be not greater than can be decomposed 

 by the leaves. We know that an excess of carbonic 

 acid kills plants, but we know also that nitrogen, 

 to a certain degree, is not essential for the decom- 

 position of carbonic acid. All the experiments 

 hitherto instituted, prove that fresh leaves placed in 

 water, impregnated with carbonic acid, and exposed 

 to the influence of solar light, emit oxygen gas, 

 whilst the carbonic acid disappears. Now, in these 

 experiments no nitrogen is supplied at the same 

 time with the carbonic acid ; hence no other con- 

 clusion can be drawn from them, than that nitrogen 

 is not necessary for the decomposition of carbonic 

 acid, for the exercise, therefore, of one of the 

 functions of plants. And yet the presence of a 

 substance containing this element appears to be 

 indispensable for the assimilation of the products 

 newly formed by the decomposition of the carbonic 



