NOT INDISPENSABLE FOR PLANTS. 33 



carbon in this form, than its decaying humus had formerly lost 

 in carbonic acid. 



Plants do not exhaust the carbon of a soil in the normal con- 

 dition of their growth ; on the contrary, they add to its quantity. 

 But if it be true that plants give back more carbon to a soil than 

 they take from it, it is evident that the amount of carbon which 

 is removed in any shape in the crop must have been derived from 

 the atmosphere in the form of carbonic acid. It is well known 

 that springs occurring in gardens of the richest vegetable mould, 

 furnish clear and perfectly colorless water destitute both of 

 humus and of salts of humic acid. It is likewise known that 

 humates cannot be detected in the springs of meadows, in the 

 waters of our rivers, or even in acidulous mineral waters, 

 although they contain a considerable quantity of alkaline salts. 

 Now a simple consideration of these facts proves to us either that 

 the richest vegetable mould is free from humic acid, or that this 

 acid cannot be absorbed by plants through the agency of water. 

 Hence it follows that the common view of the action of humus is 

 erroneous. The water resting upon a meadow is found to be 

 uch in carbonic acid and alkaline bases. Well-water also gene- 

 rally contains much of the former ingredient. The influence, 

 then, of humus or decaying vegetable matter upon vegetation, is 

 explained by these facts in the most clear and satisfactory man- 

 ner. Humus, therefore, does not nourish plants by being assimi- 

 lated in its soluble state, but by furnishing a gradual and conti- 

 nued source of carbonic acid. This gas forms the chief means of 

 nourishment to the roots of plants, and is constantly formed anew 

 as long as the soil admits the free access of air and moisture, 

 these being the necessary conditions for effecting the decay ot 

 vegetable matter. 



The verdant plants of warm climates are very often such as 

 obtain from the soil only a point of attachment, and are not de- 

 pendent on it for their growth. How extremely small are the 

 roots of the various species of Cactus,* Sedum, and Sempervivum, 



* The Cactus" was probably introduced into Sicily by the Spaniards. It 



forms as important an article of diet with the inhabitants of that island as 



the potatoe does with ourselves. This abundant, cooling, and juicy fruit 



fcrms the principal food of the lower classes for three months, and is con- 



3* 



