134 



The Great World's Farm 



body, of which every part is dependent upon the rest. 

 But while the roots can do their work in the dark, the 

 leaves are perfectly helpless without light. 



Give the plant light, however, together with the proper 

 food which the roots collect from the soil, and then the 

 leaves have no difficulty in adding the carbon which is 

 their share. 



And why, it may be asked, should they have any diffi- 

 culty? Since the whole plant wants it, and has to get it 

 through the leaves, surely it would be more strange if the 

 leaves could not find it, since they live in the air, where it is. 



Perhaps; but the proportion in the air is extremely 

 small, though the amount sounds large; and leaves can- 

 not wander in search of food, as roots do. The food 

 must come to them, as they cannot go to the food. Car- 

 bon exists in the air, combined with oxygen, as the gas 

 carbon dioxide, or carbonic acid; and there are about 

 three billion four hundred million tons of the gas in the 

 atmosphere of the whole globe. The figures convey little 

 to one's mind, but at all events, the amount sounds com- 

 fortably large — sufficient, at least, to preserve the vege- 

 table world from all risk of a dearth of this species of food. 



And yet it has been calculated, that if used at the 

 present rate, the whole of this enormous supply would be 

 exhausted in about a hundred years, after which not so 

 much as a blade of grass could exist until the supply were 

 renewed. 



Let us put it in another way. The amount is large in 

 itself, but it is enormously diluted — so much diluted, 

 indeed, as to be hardly reckoned at all; that is, in speak- 

 ing of the air, we commonly say that it consists of about 

 four-fifths nitrogen and one-fifth oxygen, leaving the car- 



