Leaves and their Work 161 



compounds to which plants and flowers owe their 

 fragrance. 



And this carbon the leaves have no difficulty in pro- 

 viding, so long as the roots do their part ; but if they 

 fail, the leaves must fail too. For the plant is a whole, 

 a 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. 



Cfive 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 

 difficulty ? 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 

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

 food must come to them, as they cannot go to the 

 food. Carbon 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 comfortably large sufficient, at 

 least, to preserve the vegetable 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 



II 



