THE GRF.ENn'OOD TREE. 147 



takes into account practically this most important factoi* 

 in human and animal life. Wo toil for hruacl, but wo 

 ignore tho supply of oxygen. And why ? Simply because 

 oxy'.;en is universally diffused everywhere. It costs 

 nothing. Only in the Black Hole of Calcutta or in a 

 broken tunnel shaft do men ever begin to find themselves 

 practically short of that life-sustaining gas, and then they 

 know the want of it far sooner and far more sharply than 

 they know the want of food on a shipwreck raft, or the 

 want of water in the thirsty desert. Yet antiquity never 

 even heard of oxygen. A prime necessary of life passed 

 unnoticed for ages in human history, only because there 

 was abundance of it to be had everywhere. 



Now it isn't quite the same, I admit, with the 

 carbonaceous food of plants. Carbonic acid isn't quite 

 so universally distributed as oxygen, nor can every plant 

 always get as much as it wants of it. I shall show by- 

 and-by that a real struggle for food takes place between 

 plants, exactly as it takes place between animals ; and 

 that certain plants, like Oliver Twist in the workhouse, 

 never practically get enough to eat. Still, carbonic acid 

 is present in very large quantities in the air in most 

 situations, and is freely brought by the wind to all the 

 open spaces which alone man uses for his crops and 

 his gardening. The most important element in the food 

 of plants is thus in effect almost everywhere available, 

 especially from the point of view of the mere practical 

 everyday human agriculturist. The wind that bloweth 

 where it listeth brings fresh supplies of carbon on its 

 wings with every breeze to the mouths and throats of 

 the greedy and eager plants that long to absorb it. 



L 2 



