WORKING OF GREEN LEAVES 



countries, such as Great Britain, the cultivation of plants is 

 still the largest national industry. In 1897 we grew enough 

 corn to give a ration of lib. per diem to every inhabitant 

 for 68 days, and we manage to get a large amount from 

 every acre (28 to 33 bushels per acre). In most other 

 countries the relative importance of land and of agriculture 

 generally is very much greater than it is in Britain. 



Moreover, it seems at first sight as if all this harvest had 

 been made out of nothing at all. Plants do take in a small 

 amount of mineral matter from the earth, but these minerals 

 form but a very little part of the bulk of a tree or any 

 vegetable substance. 



A piece of wood can be burnt up in a fire and very little 

 indeed of it is left. A few ashes will indeed remain, which 

 are the minerals taken in from the earth, but all the rest has 

 vanished into the atmosphere. The water which was con- 

 tained in the wood has become steam and is evaporated ; the 

 woody matter consisted chiefly of compounds of a chemical 

 substance, carbon, which also becomes an invisible gas 

 (carbonic acid gas) in a fire and goes back into the atmo- 

 sphere. 



When the piece of wood was formed in a growing tree, it 

 is easy to see where the water came from : it was taken in by 

 the roots. Just as flowers drink up the water in a vase, and 

 wither if they do not receive enough, so all plants suck up 

 water by their roots. The carbonic acid gas is taken into 

 plants through their leaves and is worked up into sugar, 

 starch, wood, and other matters inside the plant. 



But there is another very interesting point about the way 

 in which wood is burnt in a fire ; heat and light are obtained 

 from a wood fire. Where did that heat and light come 

 from ? 



i6 



