64 THE BTOllV 01? 1^1113 PLANTS. 



how necessary a part it is of every lire. Even 

 at present we are obliged to provide for its free 

 admission by the bars of the grate, and by 

 checking or regulating its ingress we can 

 slacken or quicken the burning of the lire. 



Or, to take another analogy, oxygen is just as 

 necessary to human beings and other animals as 

 food and drink are. But, as a rule, we get 

 oxygen everywhere in such great abundance 

 that we never think of taking it into practical 

 consideration. Still, in the Black Hole of Cal- 

 cutta, the unhappy prisoners thoroughly realised 

 the full value of oxygen, and would gladly have 

 paid its weight in gold for the life-giving 

 element. 



Now, carbonic acid, on which plants mainly 

 live, is not so common or so abundant a gas 

 as oxygen ; but still, it exists in considerable 

 quantities in the air everywhere. So most plants 

 are able to get almost as much as they need of 

 it. Nevertheless, submerged plants, and plants 

 that grow in very crowded places, seem to com- 

 pete hard with one another for this aerial food ; 

 and in certain cases they appear to live, as it 

 were, in a very Black Hole of Calcutta, so far as 

 regards the supply of this necessary material. 

 In farms and gardens, however, the farmer takes 

 care that every plant shall have plenty of room 

 and space — in other words, free access to sun- 

 light and carbonic acid. He " gives the plants 

 air," as he says, not knowing that he is really 

 supplying them with their aerial food- stuff. Ho 

 does this by keeping down weeds — by ploughing, 

 by digging, by hoeing, or tilling. Indeed, what 



