6o THE STORY OF THE PLANTS. 



live, is not so common or so abundant a gas as 

 oxygen ; but still, it exists in considerable quanti- 

 ties 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 compete 

 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 sunlight and carbonic 

 acid. He " gives the plants air," as he says, not 

 knowing that he is really supplying them with 

 their aerial food-stuff. He does this by keeping 

 down weeds — by ploughing, by digging, by hoe- 

 ing, by tilling. Indeed, what do we really mean 

 by cultivation ? Nothing more than destroying 

 the native vegetation of a place, in order to make 

 room for other plants that we desire to multiply. 

 We plough out the grasses and herbs that occupy 

 the soil ; we sow or plant thinly seeds or cuttings 

 of corn or vines or potatoes that we desire to 

 propagate. We give these new plants plenty of 

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

 light and carbonic acid. And that is the funda- 

 mental basis of cultivation — to keep down certain 

 natural plants of the place, in order to give free 

 room to others. 



But as the crop-plants require to root them- 

 selves, the farmer naturally thinks most x)i the 

 soil they root in — which he has to buy or rent, 

 while the carbonic acid comes freely to him, un- 

 perceived, with the breath of heaven. Where 

 water is scarce, as in irrigated desert lands, the 



