How PLANTS FEED. 169 



moment of sunshine in drinking in this, to us, poisonous 

 gas, and they do this so rapidly that there is actually rather 

 less of the gas close to the earth's surface, where it is gene- 

 rated, than there is higher up. 



One plant of colza-rape, for instance, will drink more 

 than two quarts of the gas in the course of one day's sun- 

 shine. An acre of beech-forest takes about three and a half 

 tons of gas, or one ton of solid carbon, every year; and if 

 the whole earth were covered with beech-trees, the supply of 

 gas would be altogether exhausted in about eight years, 

 supposing there were no means of renewing it. 



As, however, three-fourths of the globe are covered with 

 water, and as the vegetation of the fourth quarter is less 

 than a third of what it would be if covered with forest-trees, 

 the present supply would last a hundred years. 



It is the leaves, not the roots of plants which take up 

 carbonic acid ; for though the soil contains much, plants 

 can be grown and brought to perfection in water which is 

 quite free from it, provided, of course, the other food they 

 need be supplied to them; and they will be found to 

 contain quite as much carbon as those grown in earth. 

 They must therefore take it from the air, and as fast as 

 they remove it from their own immediate neighbourhood, 

 its place is supplied by more, in obedience to that law of dif- 

 fusion already mentioned ; so that we may imagine streams 

 of the gas to be constantly flowing towards every leaf and 

 blade. 



It must not be supposed that the breath and fires of 

 England necessarily feed only the English crops and trees, 

 for then what would happen during the winter months? 



