174 THE WORLDS LUMBER ROOM. 



naturally more abundant in the air of towns and cities, 

 the supply from this source falls far short of the demand. 

 For all plants require nitrogen (and without it those used 

 for food would lose their nutritive qualities): yet, though 

 there is an unlimited supply of it in the air, they are unable 

 to take it up with their leaves, except in the compound 

 forms of nitric acid (nitrogen, hydrogen, and oxygen) and 

 ammonia (nitrogen and hydrogen). 



From two to twenty-one pounds to the acre are washed 

 down from the air on an average every year ; but a single 

 acre of clover-hay requires about 108 pounds, and in 

 twenty-eight bushels of wheat there are 4$J pounds of 

 nitrogen. The remaining quantity is, therefore, derived from 

 the decay of animal or vegetable matter naturally present in 

 or added to the soil.* 



Vegetables, then, derive all their nourishment from the 

 air and from the soil, and convert gases and minerals into 

 food for the animal creation, which is altogether dependent 

 upon them for its means of subsistence. 



But there is one group of vegetables which are like ani- 

 mals in this respect, that they must have animal or vegetable 

 matter to feed on. These are the fungi, and wherever they 

 are seen there we may be sure is some decomposing or- 

 ganic matter. 



Fungi do not contribute to the purification of the air 

 by taking up carbonic acid, for they resemble animals in 

 their way of breathing, as well as of feeding ; but they are 

 scavengers, and some of them, such as the mushroom, are 

 very perfect ones, since they convert refuse into wholesome 

 * Chapter VII. 



