CHAP, vii NITROGEN REQUIRED BY PLANTS 59 



If we treat a piece of dried horse dung and a 

 piece of meat in the same way we shall in each case 

 obtain ammonia gas. 



We should expect to find that as plants require 

 nitrogen they would absorb this gas from the air, as 

 they do carbonic acid gas, through the leaves. 



Careful experiments, however, have shown that 

 they cannot do this, and have to obtain their 

 nitrogen in other ways. 



There is, however, one natural order of plants 

 which can take up nitrogen from the air in a very 

 curious way indeed. These plants belong to the 

 order leguminosse, and some of the commonest 

 varieties of the order are vetches, peas, lupins, etc. 



If we pull up one of these plants and examine 

 the roots we shall find them covered with little 

 lumps. On cutting through one of these lumps and 

 examining it under a powerful microscope, we find 

 it full of minute creatures, to which the name of 

 bacteria have been given. These bacteria are able 

 to absorb nitrogen gas, which surrounds the roots, 

 and make it up into such compounds as the plant 

 can absorb and use. 



In this way the plant is fed by these bacteria 

 from the free or uncombined nitrogen of the air. 



No other common crop plants have, however, so 

 convenient an arrangement, and, being unable to 

 obtain nitrogen direct from the immense stock of 

 free nitrogen in the air, they absorb the compounds 

 of nitrogen which are formed (in part by the action 



