201 



graphs of the roots of soy 



bean, alfalfa, and Canada field pea are 

 shown in Figs. 30, 31, and 32. The 

 nodules on the roots are of the nat- 

 ural size. It is seen that the size 

 and the shape of the nodules vary 

 with the different 

 certain conditions 

 ules may develop. 



legumes. Under 

 very large nod- 

 In Fio-. 33 are 



shown roots of the Canada field pea 

 grown in a loamy soil, the nodules 

 being of the natural size. 



Another point of interest, espe- 

 cially to farmers, is the fact that 

 leguminous crops seem in some way 

 to add a little fertility to the soil on 

 which they are grown. For many 

 centuries past, farmers have ob- 

 served that non-leguminous crops, as 

 wheat, corn, potatoes, and the like, 

 grown on land on which clover or 

 some other legume was raised the 

 year before, invariably gave a better 

 yield. It was not understood at 

 first, but scientific study in recent 

 years has shown that legumes may 

 add a certain amount of nitrogen to 

 the soil. 



There are, then, three features 

 that distinguish leguminous crops 

 from other crops : 



1. Legume crops are very rich in 

 nitrogen. 



2. Legumes have nodules on the 

 roots. 



3. Legumes add fertility to the 

 soil. 



NODULES CAUSED BY BACTERIA. 



Fig. 31.— Boot of alfalfa, show- If an extremely thin slice is cut 

 ing nodules. Natural size. from one of the nodules and is mag- 

 nified under the microscope about 

 one thousand times, a large number of little rod-like bodies can 

 be seen. Some of them are sausage-like in shape, and others 

 may send out short outgrowths so that they are often called X 

 and Y forms. These bodies are bacteria. In Fig. 34 are shown 

 a few of the forms of the bacteria found in the legume nodule. 

 They are living plants and, like other living organisms, thev can 



