LEGUMES IN GENERAL 247 



It is very interesting to know that while the soil-enrich- 

 ing value of the legumes was known soon after the dawn 

 of the Christian era, it has taken almost 1900 years to 

 discover to what this peculiar virtue is due. 



246. Bacteria in relation to legumes. — The exact 

 relation that exists between the bacteria in the nodules 

 and the host plant is not definitely known. Present 

 information on this subject, however, shows that the 

 plant and the bacteria enter into a partnership, the result 

 of which is mutual benefit. This relationship is called 

 sjTubiosis. The bacteria which are in the soil enter the 

 plant root through the root hair and work their way 

 further into the root, which, because of their presence, 

 makes an abnormal growth, forming a nodule. When 

 once in the root, the bacteria rapidly increase in numbers 

 by division, and as they increase the nodule enlarges. 

 The mutual benefit of this partnership is derived from 

 barter, in which the bacteria trade nitrogen to the plant, 

 in return for all of the other elements necessary to their 

 growth. The advantage of this partnership is to be found 

 in the fact that the plant is unable to use the free nitrogen 

 of the air, while the bacteria draw it in large amounts 

 from this source and use it to build up their own structure, 

 and when they die the nitrogen from their decomposed 

 bodies becomes available to the plant. It seems that the 

 greater percentage of the bacteria are not long lived, but 

 soon die, supplying the plant with this necessary element 

 of plant food. Support for this belief is to be found in the 

 fact that the nodules are smaller, softer, and nearly empty 

 of bacteria at the end of the growing season. However, 

 not all of them die. Some few remain alive, and as the 

 roots decay, find their way back into the soil until another 

 plant comes forth with which to form a new partnership. 



