380 LEGUMES 



were well known to these early writers, as (1) clover needs a lime- 

 stone soil ; (2) that land may become " clover sick " ; (3) and that 

 it increased production of the soil. 



Several theories were advanced as to why clover improved the 

 soil. Some thought it to be deeper rooted. Liebig (1840-1850), 

 a German chemist, thought all plants took nitrogen from the air 

 through the leaves, and legumes, owing to the broader leaves, took up 

 much more than grasses. 



From this time on various scientists investigated the problem, 

 but it was not until 1886 that Hellriegel, another German chemist, 

 was able to explain the matter. Among his later experiments the 

 following may be noted : 



When he planted peas in pots filled with sterile soil, they would 

 germinate, grow a short time, and die, though occasionally one would 

 live and do well. He then took forty pots and filled them with ster- 

 ilized soil, but ten of these he wet with leachings from a fertile soil. 

 In these ten pots the peas did well, but twenty-eight out of the thirty 

 remaining died. On the plants that lived he found root nodules. 

 He concluded that the nodules were necessary for growth. As it had 

 earlier been proved that these nodules contained bacteria, it was soon 

 established that the bacteria caused the nodules, and that it was 

 through the agency of these bacteria that nitrogen was taken from 

 the air. 



The bacteria are minute, rod-shaped bodies called Bacillus 

 radicicola. They may live in the soil for many years. When the 

 bacteria come in contact with a legume root, they enter the root hairs. 

 This irritation causes the growth of a nodule, enclosing the bacteria, 

 which multiply rapidly. The bacteria take up nitrogen from the air 

 and supply it to the plant, while the plant furnishes other food matter 

 for the bacteria. The plant and bacteria are, therefore, mutually 

 helpful to each other, and this relation is called symbiosis. 



Forms for Different Legumes. While the same species of 

 bacteria is apparently found on all legumes, yet there is some slight 

 difference in the forms, as the bacteria for one kind of legume will 

 not inoculate another variety of legume except in a few cases. Clover 

 is not benefited by the bacteria from peas or alfalfa. However, 

 alfalfa, sweet clover, and burr clover do have the same form, and 



