122 AGRICULTURAL BACTERIOLOGY 



chemistry, that in some unknown manner the leguminous 

 plants were able to increase the content of the soil in nitro- 

 gen, and that they seemed to have sources of nitrogen that 

 were not open to other classes of plants. 



It had also long been known that there were commonly 

 on the roots of the leguminous plants, nodules or tubercles, 

 which were usually looked upon as galls similar to those 

 produced on many plants by the stings of insects, and by 

 other causes that stimulate the growth of the plant cells in 

 the immediate vicinity to which the stimulus is applied. 

 The tubercles were thought to be injurious, or at least not 

 of service to the plant. It had been found that the tu- 

 bercles contained bacteria. Hellriegel and Wilfarth, Ger- 

 man investigators, found, in their study of the ability of 

 different plants to grow in the absence of some one element, 

 that the non-leguminous plants were able to make but a 

 slight growth in the absence of combined nitrogen. Some 

 growth would always take place because of the content of 

 nitrogen in the seed, but when this was exhausted, the 

 plant would die of nitrogen starvation. When legumes 

 were studied, the results in some instances were identical 

 with those obtained with the non-legumes, while at other 

 times the legumes showed an ability to grow in the absence 

 of combined nitrogen in the soil. No prediction could be 

 made as to the outcome of any experiment when legumes 

 were used, as could be done with the other kinds of plants. 



These investigators found that the ability of the legumi- 

 nous plants to grow in the absence of combined nitrogen was 

 correlated with the presence of tubercles on the roots of the 

 plant. If the soil was sterilized, no tubercles appeared, 

 and the plant was unable to grow except when nitrogenous 

 fertilizers had been added to the nitrogen-free soil. A few 

 drops of the leachings from the soil on which the legume in 

 question had been grown was sufficient to induce tubercle 



