348 The Physiology of Plants BOOK in 



been definitely ascertained. Nobbe and Hiltner infected 

 plants of Robinia with the germs of B. radicicola, and 

 cultivated them in solutions free from combined nitrogen. 

 When the tubercles were established on the roots it was 

 found that the plant could not appropriate nitrogen so 

 long as they remained submerged ; on being brought into 

 the air, the fixation of the atmospheric nitrogen began. 



The last contribution of the century to the subject was 

 made by Miss Dawson, who published a very complete 

 investigation into the structure and life-history of the organ- 

 ism in 1899 an d 1900. Confirming in the main the observa- 

 tions of Marshall Ward much earlier, she showed that the 

 hypha-like structures which penetrate to the tubercle are 

 strands of rodlets lying side by side in a homogeneous 

 matrix which contains neither cellulose nor chitin, and 

 which is probably zoogloeal in nature as Prasmowski sug- 

 gested. The bacteroids budded off from them are of 

 various shapes, often simulating the appearance of a V, 

 an X, or a Y. The authoress left it undecided whether 

 these forms are the results of branching or of the union 

 of two or more individuals. In some cases, in addition to 

 these, certain motile forms were observed. 



A very complete investigation of the process of infection 

 possessed by B. radicicola was made in the course of Miss 

 Dawson's experiments, and led her to the conclusion that 

 there is only one organism capable of forming nodules on 

 the roots of leguminous plants and working symbiotically 

 with them. She investigated a large number of genera 

 selected from all sections of the Natural Order. She held 

 the view, in opposition to Hellriegel and Wilfarth that the 

 difficulty of inoculating one genus with organisms from 

 another is probably due to special physiological peculiarities 

 of the several hosts and not to the organisms being specific 

 in each case. 



The existence of this form of symbiosis and its great 



