138 PRINCIPLES OF SOIL MICROBIOLOGY 



differences between the members of the population, physiological reac- 

 tions and the even more sensitive serological reactions can bring out 

 these variations. This explains the modification of a strain when 

 grown on artificial culture media or as a result of repeated passage 

 through the host plant. It also suggests the possibility of improving 

 or deteriorating a strain by the proper selection of the types of cell. 

 This phenomenon explains the increase in activity and fixation of nitro- 

 gen by repeated passage through plants. 168 The process of adaptation 

 to a particular host plant is longer in case of vegetatively weak organ- 

 isms than for vegetatively strong organisms. 



A detailed study of the chemistry of nitrogen fixation by nodule 

 bacteria (588) and the artificial inoculation of soil with bacterial cul- 

 tures (817) will be discussed elsewhere. 



Nodule formation by non-leguminous plants. In addition to the 

 legumes, a number of non-legumes are found possessing nodules on their 

 roots. Of these, most attention has been paid to Ceanothus (red-root) , 

 Elaeagnus (silver berry), Alnus (alder), Podocarpus, Cycas and Myrica 

 (sweet gale). 



At first these nodules were thought to be of fungus origin. The 

 nodules of Alnus, Elaeagnus and Ceanothus were found 169-171 to be 

 caused by bacteria belonging to the Bad. radicicola group and capable 

 of fixing nitrogen. In some plants at least (Myrica) the organism is of 

 the nature of an Actinomyces. 172 Coriaria japonica produces nodules 

 similar to those produced by the Alder, due probably to an Actinomyces 

 (Act. myricae according to Peklo) in both cases. 173 In the roots of 

 cycads, Bad. radicicola, Azotobacter and an alga ( Anabaena) were demon- 



168 Wunschik, H. Erhohung der Wirksamkeit der Knollchenerreger unserer 

 Schmetterlingsblutler durch Passieren der Wirtpflanze. Centrbl. Bakt. II, 64: 

 395-445. 1925. 



169 Hiltner, L. Uber die Bedeutung der Wurzelknollchen von Alnus glutinosa 

 fur die Stickstoffernahrung dieser Pflanze. Landw. Vers. Sta. 46: 153-161. 1896. 



170 Kellerman, K. F. Nitrogen-gathering plants. Yearb. Dept. Agr. U. S. A., 

 1910, 213-218. 



171 Bottomley, W. B. The root nodules of Ceanothus americanus. Ann. Bot. 

 29: 605-610. 1915. 



172 Arzberger, E. G. The fungous root-tubercles of Ceanothus americanus, 

 Elaeagnus argentea, and Myrica cerifera. Mo. Bot. Gard. 21 Ann. Rpt. : 60-103, 

 1910. 



173 Shibata, K., and Tahara, M. Studien uber die Wurzelknollchen. Bot. 

 Mag. Tokyo, 31: 157-182. 1917. 



