BACTERIA FIXING ATMOSPHERIC NITROGEN 139 



strated. 174 Burrill and Hansen 175 came to the conclusion that the 

 root-nodules of Ceanothus (C. americanus), Alnus, Cycas revoluta, and 

 Myrica are not caused by Bad. radicicola. The evidence that Elaeagnus 

 and Podocarpus nodules are caused by B. radicicola is not conclusive. 

 It is still questionable whether nitrogen fixation by any of these plants 

 takes place, 176 although it is claimed 176 * that some plants (like Casua- 

 rina) are thus able to grow readily in very poor sandy soil. The ques- 

 tion of symbiosis with fungi (mycorrhiza formation) is discussed else- 

 where. 



It is of interest to point out, in this connection, that there are legumi- 

 nous plants, which do not form any nodules. These include Gymno- 

 cladus, Carcis, Gleditsia and the Cassias of the subfamily Caesal- 

 pinaceae. 



Nodule formation in the leaves of some plants. A condition similar to 

 nodule formation by bacteria on the roots of leguminous plants has been 

 observed on the leaves of certain tropical plants, namely the Myrsina- 

 ceae, such as Ardisia, certain Rubiaceae, such as Pavetta and Grumilea. 

 Koorders 177 demonstrated the presence of either bacteria or fungi in the 

 bloom bud hydathodes of nineteen species of tropical plants, repre- 

 senting six genera; a symbiotic relation was found to exist between the 

 host plant and the microorganisms. Zimmermann 178 was the first to 

 show that the nodules on the leaves of the Rubiaceae (four species 

 examined) are filled with bacteria. He also found nodules on the upper 

 side of the leaf of Pavetta lanceolota and on the under side of P. angusti- 

 folia. The bacteria were present in chains and as longer forms. P. 

 indica had even a greater number of nodules scattered over the whole 

 surface of both sides of the leaf and formed dark green spots. The 

 bacteria do not penetrate the cell but are found in the intra-cellular 



174 Spratt, E. R. The formation and physiological significance of root nodules 

 in the Podocarpineae. Ann. Bot. 26: S01-814. 1912; The root nodules of the 

 Cycadaceae. Ibid. 29: 619-626. 1915. 



178 Burrill and Hansen, 1917 (p. 126). 



176 Miehe, H. Anatomische Untersuchung der Pilzsymbiose bei Casuarina 

 equisetifolia nebst einigen Bemerkungen liber das Mykorhizenproblem. Flora, 

 111-112: 431-449. 1918. 



176a Rao, R. A. Casuarina root nodules and nitrogen fixation. Madras Agr. 

 Dept. Yearbook, 1923, 60-67. 



177 Koorders, S. H. Uber die Bluthenknospen Hydathoden einiger tropischen 

 Pflanzen. Ann. Jard. Bot. Buitenzorg, 14: 354-477. 1897. 



178 Zimmermann, A. tlber Bakterienknoten in den Blattern einiger Rubiaceen. 

 Jahrb. wiss. Bot. 37: 1-11. 1902. 



