338 ROOT-NODULE BACTERIA 



is not very satisfactory for the growth of these organisms. Much better results are 

 obtained if special sugar media almost free of combined nitrogen or if sugar media 

 plus extracts of plants are used. On mannitol phosphate agar at 2o°-25° C. well- 

 developed colonies are obtained within five to ten days. 



The colonies of bacteria vary in appearance depending on the kind of leguminous 

 plant from which they were obtained. From alfalfa, clover, and pea nodules the col- 

 onies are usually large, raised, opaque, and sticky, while those from the nodules of 

 soy bean are usually small, slightly raised, producing an almost transparent film which 

 slowly spreads over the surface of the agar medium.' 



Morphology. — When cultures from a synthetic nitrogen-free agar medium are ex- 

 amined the cells usually appear as short rods. In addition to the rods, cocci and large 



unevenly stained rods are found. These 

 banded rod- forms may in turn become 

 greatly swollen, and are then known as 

 "bacteroids." The coccus forms which 

 arise from the banded mother-cells are 

 at first non-motile, but later become 

 actively motile. As these cocci become 

 older they elongate and return to the rod 

 form, and are often motile. Still later 

 they appear as the banded non-motile 

 rods. According to Lohnis and Smith,^ 

 Bewley and Hutchinson,-^ and more 

 recently Thornton and Gangulee,^ the 

 various forms through which the organ- 

 isms naturally pass make up the so- 



„ -n . • r . J 1 r called "life-cycle." Figure =; is a photo- 



FiG. 5. — Bacteria from root nodule of pea -^ . 



plant showing large vacuolated forms. Xisoo. micrograph of SOme of the typical bac- 



teroid forms. 



The bacteroid form may not appear on culture media, but in the nodule this greatly 

 enlarged and banded or highly vacuolated form is of common occurrence and some- 

 what characteristic of the plant species. Small amounts of caffeine, cumarine, and 

 other vegetable alkaloids added to the culture medium promote the formation of 

 bacteroids.s The bacteroids produced in this way are not so numerous as those found 

 in the nodule. Many of the small coccus forms may develop from one of the branched 

 swollen rods. As the nodules disintegrate the root-nodule bacteria pass back into the 

 soil as these small cocci. 



Flagellation. — According to the number and arrangement of the flagella these bac- 

 teria may be divided into two great groups.** Those of the alfalfa, clover, pea, and 



' Burrill, T. J., and Hansen, R.: loc. cit.; Fred, E. B., Whiting, A. L., and Hastings, E. G.: loc.cit. 



= Lohnis, F., and Smith, N.R.: J. A gr. Research, 6, 6^$. 1916. 



3 Bewley, W. F., and Hutchinson, H. B.: J. Agr. Sc, 10, 144. 1920. 



''Thornton, H. G., and Gangulee, N.: loc. cit. 



5 Barthcl, C: Ann. deVInst. Pasteur, 35, 634. 1921. 



•^ Shunk, I. v.: J. Bact., 6, 239. 1921; ibid., 5, 181. 1920. 



