22 



MICROBES AND TOXINS 



have been made : those of the vetches, clovers, and peas, and 

 those of the beans and lupines. Maz^ distinguishes those 

 adapted to calcareous soil, the ' calcicolje ' from those adapted to 

 acid soils, the 'calcifugee.' They are aerobic, and pure cultures 

 can be obtained. They require carbohydrate food, and are 

 not fond of nitrates. Grown in pots, the nodules are 

 encouraged by the addition of chromium, manganese, nickel, 

 and cobalt. About loo grams of sugar are consumed for each 

 gram of nitrogen fixed. The bacteroidia only act well when 

 their food supply is regulated in quantity as well as in quality. 

 The broth for the cultures should contain i per 10,000 at 



Fig. 8. — Bacteria and Eacterioidia from the nodules 

 of Leguminosse (after Beijerinck). 



least and i per 3,000 at most of combined nitrogen • the 

 proportions of saccharose should lie between 2 and 6 per 

 cent. 



The virulence of the bacteroidia varies ; those which have 

 undergone several passages in the roots attack the new roots 

 more easily. It seems that the weaker bacteria induce a 

 certain immunity in the rootlets towards the more virulent. 

 A root already bearing nodules does not produce new ones 

 except when inoculated with very virulent bacteroidia. On the 

 same plant the recent nodules of the lateral roots contain 

 bacteria much more virulent than those of the main root. 



