EDWIN BROUN FRED 



337 



gen content and yield, in the case of canning peas the presence of the nodule bac- 

 teria often results in more uniform growth and hence an improvement in the 

 quality of the peas. 



FACTORS THAT INFLUENCE NODULE FORMATION 



When leguminous plants are grown in soil abundantly supplied with nitrate ni- 

 trogen, the plants may develop luxuriantly and yet fail to show nodules, although the 

 soil and seed have been richly inoculated with the proper bacteria. That this lack of 

 nodules is not due to a toxic effect on the bacteria is well established. From all of the 

 tests carried out it has been found that the suppression of nodule formation is not a 

 direct effect of the nitrates but rather a case of change in the metabolism of the higher 

 plants.' The addition of certain salts, such as phosphates and sulphates, and also car- 

 bon compounds, such as sugars and organic acids, have been found beneficial to 



nodule production.^ 



TABLE III 



Nodule formation and consequently nitrogen fixation are also greatly influenced 

 by soil reaction, temperature, moisture, and oxygen relations. 



CHEMISTRY OF NITROGEN FIXATION 



If the average amount of nitrogen fixed per acre per year is taken as 100 lb., then 

 the energy required by the bacteria would be that from about 10,000 lb. of car- 

 bohydrate. This estimate is based on the results of pure-culture studies assuming 

 that the nodule bacteria fix 1-2 mgm. of nitrogen for 100 mgm. of sugar consumed. 

 Obviously this requirement of carbohydrate is far in excess of the amount supplied to 

 the root nodules by the plant. It is highly probable, therefore, that the organisms in 

 the plant nodule function much more efficiently than outside of the host. Christian- 

 sen- Weniger'' suggests that the process is exothermic. Just how this fixation takes 

 place is not known. It is probably a reduction process which results in the formation 

 of ammonia. 



PURE-CULTURE STUDIES 



If the roots of a normal leguminous plant bearing nodules are thoroughly washed 

 and the contents of a young nodule pressed out on a glass slide, fixed, and stained, 

 great masses of variously shaped bacterial cells will be seen. 



From these active young nodules, it is compartively easy to isolate pure cultures 

 of the nodule-forming bacteria. The ordinary culture media, nutrient or plain agar, 



'Fred, E. B., and Graul, E. J.: /. Af?i. Soc. Agron., 8, 316. 1916 



^Wilson, J. K.: Cornell Agr. Exper. Sla. Bull. j,S6. 1917. 



3 Christiansen-Weniger, F.: Centralbl.f. Bakteriol., Abt. II, 58, 41. 1923. 



