122 PRINCIPLES OF SOIL MICROBIOLOGY 



active show less development of nitrogen-fixing bacteria and a re- 

 tarded or no development of spontaneous cultures. 3. Inactive soils 

 are free from aerobic nitrogen fixing bacteria. The methods of analyses 

 are described elsewhere (p. 732). 



Symbiotic nitrogen-fixation by nodule bacteria. Historical. Many 

 centuries before the discovery of the nodule bacteria and their part in 

 the enrichment of the soil with combined nitrogen, due to their symbiotic 

 action with leguminous plants, had been established, the practical 

 agriculturist came to consider the growth of these plants in the soil 

 equivalent to manuring the soil for the succeeding crop. 92 



In the early part of the 19th century Davy 93 stated that "peas and 



beans seem to be well adapted to prepare the soil for wheat 



Peas and beans contain a substance similar to proteins; but it seems that 

 the nitrogen, which is one of the constituents of this substance, takes its 



92 Use of legumes by Romans is found in the work of Plinius-Historia naturalis 

 LVIII; Varro-De re rustica. Lib. I, Chap. 23; by ancient Chinese, in the book 

 of F. H. King — Farmers of forty centuries. Madison, Wis. 1911. 



93 Davy, H. Elements of agricultural chemistry. 1814, p. 412. 



PLATE VII 

 Symbiotic Nitrogen Fixing Bacteria 



37. Types of nodules on leguminous and non-leguminous plants: A, Trifolium 

 pratense; B, Garden pea; C, Cowpea; D, Soja hispida; E, Vicia faba; F, Ceano- 

 thus americanus; G, nodules on leaf of Paivetta indica; H , Alnus. (A, after Mak- 

 rinov and Omeliansky; C, D, after Albrecht; E, after Straszburger and Omelian- 

 sky; F, after Burrill; G, after Faber and Omeliansky; II, after de Rossi.) 



38. Detailed examination of Bad. radicicola on lupines : a, nodule, natural size; 

 b, cross section of root and nodule; c, cell of nodule filled with bacteria, X 400; 

 d, nodule bacteria, X 1200; e-/, bacteroids, X 1200 (after Woronin, Fischer and 

 Omeliansky). 



39. Nodule bacteria of Vicia saliva: a-d, transformation of rods into bacteroids 

 (after Beijerinck). 



40. Nodule bacteria from nodules of alfalfa, X 1200 (from Edwards and 

 Barlow). 



41. Flagellation of bacteria of leguminosae: 1, Phaseolus vulgaris; 2, Cracca 

 virginiana; 3, Vicia sativa; 4, Medicago saliva; 5, Melilolus alba; 6, Lespedeza 

 striata (from Shunk). 



42. Bad. rubiacearum of Pavetta zimmermanni: A, contents of nodule; B, 

 preparation of colony grown on agar; C, various forms of different stages of devel- 

 opment of pure culture: A and B, X 1660; C, X 1300 (after von Faber and de 

 Rossi). 



