214 



Bacteria in Relation to Country Life 



invading germs. Tlie nitrogen compounds manufactured 

 in the nodules are carried to otiier parts of tlie plant 

 where they are employed for the building of new tissue. 

 The bacteria of legumes. — The legume bacteria are 

 widely distributed in the soil and are present in streams 

 and lakes. The particles of dry soil which are carried 

 by the wind may have adhering to them one or more 



of these organisms ; 

 hence wild legumes 

 may be enabled to 

 establish themselves 

 in new places because 

 of the occasional in- 

 fection by bacteria 

 brought by the wind. 

 The formation of 

 nodules, or tubercles, 

 as they are fre- 

 quently called, occurs 

 only on the younger 

 parts of the roots. 

 Under proper con- 

 ditions, the germs that come in contact with the root- 

 hairs penetrate the latter, and, by means of the filaments 

 already mentioned, make their way toward the interior 

 of the root-branch. There is no regularity in the ar- 

 rangement of the tubercles on the roots, since infection 

 may occur in different places and at different times. 

 This bacteria that enter the legume roots are very small 

 and rod-shaped, but within the nodules they subse- 

 quently assume various shapes and sizes. In different 



Fig. 33 



Bacteroids from legume tubercles. — 

 1. From Melilolv^ alba; X 3,000. 2, 3, 

 and 5. From Medicago sativa; X 3,000. 

 4. From Vicia villosa; X 3,000. (Har- 

 rison and Barlow.) 



