304 SYMBIOTIC NITROGEN FIXATION 



a very necessary stage in the life of the organism. Its significance 

 in the actual fixation of nitrogen, however, is pure speculation." 



The organisms are actively motile and when viewed under the 

 microscope may be seen darting about with amazing rapidity, now 

 tumbling end over end, now spinning violently on the short axis 

 and then sweeping across the field in a darting, jerking course. 

 They contain from 6 to 20 flagella. The number and distribution 

 of the flagella are variously given by the different investigators due 

 probably to either variation in organisms or to the difficulty with 

 which flagella is demonstrated owing to the gum or slime produced 

 by the organism. 



Mode of Entrance into Host. The method of inoculation and the 

 growth of the nodule is described as follows by Whiting : 



" As the tip of the root hair of the legume pushes itself out into the 

 soil, it chances to come into intimate contact with the organism, 

 B. radicicola. Some scientists have exploited the view that the 

 organism is attracted to the plant by chemotaxis, believing that the 

 plant excretes a substance, probably a carbohydrate, which diffuses 

 into the soil solution and attracts the motile organism. While it 

 has been rather definitely shown that this organism progresses in 

 the soil at a rapid rate, nevertheless the number of root hairs infected 

 is too small to lend support to a chemotactic theory. However the 

 case may be, the organisms cluster at the tip of the hair and by 

 means of an enzyme (or otherwise) rapidly dissolve the cellulose of 

 the cell wall, thus enabling the organism to enter the root hair. As 

 a result, there is a decided bending of the tip, causing it to resemble 

 a shepherd's crook. This was early observed as a sign of complete 

 infection. It is claimed that other root hairs which form after infec- 

 tion are immune to the attack of other leguminous bacteria. 



"The organisms, by rapid division and growth, advance through 

 the center of the infected root hair. Prazmowski found organisms 

 in the cell sap and even in the epidermis only two days after inocula- 

 tion. In this advance an infection strand (Infektion-schlauche) is 

 formed, which consists of gelatinous material, and in the earlier 

 stages of development this strand may be traced from the root hair 

 into the inner tissue of the root and from cell to cell throughout the 

 nodule. This infecting strand is not supposed to constitute a 

 portion of the living tissue, nor is it a well-defined tube; but, as Fred 

 has recently shown, it consists of a large number of zooglea occurring 

 adjacent to one another, in which separate bacteria can be distin- 

 guished. The infecting strand branches profusely and it was this 

 habit of growth which caused the early investigators to consider it 

 the mycelium of a fungous growth. 



"Growth of the Nodule. The presence of B. radicicola in the 

 tissues of the root causes a rapid cell division in the pericycle. 

 These cells become larger and contain more protoplasm than the 



