214 Nitrogen Fixation— Symbiotic 



of the roots, the bacteria favor the multiphcation of the surrounding 

 cells. This leads to the formation of a young nodule, which pro- 

 duces a swelling on the side of the root by pushing out the overlying 

 cortical parenchyma and epidermis. 



In some plants, bacterial infection results in a rapid division of the 

 infested cells, which give rise to bacteroidal tissue. In these plants 

 the nodules usually arise in the cambium layers. 



A third type of infection is known, in which the intercellular zo- 

 ogloea plays the important part. The bacteria which enter the root 

 change into rods and multiply rapidly in the slime filament or in the 

 zoogloeal mass. Many of these rods change into bacteriods. 



Wilson reported three types of plant response with respect to 

 nitrogen fixation: 



1. The association of certain strains of bacteria with all the species 

 of host plant tested. 



2. The association of certain strains of bacteria with only one 

 species of host, not with another. 



3. The association of certain strains of bacteria with certain species 

 of plants, producing erratic responses due to the carbohydrate-nitro- 

 gen relationship in the plant. 



Morphology and Life Cycle of Nodule Bacteria 



The bacteria responsible for the formation of root nodules vary 

 greatly in size and shape. Beijerinck described the small, oval forms 

 as "swarmers." In the young nodules, there are present normal rods 

 together with large club-shaped or branching forms (bacteroids). 

 In old decomposing nodules, the branching forms are vacuolated, 

 showing small, oval, deep-staining bodies within, which are the 

 motile swarmers or the branching form dividing into bacilli. 



The organism produces short Gram-negative rods, motile by means 

 of flagella when young. The bacteroids may also be formed on 

 artificial media, when such substances as acid phosphate, sodium 

 succinate and glycerol, caffeine and cumarine are present. Caffeine 

 and other vegetable alkaloids, like guanidine, pyridine, and chinoline, 

 will stimulate the formation of involution forms in pure culture. It 

 was suggested, therefore, that the formation of the bacteroids in root 

 nodules is due to the presence of alkaloids in the plant. The bac- 

 teroids are produced in the medium or in the nodule as a result of 

 specific nutrition or of unfavorable conditions. According to Zipfel, 



