Strain of Organisms and Nodule Formation 213 



Strain ok Organisms and Nooule Formation 



Numerous studies were made of strain variations of l:)acteria as 

 iiiHuencing the fixation of nitrogen. It is sufficient to cite the results 

 of Fred, Baldwin, and McCoy. 



1. Different isolations of organisms from a nodule or from the soil 

 may varx- widely in their ability to benefit the host plant through 

 the fixation of nitrogen, even though nodules are readily formed. A 

 strain that fixes little nitrogen in association with the host is called 

 "poor," and one that is beneficial is considered a "good" strain. 



2. The nodules formed by poor strains are usually small, round, 

 and white and are scattered over the entire root system. Nodules 

 from a good strain are fewer in number but much larger; they are 

 pink in color and elongated, and are located near the main roots. 

 This is true of certain legumes but not necessarily of others. 



3. The plant species determines largely whether a given strain is 

 poor or good. Cultivation on certain media often causes a good 

 strain to lose its effectiveness. Successive passage through a host 

 plant modifies the effectiveness of a strain. A poor strain may im- 

 prove, and a good one may deteriorate. 



4. Many of the strains found in the natural habitat, either in the 

 soil or in the nodules of wild legumes, are of the poor type. 



5. Although a given host may possess nodules of both effective and 

 ineffective t\^pes, plants already infected with one strain of the or- 

 ganism resist infection of a contrasting strain to a greater degree 

 than do nodule-free plants. 



The plants exert a chemotactic effect upon the bacteria, which 

 congregate around the plant roots; the bacteria, in their turn, secrete 

 a substance of the nature of an auxin, which causes the curling of 

 the root hairs of the plant. During the early stages of growth of 

 the plant the bacteria act as parasites and enter the host through the 

 root hairs or through ordinary epidermal cells. When nitrates are 

 present, formation of nodules is repressed. The plant may also form 

 a substance which inhibits the growth of the bacteria and causes 

 their destruction, which may explain the inefficiency of certain bac- 

 terial strains. On entering the root, the bacteria multiply, forming 

 a thread of infection, which branches out into the parenchymatous 

 cells of the root. The bacteria elaborate certain stimulating sub- 

 stances which cause the cells to enlarge. On reaching the inner cells 



