186 Perspectives in Microbiology 



One can indicate what seems to be a logical sequence 

 of events from the time the bacterium affects the root hair 

 and gains entrance, until the nodule is developed and the 

 relationship between the bacterium and the plant is con- 

 summated (45). The story is far from complete, however. 

 One may inquire: why do members of the genus Rhizo- 

 bium develop an association with legumes, whereas other 

 bacteria are unable to do this? Furthermore, what is it 

 that prevents not only legume bacteria, but all bacteria 

 from developing symbiotically with other plants? These 

 questions have puzzled microbiologists and plant physi- 

 ologists since the time that nitrogen fixation was found to 

 be the result of the symbiosis. We seem little nearer to 

 the answers now than before. 



With a few exceptions, such as certain leaf endophytes 

 and the association between bacteria and legumes, the 

 tissues of higher plants appear to be unfavorable for bac- 

 terial growth. This is particularly the case with the aerial 

 portions of the plants. An interesting contribution to the 

 subject was made by Russell (37) before the turn of the 

 century. In his dissertation at Johns Hopkins University 

 in 1892, he reported on the effects of injecting cultures 

 of bacteria into stems of plants. Although some of the sapro- 

 phytic bacteria persisted for more than a month and seemed 

 to multiply, they failed to destroy the tissue. Animal patho- 

 gens survived in tissues for much shorter periods. Most 

 of the penetration of the bacteria was upward. This varied 

 from 30 to 50 mm. Downward movement never exceeded 

 2 to 3 mm from the point of inoculation. The following 

 citation indicates that he found the plant an unfavorable 

 substrate for saprophytic bacteria: 'Trom the results of 

 my own experiments, the conclusion seems evident, that, 

 normally, the healthy plant, with intact outer membranes, 

 is free from bacteria within its tissues." 



Bacteria have been recovered from plant tissues by vari- 

 ous investigators since the studies made by Russell. They 



