BACTERIA OF THE SOIL 253 



The young bacteria are motile, Gram-negative rods. 

 There is some question as to the distribution of flagella. 

 Some authors have claimed that a single polar flagellum is 

 to be demonstrated, others that there are several flagella 

 distributed over the surface of the cell. This may be due 

 to the fact that in reality several species of bacteria have 

 been studied, or because the cultures examined were of dif- 

 ferent ages. In many media the older organisms become 

 considerably enlarged and more or less vacuolate. Fre- 

 quently they are decidedly irregular. Branched forms and 

 forms showing irregularities in staining are common. The 

 appearance frequently leads to the inference that multi- 

 plication is by a process of budding and not by normal 

 transverse fission. 



While there are some points not well understood, in 

 general the method of root infection has been fairly well 

 worked out. The bacteria in the soil apparently come in 

 contact with the root hairs. It will be recalled that root 

 hairs develop just back of the root caps of the young grow- 

 ing roots and are somewhat evanescent. Older roots do not 

 show them. In some way this species of organism finds a 

 way to penetrate the wall of the root hair. Apparently it - 

 digests its way through. Once inside the root hair it begins 

 multiplying relatively rapidly and produces a gelatinous 

 strand which was formerly regarded as a fungous filament. 

 In fact, this organism was originally described as a root- 

 dwelling fungus. The strand of bacteria increases in length 

 until it comes in contact with the cell wall toward the 

 interior of the root. Here a buttonlike enlargement in the 

 strand may take place. Eventually the bacteria make their 

 way through the cell wall and invade the next ceU. Infec- 

 tion of the root has now been established. 



The presence of the organisms in the cells of the roots 

 leads to an increased growth. It is probable that some 

 substance excreted by the bacteria possesses the property 



