Microorganisms and Plant Life 181 



Bacteria may be 5, 10, 20, or more times as abundant close 

 to or on root surfaces as in soil close by but free from root 

 development. The effects are evident on root hairs and 

 young roots as well as on older roots and are most promi- 

 nent during periods of active plant development. Further- 

 more, upon death of the plant, the bacterial population 

 reverts promptly to its original low level. Studies of the 

 rhizosphere were initiated in our laboratories some years 

 ago and have been expanded by many others, particularly 

 by Lochhead and associates (19) and by Clark (5). 



The rhizosphere effect has two general implications: 

 first, the extensive development of microorganisms in the 

 rhizosphere is the result of plant growth and is due princi- 

 pally to organic materials coming from the roots; second, 

 microorganisms that have made extensive development on 

 the root surfaces have important effects on the plants be- 

 cause of products formed close to the absorbing root 

 surfaces. 



Unfortunately, there is insufficient information to show 

 precisely what happens at the root surface. It is presumed 

 that considerable amounts of readily decomposable organic 

 materials come from the intact roots, and they, together 

 with diverse dead root parts, support this large population 

 of microorganisms. There is only fragmentary information, 

 however, as to whether organic materials are excreted from 

 roots, and little to indicate what the compounds are and 

 the conditions under which they are excreted. There is 

 some evidence that root penetration by the nodule bacteria 

 is conditioned by root excretions, and Virtanen and his 

 associates made proposals regarding the mechanism of ni- 

 trogen fixation by leguminous plants on the basis of the 

 amino acids excreted from root nodules (42, 45). Some in- 

 formation presented by Halleck and Cochrane (14) was 

 believed to indicate that certain fungistatic substances 

 applied to the aerial parts of plants affected the develop- 

 ment of bacteria in the rhizosphere, presumably through 



