THE BIOLOGICAL ENVIRONMENT OF ROOTS 659 



substrate uniformity and continuity, in contrast to the discontinuity of 

 the micro-habitats in the zone not invaded by roots. These arQ two 

 diflFerent microbiological worlds— different in physical arrangement and 

 in nutritional status. One must not overemphasize this point, however, 

 because although the extension of a root into soil may result in the in- 

 troduction of new and more readily available energy sources, it cannot 

 remove what was already present. The rhizosphere flora is, therefore, a 

 superimposed population, arising in response to this new circumstance. 

 One can suggest, as an analogy, the many changes that result in human 

 populations when a large industrial plant is established in an area 

 previously wholly rural. Perhaps there is a further lesson in this anal- 

 ogy, because a sociologist, charged with the responsibility of studying 

 human factors in relation to the industrial operations, would surely not 

 concentrate on the number and occupations of the rural inhabitants 

 prior to this erection of the new plant. 



The rhizosphere population and the population at some distance 

 from the roots are both in contact with the same array of clay minerals 

 —a circumstance which may impose some overriding physico-chemical 

 similarities. Further, it would be misleading to paint too static a picture 

 of the differences between these two zones. Roots, rootlets, and root 

 hairs are constantly being produced as the root system enlarges and 

 ramifies. At the same time, older roots become senescent, die, and are 

 decomposed, particularly if the plant has a spreading adventitious root 

 system. 



Microbial antagonisms 



Up to this point emphasis has been placed primarily on the in- 

 fluence of the nature of the substrate in determining those organisms 

 that are currently active in micro-habitats or the rhizosphere zone. 

 When the initial substrate is insoluble, there may be a sequence of ac- 

 tive forms with physiological capabilities which fit together efficiently. 

 In the rhizosphere such dependent sequences may be much less im- 

 portant, but in both situations antagonistic or antibiotic effects between 

 organisms may be a potent factor in modifying the population. A great 

 deal of attention has been given by pathologists to the study of patho- 

 genic fungi in the root zone, particularly those that can live a normal 

 saprophytic existence but are given a competitive advantage by their 

 ability to invade roots. This is pictured as being an evolutionary escape 

 from the competitive vigor of saprophytic life ( Garrett, 1950 ) . Implied 

 in this theory also is the opinion that root-invading fungi may not be 

 as vigorously competitive as saprophytes, and therefore may tend to 

 be displaced and to disappear if over a period of time the opportunity 

 of invading roots is not presented. The competition involved here 



