662 PLANT GROWTH AND PLANT COMMUNITIES 



space, and it brings about at once some change in permeability, so that 

 solutes, both inorganic and organic, leak from the tissue (Norman, 

 1955, 1960). Such injury is irreversible or irreparable, but in some 

 cases it may be mitigated by simultaneous presentation of divalent ca- 

 tions. The induced leakage presumably resembles that which occurs 

 normally and which supports the rhizosphere population, but it can be 

 vastly greater in amount, z^ffected areas, perhaps occurring initially 

 only at point loci, would probably enlarge, because the solutes released 

 would support additional microbial growth and polypeptide produc- 

 tion. These injured areas could constitute ready portals of entry for 

 weakly invasive pathogens, particularly those of ectotrophic habit, if 

 they themselves are not antagonized by the antibiotic. The apparent 

 abrupt collapse in resistance to damping-off organisms exhibited by 

 certain seedlings may have its origin in this phenomenon, which has 

 not hitherto been considered in discussions of root invasion by weakly 

 pathogenic fungi. 



Epilogue 



In summation, the picture of the microbiological world of the soil 

 presented in this review is one of complex communities fiercely com- 

 petitive for food, at times greatly modified or limited by antibiotic 

 effects. This population is seen in its simplest form in subsurface hori- 

 zons of fallow or uncropped soil. When higher plants are present, 

 either in the natural vegetation or through cropping, the economy is 

 changed, and new and quite different communities develop in the 

 immediate vicinity of root surfaces. The plant itself is the major mem- 

 ber of these communities, which arise as a result of its presence. The 

 intimate ecology and biochemistry of the rhizosphere zone is much 

 more involved than that of soil at a distance from roots, because of the 

 phenomena of root leakage and antibiotic production. There are inter- 

 actions among the dominant saprophytic forms. There are interactions 

 between saprophytes and root pathogens which may determine 

 whether or not the roots are invaded. There are interactions between 

 the saprophytes and the higher plants, in that products of the former 

 may adversely affect the growth and possibly the functioning of roots, 

 or, if adsorbed, may cause tissue damage which can increase the proba- 

 bility of invasion by pathogens. 



Some soil chemists and plant physiologists tacitly limit themselves 

 to the study of a system in which the plant is the only living member. 

 They deal either with the 3-phase system of root, water, and clay 

 minerals, or the even simpler 2-phase system of root and water with 

 ions in solution. It may be appropriate to conclude with a plea that the 

 presence of microorganisms in the root zone not be overlooked, either 



