The Cvoncopl of Hliizosplu'io 247 



The Concept of RhiJiiosphere 



In 1904 Hiltncr introduccxl tlic concept oF the "ihi/osplu'it'" to 

 express tlie zone of increased niierohiological activity ininiechately 

 around the roots (jf higher phuits. This term came, in time, to desig- 

 nate the intimate relations between soil microorganisms and the root 

 s\ stems of higher plants. More recently, two zones of influence of 

 plant roots upon the microbiological population came to be recog- 

 nized, the root surface and the rhizosphere, both being often grouped 

 under the term "root region." Although considerable information has 

 now accumulated concerning the mutual effects of these two bio- 

 logical systems, we are still lacking a clear idea of the importance of 

 this phenomenon in the growth of higher plants. 



These relationships may be considered midway between those of 

 true S)mbiosis, on the one hand, as in the case of the root-nodule 

 bacteria and the leguminous plants and of some of the mycorrhiza 

 formations, and, on the other, of the phenomena of parasitism. Some 

 of the relationships, like the utilization by the plants of the metabolic 

 products of microorganisms, are no doubt highly beneficial; others, 

 such as the possible curling and even more toxic effects of certain 

 antibiotics on the leaves of some plants, may be injurious. 



The rhizosphere can be studied conveniently by means of the 

 contact slide method of Rossi and Cholodny. This consists in bury- 

 ing glass slides or cover glasses in contact with the plant roots; these 

 slides or cover slips are removed at various intervals, stained, and 

 examined microscopically. This method allows us to study the effect 

 of root growth upon the development of specific microorganisms. 

 Unfortunately, the larger forms like the nematodes and protozoa do 

 not adhere to the slides. This method established the fact, however, 

 that various bacteria, actinomycetes, and fungi find the root zone 

 a highly favorable medium for their development. 



The nature of the plant and its age and the nature of the soil and 

 its treatment will influence considerably the nature and abundance 

 of the organisms. Starkey showed that alfalfa roots had only a slight 

 stimulating effect upon filamentous fungi, whereas eggplants had an 

 appreciable effect; sugar beet and corn were least effective. Legumes 

 exerted a particularly marked effect upon bacteria. Starkey was able 

 to distinguish two sources of nutrients provided by roots: (a) soluble 

 excretions, (h) sloughed-off dead root cells. Although root excre- 

 tions and detritus are the principal cause of the rhizosphere effect. 



