THE BIOLOGICAL ENVIRONMENT OF ROOTS 657 



that the soil is analogous to a tissue, in which energy sources are 

 metabolized and in which the over-all transformations can be ascer- 

 tained without specific knowledge of the pathways, the enzymes, or 

 (in the case of the soil) the organisms bringing them about. Some of 

 the end-products, particularly nitrate, are directly available to plants, 

 but, of course, they are removed only if and when roots penetrate into 

 the soil mass and, as it were, tap the accumulated reserve. 



This concept of soil microbiology, however, is not adequate, be- 

 cause there is now ample evidence to support the view that the micro- 

 biologies of cropped and uncropped soils are not identical. In other 

 words, the invasion of previously root-free soil by an expanding root 

 system is followed by great changes in the microbiology and biochem- 

 istry of the invaded soil. The microbiological environment of the roots 

 is changed in response to the presence of the roots themselves. To a 

 degree, therefore, roots determine their own immediate microbiological 

 environment and microbial associates. One might say that in soil which 

 supports vegetation there are two co-existing microbiological commu- 

 nities or systems: one in the immediate vicinity of roots, and the other 

 in the zone not yet invaded by roots and' at some distance from them. 

 Just how far this distance may be is the subject of some controversy. 

 The question, in effect, is: How far from a root does its significant in- 

 fluence extend? Certainly not far in millimeters, or perhaps in fractions 

 of a millimeter; but the distance is not to be underestimated in the 

 aggregate, because of the enormous total length and surface area of the 

 root system of most plants ( Dittmer, 1937; Pavlychenko, 1937 ) . 



It is unnecessary to recapitulate the evidence establishing the 

 presence of a different soil flora in the immediate vicinity of roots. The 

 discriptive microbiology of the rhizosphere has been well explored by 

 cultural procedures and microscopic techniques (see Katznelson et al., 

 1948; Clark, 1949). Healthy roots are virtually encompassed by a 

 microbial mantle which is predominantly, but not exclusively, bacterial. 

 The density is generally much greater than in uncropped soil, and 

 manv of the active forms seem characteristicallv to be somewhat less 

 versatile nutritionally, to have lower synthetic capabilities, and to be 

 more dependent on an external supply of growth substances and or- 

 ganic nitrogen compounds. Much of the information on the nutrition of 

 rhizosphere organisms comes from Canadian workers directed or in- 

 spired by A. G. Lochhead. Their findings attest not merely to the rich- 

 ness and density of the rhizosphere flora but also to differences among 

 the array of organisms present on the roots of different species in the 

 same soil— an observation foreshadowed by R. L. Starkey many years 

 earlier. - 



Now this is a circumstance of great significance, because it points 

 to the underlying nutritional basis for the very existence of the rhizo- 



