Microbioli4 



for example, states that there are three components in the soil and 

 root microbial population, namely, (a) that of the surface of plant 

 roots, (b) that of the rhizosphere soil, and (c) that of the soil 

 proper. Observations of Rovia (13) and others are cited to the effect 

 that the root surface population mainly originates from the seed coat, 

 while the rhizosphere flora is of mixed origin, arising partly from 

 the seed coat and partly from the soil. 



Without doubt, root surfaces of seedling plants grown in sand and 

 from seeds not surface sterilized do seciire many of their colonizing 

 bacteria from the seed coat. However, I am not willing to agree that 

 the seed coat is of any special importance in detennining the rhizo- 

 planal microflora of plants as they commonly are found in the ordinary 

 soil. I am convinced that with few exceptions the root surface micro- 

 floras of seedlings more than a few days old, in short, of older plants 

 generally, will be found indistinguishable regardless of whether the 

 plants were grown from unste^ilized or from svirf ace-sterilized seed. 

 Placing any undue emphasis on the flora of the seed coat as the source 

 of the root surface microflora is an over-simplification of a difficult 

 problem . 



Dissimilarities between Root and Soil Microflora 



Does the rhizosphere microflora differ appreciably from that found in 

 soil apart from plant roots? This question can unhesitatingly be 

 answered yes. The rhizosphere is not simply a site of profuse develop- 

 ment of the soil flora en masse . It is rather a site wherein some 

 species are much more numerous than in soil, and other species much 

 less numerous. To illustrate with some specific data, we have observed 

 that in soix one to two inches distant from the cotton root, the aerobic 

 spore-forming bacteria of the genus Bacillus comprised 37 percent of 

 the total bacterial population, while at the root surface this genus 

 comprised only one-tenth of one percent of the total. In contrast, the 

 gram-negative dye-tolerant bacteria amounted to only 16 percent of the 

 total bacterial flora one to two inches distant, and 86 percent of the 

 flora at the root sxirface. 



For the most part, published information concerning differences between 

 soil and root floras is given in terms of morphological and taxonomic 

 types. Lochhead and associates (lU,l5), however, have made extensive 

 use of a nutritional grouping of bacteria in characterizing the rhizo- 

 sphere. They have determined the growth responses of bacterial isolates 

 on seven cultiiral media of increasing complexity. They have found that 

 the rhizosphere contains an unusually high proportion of bacteria that 

 require amino acids for their nutrition. Bacteria reqiiiring growth 

 factors such as are provided by yeast extract either were proportion- 

 ately no more numerous, or were proportionately less numerous, at the 

 root surface than in the soil. 



A nutritional grouping has some merit in that it attempts to relate 



