658 PLANT GROWTH AND PLANT COMMUNITIES 



Sphere microflora. Clearly, in conventional bacteriological terminology, 

 this is an "enrichment" phenomenon in response to an abrupt change in 

 the nutritional state. Invasion by plant roots is hardly likely to be ac- 

 companied by the introduction of new organisms, though one has to 

 be cautious about generalizations of this sort because one remembers 

 that seed inoculation with rhizobia does result in the development of 

 nodules on the roots in soils in which this organism is absent. However, 

 the normal surface flora of seed and the rhizosphere population of the 

 roots of seedlings grown therefrom do not seem to have much in 

 common (Peterson, 1959; Rouatt, 1959). The rhizosphere population 

 arises in response to, and is supported by, solutes leaking or exuding 

 from the roots. This leakage is apparently a normal phenomenon, 

 though poorly understood. The physiological aspects of this character- 

 istic of plants have not been adequately investigated. 



It is not known whether leakage occurs at a constant rate or is in- 

 fluenced by the nutritional status of the plant, its stage of maturity, or 

 the condition of the root. It may well be an inevitable consequence of 

 the existence, within roots, of apparent "free space" or "outer space," 

 which is that part of the root volume readily accessible by diffusion to 

 external ions in solution and from which active ion accumulation may 

 take place. The free space may normally contain organic solutes origi- 

 nating in the cytoplasm of adjacent cells, these solutes in turn being lost 

 by diffusion from the roots. From media containing sterile root systems, 

 numerous amino acids and sugars have been identified ( Rovira, 1956 ) . 

 Though there is no information on this point, a reasonable assumption 

 might be that those regions of the root that are most active in water 

 and ion uptake are also those from which the greatest amount of 

 solute loss occurs. 



On root surfaces and in the immediate vicinity of roots the distri- 

 bution of organisms is less clearly colonial than in the soil remote from 

 roots, because the leaking solutes or exudates presumably form a 

 more or less continuous system in the water film between the roots and 

 contiguous colloidal particles (Starkey, 1938; Rovira, 1956). Another 

 consequence of the extension of roots into soil is that water is with- 

 drawn in amounts controlled by the plant demand in relation to the 

 soil water tension. It has already been pointed out that soil moisture 

 changes may have disproportionate effects on the space available for 

 growth of bacterial colonies. This must apply also to the rhizosphere, 

 though as yet little attention has been given to the significance of mois- 

 ture changes in the immediate vicinity of roots. 



Recapitulating, then, the extension of roots into soil is accompanied 

 by the introduction of soluble energy material and the withdrawal of 

 water— both events which may be expected to cause major microbial 

 responses. Nutritionally the rhizosphere has a considerable degree of 



