THE BIOLOGICAL ENVIRONMENT OF ROOTS 661 



nisms in soil, which may greatly modify or limit the forms that develop, 

 arise because of the production of microbial products with antibiotic 

 properties. The affected zone may be very restricted; the effect of the 

 antibiotic may well be only transitory; but at a particular locus the 

 ecological result may be substantial. 



It is necessary next to think out the implications this may have in 

 the rhizosphere. Here the plant provides the energy sources in the 

 form of solutes leaking from its roots. As a root elongates, a heavy 

 population is rather quickly established on its surface, though the root 

 tip itself seems to remain relatively free from microorganisms. Be- 

 cause solutes continue to leak from roots, any organisms on the surface 

 or in the moisture film encompassing the root will have a continuing 

 nutrient supply— a circumstance which is likely to provide some stabil- 

 ity. If, in addition, microbial products with antibiotic properties are 

 formed, it is unlikely that there will be the sequence of active forms 

 that is characteristic of the decomposition of plant residues in soil. Fur- 

 thermore, the rhizosphere population is to a degree protected against 

 desiccation in periods of low soil moisture. Roots will even elongate 

 into dry soil in which microbial activity, as measured by respirometry, 

 is low. Either in moist or dry soils, therefore, the microflora in the 

 immediate vicinity of the root surface is likely to persist as long as the 

 root remains functional and solute leakage occurs. The germination of 

 spores of pathogenic fungi in the vicinity may be repressed if fungi- 

 static products are present (Jackson, 1957), or it may take place nor- 

 mally if, as saprophytes, these fungi are compatible with their neigh- 

 bors. Some pathogens have an ectotrophic growth habit, ramifying 

 extensively over the outside of the moist root but invading only infre- 

 quently by branch hyphae. Some of the troublesome damping-off path- 

 ogens seem relatively non-specific but not vigorously in\'asive. 



The activities of the soil population, whether in the rhizosphere or 

 in the soil at large, have long been regarded as primarily affecting the 

 level and supply of nutrient ions, major and minor. In any discussion 

 of interactions between soil organisms and higher plants, this effect, 

 though indirect, is of prime significance. But the observation that mi- 

 crobial products may directly bring about changes in the roots intro- 

 duces another consideration deserving more attention. 



Some soluble microbial products may affect root-growth develop- 

 ment and function if absorbed and translocated ( Brian, 1957a; Nor- 

 man, 1959). Less mobile products, similarly inhibitory, may influence 

 root-hair proliferation locally and therefore adversely affect the welfare 

 of the plant, x^ number of polypeptide antibiotics, including polymyxin, 

 duramycin, circulin, novobiocin, and bacitracin, cause direct injury to 

 root tissues at low concentrations. The polypeptide becomes quickly 

 bound to absorption sites on root surfaces and in the accessible free 



