180 Perspectives m Microbiology 



on plants, whereas plants can develop in the complete 

 absence of man and even other animals. As regards de- 

 pendence, Dodge (9) made the following case for fungi: 

 "We may rest assured that as green plants and animals 

 disappear one by one from the face of the globe, some of 

 the fungi will always be present to dispose of the last 

 remains." To continue a bit further, both fungi and most 

 bacteria would encounter nutritional difficulties with the 

 loss of green plants and animals. In this connection, a good 

 case could be made for chemoautotrophic bacteria as the 

 organisms most likely to survive the longest in a world 

 being depleted of living things. 



My discussion is based on the principle that higher 

 plants are indispensable to human welfare and that mi- 

 crobial development is intimately related to plant growth. 

 Whereas most microorganisms grow separately from plants, 

 others are associated in states of symbiosis and parasitism. 



The Rhhospbere 



Among the relations of microorganisms and plants, most 

 attention has been devoted to the role of microorganisms 

 as converters of soil materials, frequently summarized in 

 cycles of the elements. In addition, attention has been di- 

 rected to structural modification of soil by microorganisms. 

 These changes do not require close association between 

 microorganisms and higher plants, yet the association is 

 frequently closer than may be realized, and the effects of 

 the microorganisms on the plants are pronounced, for the 

 organisms are in close proximity. 



The area of the soil close to plant roots was designated 

 many years ago by Hiltner as the "rhizosphere." This is 

 the region of the soil from which the plant obtains its 

 nutrients and in which each soil exerts its particular effects 

 on the plant. In this zone, one finds great numbers of 

 microorganisms, far greater than in the rest of the soil (39). 



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