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Higher Plants and Soil Microorganisms 



The microorganisms of the soil exert a variety of effects on the 

 growth of higher plants. Most of these effects are beneficial, but a 

 few can be injurious. Higher plants, on the other hand, influence 

 the growth of microorganisms in different ways, both stimulating 

 and injurious. The mutual interrelations between the higher plants 

 and the microorganisms may be summarized under the following 

 groups of reactions: 



1. Microorganisms favor the growth of higher plants by affecting 

 the availability of various nutrient elements essential for plant growth, 

 notably carbon as carbon dioxide, nitrogen, and phosphorus. 



2. Microorganisms favor plant growth through the production of 

 specific growth-stimulating or growth-regulating substances, such 

 as auxins and phytohormones. 



3. Certain groups of microorganisms form a variety of symbiotic 

 relationships with higher plants. 



4. Different microorganisms may compete with higher plants for 

 some of the nutrients present in the soil. 



5. Some microorganisms have injurious effects upon higher plants, 

 either by directly attacking them as plant parasites, or by producing 

 certain toxic substances. 



6. Certain viruses, notably bacteriophages, have the capacity to 

 attack useful bacteria and may thus prove to be indirectly injurious 

 to plant growth. 



The plants, in turn, supply microorganisms with various nutrients 

 in the form of plant residues and excretion products. They also offer 

 a favorable medium for the growth of various groups of microorgan- 

 isms, either in the immediate vicinity of the roots or directly upon 

 the roots. By the excretion of toxic products, plants may also exert 

 various injurious effects upon the growth of microorganisms. 



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