38 INTERRELATIONSHIPS AMONG MICROORGANISMS 



supply, and temperature, the one that finds conditions more suitable 

 for its development will grow more rapidly and in time be able to 

 suppress the other. According to Porter (729), the effects produced 

 by fungi in mixed culture are due either to exhaustion of nutrients or 

 to the formation of detrimental or beneficial products. When two or 

 more organisms live in close proximity they may exert antagonistic, 

 indifferent, or favorable effects upon one another. These potentialities 

 were later enlarged (1046) to include stimulating, inhibiting, over- 

 growing, and noninfluencing effects. After considerable experimenta- 

 tion and speculation, Lasseur (548) came to the conclusion that antago- 

 nism is a very complex phenomenon and is a result of numerous and 

 often little-known activities. Antagonism influences the morphology 

 of the organisms, their capacity for pigment production, and other 

 physiological processes. 



No sharp lines of demarcation can be drawn between associative and 

 antagonistic effects. Well-defined effects of two symbionts may change 

 during the various stages of their life cycles or as a result of changes in 

 the environment. It is often difficult to separate strictly symbiotic phe- 

 nomena from associations of less intimate nature, frequently desig- 

 nated as commensalisms. The various stages of transition from obligate 

 parasitism to true saprophytism can be represented as follows: 



Obligate parasitism (cer- Facultative parasitism (spe- Modified parasitism; 

 tain bacteria, smut fungi) — > cies of Fusarium, Rhizoc- —^ hosts may derive some — > 

 tonia, and Actinomyces') benefit (certain mycor- 



rhiza) 



Balanced parasitism (vari- True symbiosis (root- True saprophytism (auto- 



ous mycorrhiza) —^ nodule bacteria, lichen — > trophic and heterotrophic 



formations) bacteria and fungi). 



The phenomena of antagonism do not fit exactly into the above 

 scheme but are parallel with it: the injurious effects of one organism 

 upon another range from antagonism of varying degrees of intensity 

 to the actual living or preying of one organism upon another. The lat- 

 ter may be classified with the phenomena of parasitism and disease pro- 

 duction. 



Microorganisms inhabiting the soil live in a state of equilibrium 



