STIMULATION BY ASSOCIATIVE INTERACTION 281 



Greaney and Machacek (1935) were able to demonstrate that 

 Cephalothecium roseum inhibits Helminthosporiwn sativum. 



Garrett (1936) explains somewhat differently the relative inci- 

 dence of Ophiobolus graminis in soils. His observations led him 

 to conclude that O. graminis increases in amount only so long as 

 there are living host roots, along which it spreads. Its rate of 

 spread is hypothesized to be related to the carbon dioxide content 

 arising from respiratory processes in the microclimate along the 

 root. The presence of alkaline receptors for carbon dioxide in the 

 soil stimulates spread of the pathogen. Decline of O. graminis 

 occurs in its saprophytic phase at which time the mycelium is be- 

 ing decomposed by other soil-inhabiting species. 



Recently Weindling (1932, 1934, 1938) found that Tricho- 

 dervia lignorwn and Gliocladium fimbriatum penetrate the hyphae 

 of such soil-borne parasites of seed plants as Rhizoctonia solani, 

 Sclerothtm rolfsii, and Phytophthora parasitica. Undoubtedly 

 antagonisms of this sort are not uncommon in the fungus flora of 

 soil, and such relationships are factors in the control of diseases 

 of cultivated plants. Evidence in support of this type of antago- 

 nistic action by Trichoderma against fungi that cause damping-off 

 of cucumber seedlings is derived from the experiments of Allen 

 and Haenseler (1935). They applied cultures of Trichoderma 

 to the soil with the result that damping-off was apparently 



checked. 



It would appear to be feasible to evaluate the several factors 

 previously mentioned that are known to influence the incidence 

 of fungi in soil generally. This field of research certainly offers 

 many possibilities. As is indicated by the rate at which invasion 

 of heat-sterilized soils is accomplished, for example, by Pyronema 

 confluens, more attention should be devoted to such problems as 

 they relate to culture of plants in cold frames, hotbeds, and 

 greenhouses. 



STIMULATION BY ASSOCIATIVE INTERACTION 



Apparently one fungus may be stimulated by the presence of 

 another in either of two ways: increased assimilatory or vegetative 

 activity or else reproductive activity. The proximate cause of 

 these responses need not be the same metabolic product but may 

 be different specific entities. Much interest in recent years has 



