Seaver-Cl^rk : Studies in Pyrophilous Fungi 115 



these substrata when exposed in our laboratory where the spores 

 are present from previous experiments. 



As stated above, the extract of North Dakota soil is much 

 darker in color than those of local soils, a result which would be 

 expected since the soil itself is much blacker by reason of the 

 large amount of organic matter present. We have shown by our 

 experiments that the color of the extract is an index to the amount 

 of soluble substances they contain. It would naturally follow 

 that the extract of North Dakota soil is much richer and there- 

 fore a more favorable culture medium for Pyronema than the 

 extract of the other soils studied, as our experiments have later 

 shown. 



In order to run a parallel test on the unheated-soil extract and 

 heated-soil extract, three Petri dishes were partially filled with 

 the highly colored extract of North Dakota soil and three simi- 

 lar dishes filled with the extract of the same soil unheated. 

 These were placed side by side and loosely covered, allowing 

 abundant opportunity for air infection. In a few days, the 

 extract of the heated-soil showed an abundant infection consist- 

 ing of numerous surface colonies of various kinds of fungi and 

 a large number of immersed colonies of what appeared to be 

 Pyronema. In about a week, two of the dishes showed abundant 

 growth of Pyronema, the mycelium forming a very thin but 

 tough membrane over the surface of the extract and the fruit 

 being produced in great abundance over its surface, especially 

 around the outsides of the culture {pi. 24). Although the 

 mycelium of Pyronema had invaded our extracts continually 

 these were the first cultures in which the fruit was produced in 

 the extract. The controls containing the extract of unheated- 

 soil showed no signs of infection by fungi of any kind. 



The abundant infection of heated-soil extracts with Pyronema 

 while the extracts of the same soil unheated remained uninfected, 

 is strong evidence that the fungus appears here on account of 

 the large amount of soluble food material liberated in the soil 

 through the process of heating. 



Numerous attempts to render unheated-soil favorable to Pyro- 

 nema growth by the addition of heated-soil extract have failed to 

 yield the expected results. Assuming that the extract contains 

 food material for Pyronema it has been difficult to account for 



