114 



Mycologia 



have since found that the higher the temperature to which the soil 

 is heated (up to 200° C.) the more favorable are the conditions 

 for the production of a highly colored extract. This would at 

 least suggest that the formation of this extract is directly con- 

 cerned in the growth of Pyronema. 



IV. Biological Experiments with Soil Extracts 

 A number of extracts of heated soils were made and placed in 

 tightly closed bottles, with no intention, however, of guarding 

 against the possibility of air infection by fungi. Several of the 

 extracts were evaporated down to various stages of concentration 

 and all kept for experiment and study. 



In a short time it was observed that a number of these extracts 

 were infected with the mycelium of a fungus. In most cases the 

 mycelium consisted of globose colonies varying from a few 

 millimeters to a centimeter in diameter. Each consisted of a 

 mass of mycelial threads radiating from a central point and 

 apparently originating from a single spore. As the plants became 

 older the mycehum became more fluf¥y and in some cases almost 

 entirely filled the bottles containing the extract while in other 

 cases the extract seemed to be less favorable for their continued 

 growth. These colonies usually started near the bottom of the 

 bottle, entirely immersed in the solution. 



Some of this mycelium was removed from the bottles and 

 placed on filter paper saturated with the extract. In several 

 oases the fruit of Pyronema soon appeared and in some cases 

 was produced in abundance, while in a few experiments no 

 fruit was produced. From these experiments it became evident 

 that the fungus which was infecting our extracts was Pyronema, 

 as we had previously suspected and as the mycelium itself 

 indicated. 



So favorable are the heated-soil extracts as culture media for 

 Pyronema that it has been found almost impossible to keep the 

 extracts for any length of time in our laboratories without hav- 

 ing them thoroughly infected with the fungus {pi. 25, f. 2). 

 The same fact has already been noted with reference to heated- 

 soil itself, this being so favorable as a nutrient medium for 

 Pyronema that it is difficult to keep the fungus from invading 



