112 



Mycologia 



unheated-soil prepared in various ways proved to be fully as 

 favorable, as culture media for Pyronema, as similar soil treated 

 with distilled water, the variations being no more marked than 

 would be expected in experiments of this nature. Heated-soil 

 treated with distilled water and that treated with the extract of 

 unheated-soil both gave good results. 



These results indicate that if the failure of Pyronema to grow 

 on unheated-soil is due to toxic constituents present in the soil 

 these substances are not soluble in water, at least not in sufficient 

 quantities to render extracts of such soils toxic to Pyronema. 



III. Pyronema Growth a Food Problem 

 It had often been noted in the course of our experiments that 

 heated-soil when watered had a peculiar and characteristic pun- 

 gent odor, together with the rather pleasant odor of caramel. 

 It had also been noted that heated-soil when watered, often 

 assumed a darker color than the same kind of soil unheated and 

 watered in the same manner. This change of color was not uni- 

 versal but frequently occurred and probably depended upon the 

 intensity of the heat. 



Having repeatedly failed to show that the extract of unheated- 

 soil had any toxic influence on the growth of Pyronema, we 

 were inclined to abandon the toxin theory as an explanation of 

 the failure of this fungus to grow on unheated-soil. It then 

 occurred to us to reverse our experiments and try the effect of 

 heated-soil extract with unheated-soil (p. 115). On December 2, 

 a five-inch pot of soil was heated to a temperature of 175° C. for 

 about two hours. This soil was then cooled and the pot placed 

 over a filter in an extraction apparatus arranged for this 

 purpose. Distilled water was poured into the pot until it 

 percolated through the soil and filtered into the bottom of the 

 extraction apparatus. A similar pot of the same kind of soil 

 unheated was treated in the same manner. The extracts of the 

 heated-soil and unheated-soil made in this manner were very 

 different ; the extract of the heated-soil was of a bright amber or 

 reddish-brown color and very clear, while that of the unheated- 

 soil was clear or slightly clouded but with no trace of the color 

 characteristic of the heated-soil extract. The odor of the heated- 



