110 



Mycologia 



ing. It was not found possible by washing with water to wholly 

 remove from the unheated-soil the unfavorable constituent but the 

 soil extract contained the substances of unheated-soil which are 

 unfavorable to the growth of Pyronema. Experiments with 

 heated-soil showed that the properties favorable to Pyronema 

 growth which were developed in the soil by heating may be 

 removed from the soil by washing and in other ways. The reason 

 for this is not clear. By boiling the extract of an unheated-soil 

 its unfavorable properties may be partially removed. An extract 

 of heated-soil cannot, to any great extent, render an unheated- 

 soil a favorable nutrient medium for Pyronema but this may be 

 accomplished by the addition of kainite, while the addition of 

 charcoal, coal, and coke in various forms yields no beneficial 

 results. 



n. Toxin Theory 



The conclusions of Kasarofif as to the reasons for the failure 

 of Pyronema to grow on unheated-soil while it thrives on heated- 

 soil seem to indicate that the unheated-soil contains a substance 

 toxic to Pyronema which substance is destroyed by heat. This 

 explanation is strengthened by the fact that other investigators 

 have found in soils substances toxic to the higher plants, which 

 substances inhibit the growth of such plants even in the presence 

 of an abundance of food material. 



The idea that there are toxic organic materials in soils arising 

 from previous plant growth or otherwise, is rather a new con- 

 ception but one which has been advanced and confirmed by 

 Schreiner and his collaborators in the last few years. Accord- 

 ing to these conceptions there are definite, organic compounds 

 in the soil, which, in the case of four substances, have been 

 isolated, crystallized and analyzed, thus proving their composi- 

 tion and constitution. Some of these substances were proved to 

 be harmful to plants by water-culture experiments, and all of 

 them belong to types of substances that may well prove toxic to 

 different kinds of plants on further investigation. 



Such organic bodies may arise in a variety of ways, by root 

 excretions from the growing plants, by simple decomposition and 

 oxidation of the plant remains in the soil, and by bacterial action, 

 etc. When one remembers that the plant body contains besides 



