188 H. JOEL CONN 



on ordinary media, in spite of the fact that they are not very 

 abundant in soil. In nitrogenous culture media these bacteria 

 grow rapidly and cause a vigorous amonification. For this 

 reason they have been assumed to be the important ammonifiers 

 of the soil. 



This assumption was accepted as reasonable when I began 

 to study the bacteria of soil. The first suspicion to the contrary 

 came when it was noticed that the numbers of these spore- 

 formers in the soil remained almost constant under all conditions, 

 while the other bacteria varied in number according to the mois- 

 ture content, aeration of the soil, or other conditions. The most 

 natural explanation for this seemed to be that these bacteria 

 lived over unfavorable conditions in the form of spores. It was 

 soon realized, however, that this argument could not be carried 

 to its logical conclusion without assuming that spore-formers 

 were normally present in soil only as spores ; in which case natur- 

 ally their nximbers would not vary. 



A series cf tests to investigate this matter has been made at 

 the New York Experiment Station during the past year. The 

 method used depended upon the fact that spores can resist 

 higher temperatures than the vegetative forms. To determine 

 the number of spores and vegetative rods present in any soil, 

 one lot of diluted soil-infusion was plated in the ordinary manner, 

 while a parallel lot of the diluted infusion was heated before 

 plating ;or fifteen or twenty minutes at 75 to 85°. Then the 

 colonies of the three spore-bearers, B. nycoides, B. cereus and 

 B. megitherium, appearing on each set of plates, were counted. 

 The colonies that developed from the heated infusion were as- 

 sumed to arise from spores only; while in the case of the un- 

 heatel infusion colonies might arise from vegetative rods as well. 



T^e culture medium used in these tests was gelatin.^ On 

 this medium each of the three organisms investigated produced 

 a farly characteristic colony, so that it was ordinarily possible 

 to distinguish them with little difficulty from non-spore-formers 

 on the plates made from unheated infusion. Plates were in- 



* Twelve per cent of Gold Label gelatine dissolved in tap-water and clarified 

 wth white of egg. 



