206 H. JOEL CONN 



The interpretation of the figures hinges upon the question 

 whether these organisms should be regarded as filametous fungi 

 producing spores or as unicellular bacteria occurring in filaments. 

 On ordinary culture media they exist as branched filaments 

 that break up under certain conditions into short rods or coccus- 

 like bodies, known as conidia because of their similarity to the 

 conidia of molds in method of formation. When such cultures 

 are plated, each colony ordinarily comes from one conidium or 

 group of conidia. If they grow similarly in the soil and if the 

 conidia are actually spores, an increase in the number of colonies 

 on the plates may indicate merely an increase in spore-produc- 

 tion. A few observations are at hand, however, to indicate 

 that Actinomycetes occur in the soil not as filaments but as 

 chains of short rods or cocci closely resembling ordinary bacteria. 

 If this is the normal mode of growth in the soil and if these 

 bodies are individuals instead of spores, an increase in the num- 

 ber of colonies on the plates may be regarded as more nearly 

 representing a true increase in the number of the organisms in 

 the soil. 



Making the assumption that the latter condition actually 

 exists in the soil, which seems justified so far as the facts are 

 known, there are two explanations of the higher numbers ob- 

 served in sod soil that seem sufficiently probable to be considered. 

 One is that sod soil becomes more compact in time than culti- 

 vated soil and that poor aeration favors the Actinomycetes in 

 some way, in spite of the fact that they ordinarily seem to like 

 a good supply of oxygen. This explanation does not well fit 

 the facts, however; for it has been found that sod soil, dug up 

 and well aerated and then kept in a pile for three months, may 

 still retain its high Actinomyces content. The other explanation 

 which has been considered is that the Actinomycetes are active in 

 the decomposition of grass roots or perhaps of plant roots in 

 general. In view of the past observations as to the association 

 between Actinomycetes and plant roots, this explanation seems 

 worth bearing in mind. Experiments are now being carried 

 on which are designed to show whether or not this is the true 

 function of Actinomycetes in soil. 



