170 THE SOIL FLORA 



in the recognition of species. On gelatin there is less diversity of 

 growth than on agar. The growth is generally gray, brown, or 

 colorless; the aerial mycelium is often lacking, and if present is 

 white, gray, or colorless; and if the medium itself is colored, it 

 generally becomes a reddish brown. 



The growth in liquid media is also characteristic. The medium 

 remains clear except for small colonies that may sink to the bottom, 

 remain in suspension, float on the surface, or adhere to the walls 

 of the tube. The surface colonies often grow together and become 

 covered with a mass of aerial mycelium, sometimes forming a firm, 

 wrinkled membrane that strongly suggests the surface membrane 

 of the tubercle organism growing on broth. Pigments are often 

 produced in liquid culture, the pigment varying with the composition 

 of the medium and with the species growing in it. 



Nearly all liquefy gelatin and ammonify proteid, Hunter rnain- 

 taning that ammonification is their chief function. Nitrate reduc- 

 tion has often been observed, as has the decomposition of cellulose. 

 Some are animal pathogens, and at least one a plant pathogen. 

 Other important physiological activities will undoubtedly be worked 

 out when the technic for studying them is further developed. It 

 is not impossible that they are as diverse in physiology as are the 

 true bacteria. 



One of the most common characteristics of many members of this 

 group is their peculiar odor. They have a pungent, musty odor, 

 difficult to describe, but impossible to mistake after once having it 

 brought to the attention. It is sometimes spoken of as an earthy 

 odor, but it would be more correct to say that soil often has an 

 actinomyces-odor, as the odor of the cultures is much stronger than 

 that of soil, and the soil odor is undoubtedly due to the actinomy- 

 cetes it contains. The odor seems to be associated with the aerial 

 conidia, and does not seem to be produced by cultures that do not 

 possess aerial mycelium. Not all species of Actinomyces .have this 

 odor, however, even when an abundant aerial mycelium is produced. 



Various functions have been ascribed to the actinomycetes 

 ammonification, nitrate reduction, and cellulose-decomposition but 

 enough work has not been done to enable a definite statement as 

 to whether these are their functions in the soil. Conn has demon- 

 strated that the addition of grass roots to a soil materially increases 

 this group of organisms and Waksman and coworkers consider 

 that inasmuch as the actinomyces are strong cellulose decomposers 

 and weak producers of ammonia their probable role in soil fertility 

 lies in the formation of humus. 



REFERENCES. 



Conn, H. Joel: Soil Flora Studies, New York Agr. Exp. Sta. Tech. Buls., 57, 58, 59, 

 and 60. 



Waksman, Silman A.: Cultural Studies of Species of Actinomyces. Soil Science, 

 1919, viii, 71-215. 



