78 A MANUAL OF THE ASPERGILLI 



4. Particular species of the Aspergilli tend to mutate along certain 

 definite lines, e.g., the production in A. niger of forms with tan to light 

 brown spore heads (Schiemann, 1912; Steinberg and Thorn, 1940; Whelden, 

 1940; and Raper, Coghill, and Hollaender, in press), and the production 

 mA.fumigatus of forms with colorless spore heads (Yuill, 1939; and Stein- 

 berg and Thorn, 1940a). The type of mutant produced is not governed 

 by the type of treatment given, although the number of mutations produced 

 is strongly influenced by this factor. 



5. Great variability in biochemical activity is encountered among strains 

 isolated from nature, but these differences are rarely linked with specific 

 morphological changes. 



6. The Aspergilli can be made to mutate physiologically as well 

 as morphologically by the application of various stimuli. Physiological 

 mutations may conceivably be of tremendous importance in the develop- 

 ment of improved strains for fermentation processes. 



7. Despite natural variation, most strains of Aspergillus when subjected 

 to critical transfer and maintenance under rigorous culture conditions can 

 be kept for many years with constant colony appearance, stable 

 morphology, and dependable biochemical activity. 



