VARIATION 



77 



workers, certain observations of a summary character can be made regard- 

 ing variation in the Aspergilli: 



1 . Aspergilli include strains and species adapted to a very wide range of 

 environmental conditions. Such conditions may influence materially the 

 cultural and morphological characteristics of these molds. 





ft ' 



B 



,A 



c 



Fig. 19. Photomicrographs showing details of structure in the conidial heads of 

 the parent strain, and in two selected mutations of Aspergillus lerreus (NRRL No. 

 265) produced by ultra-violet, radiation, X 600 A, Typical heads of non-irradiated 

 parent strain. B, Mutation in which conidium formation is incomplete and cells 

 adhere in long chains in liquid mounts. C, Mutation in which many fruiting struc- 

 tures develop vesicles but often fail to produce sterigmata and spores. 



2. Under natural conditions, great numbers of variants appear along with 

 occasional sharply separable forms, or mutants, which are definitely of 

 species rank. The extent to which natural mutations may account for 

 described species is a matter of conjecture. 



3. The Aspergilli can be made to mutate in the laboratory by subjecting 

 them to a variety of different excitants, or stimuli. Induced mutations may 

 parallel some of those found in nature and described as species. 



