68 A MANUAL OF THE ASPERGILLI 



reduced conidial apparatus is observed in all of the strains of A. sydowi 

 examined. Over a period of many years variants have exhibited all 

 gradations in colony appearance from typical A. sydowi to the aspect of P. 

 restrictum of Abbott except for the presence of an occasional conidiophore 

 and head of .4. sydowi. Repeated cultural tests exclude contamination. 

 These variants occur in nature, they produce abundant conidia, and they 

 undoubtedly maintain themselves successfully in the field. 



Aspergillus fumigatus presents a somewhat similar condition. In this 

 species, typical cultures produce heavily sporing, velvety colonies that are 

 dark green in color and show almost no aerial mycelium. Other strains 

 commonly isolated from nature produce very floccose colonies and bear 

 comparatively few conidial heads (fig. 37). These heads, however, are 

 typical in form and in dimension. Strains possessing this contrasting 

 character are relatively stable in culture, but in this species, as in A. terreus, 

 all degrees of intergradation are found between this floccose type and strains 

 entirely typical of the species. The same story is repeated in other species. 



Appreciable variation can normally be expected among the isolates of 

 any of the very abundant and cosmopolitan species. Such variant strains, 

 however, are the exception rather than the rule, since the great majority of 

 isolates are quite typical of the species. Attention was called to this fact 

 in our study of the A. glaucus group (1941). 



When grown in comparative culture, strains successively isolated as repre- 

 senting a particular species usually show enough difference to give each 

 strain a kind of individuality. Exact identity, point by point, is not ex- 

 pected. Such strain variation may be incidental and unimportant, or it 

 may be correlated with activities which make one strain a valuable agent 

 in an industrial process and the other worthless. 



Intra-group Variation 



Inside the different groups of Aspergilli one normally finds somewhat 

 similar but wider variations than those seen among the strains constituting 

 any particular species. To what degree species in nature have developed 

 by mutations and by progressive variation can only be guessed. We do 

 know, however, that the species within a group, like the strains or varieties 

 within the species, are regularly bridged by intermediate forms which render 

 it difficult to establish sharp and immutable lines of separation. One can 

 almost cite it as a rule that the definiteness with which one regards a species 

 is inversely proportional to the number of strains of that species which have 

 been examined. Still species are necessary as guide-posts — as fixed points 

 around which closely related organisms showing a certain but limited 

 amount of variation can be grouped. 



The Aspergillus flavus-oryzae group can be taken as illustrative of the 

 type of variation to be expected within a group of the Aspergilli. Thorn's 



