88 A MANUAL OF THE ASPERGILLI 



In using this manual, and in studying the Aspergilli generally, it is im- 

 portant that the worker should realize that these organisms vary within 

 the species, the species vary within the group, and to a lesser extent, the 

 groups themselves vary within the genus. In other words, nature did 

 not realize that we were going to write this manual when the various 

 species and groups were being developed, hence not all of the forms one 

 encounters will fit into the various compartments which have been con- 

 structed, although these are, on the whole, comparatively elastic. This can 

 be illustrated in another way. If we completely disregard color, the genus 

 Aspergillus, as depicted in figure 20, can be likened to the spectrum. There 

 is a green region and a yellow region in the spectrum and the two regions are, 

 on the whole, distinct. Furthermore, within each of these, certain fixed 

 and definite lines can be identified. It is, however, extremely difficult, if 

 not impossible, to say where the green region ceases and the yellow begins. 

 So it is with the species and groups of the Aspergilli. There is a definite 

 nidulans group and a definite ustus group, but the line separating the two is 

 extremely tenuous. 



We do not, in any sense, infer that the Aspergilli cannot be classified — 

 that they cannot be separated into groups, species, and even varieties. 

 This manual is evidence that they can. But we do wish to emphasize that 

 we are dealing with living and variable organisms, and that in describing 

 them, we should be as explicit as possible and still keep our concepts reason- 

 ably elastic. 



Key to Groups — Based Primarily Upon Color 



The second key is based primarily upon color and is entirely artificial 

 in its construction. The various groups are separated by contrasting 

 coloration, and closely related groups may appear widely separated in the 

 key. In practice, such a key is very useful since color is the most obvious 

 character of an Aspergillus, and since the species comprising a particular 

 group, with but few exceptions, are characterized by variations in shade of 

 color rather than differences in basic coloration. Presumptive assignment 

 of the Aspergilli to groups can usually be made from this type of key which 

 is based primarily on color supplemented by the use of a handlens or dis- 

 secting microscope. 



A. Conidial heads in definitely green, blue-green, or yellow- 



green shades in young fruiting colonies B. 



AA. Conidial heads lacking green colors (Greenish in excep- 

 tional cases) K. 



B. Conidial heads in green and blue-green shades C. 



BB. Conidial heads in yellow-green shades A. flavus group 



