38 A MANUAL OJ* THE PENfCILLlA 



Penicillium decumhens regularly produces a perfume-like odor suggesting 

 castile soap. Suffice it to say that in a limited number of cases odors are 

 sufficiently distinctive to indicate presumptive relationships, but in the 

 great majority of cases they afford unreliable and questionably useful 

 criteria for species identification or separation. 



Variations and Mutations 



The Penicillia represent an unusually variable group of molds, and al- 

 though successful taxonomy and nomenclature must be based upon strains 

 considered to be "normal," the worker must recognize that variations and 

 mutations often occur. In laboratory cultures, these usually arise in one 

 of two ways. They may develop as a result of progressive change under 

 continued laboratory cultivation and appear as sectors or areas of altered 

 coloration, colony texture, or rate and habit of growth (Col. PI. I). On 

 the other hand, they may appear as limited sectors or areas showing an 

 abrupt change in some conspicuous character such as color of ripe conidia 

 or an inability to develop normal conidial structures upon the substratum 

 employed. The latter type of change commonly remains stable through 

 subsequent recultivations, hence may be regarded as a true mutation; the 

 former type often continues to show further change in the same, or in some 

 other direction, hence is usually regarded as representing a sort of step in 

 a process of progressive variation. Strains believed to represent both 

 types of development may be isolated from natural sources. . 



If reasons exist for believing them to represent merely different aspects 

 assumed by a common and cosmopolitan species such variations or muta- 

 tions are usually not accorded taxonomic status. For example, in the 

 species Penicillium citrinum Thom, some strains show marked differences 

 in colony texture and in the amount of sporulation under the usual con- 

 ditions of culture. Another strain produces conidia that are tan to avel- 

 laneous in color. Still another fails to utilize nitrate nitrogen. None of 

 these are, however, regarded as warranting specific or varietal status since 

 they apparently conform with P. citrinum in every other way, and since 

 variant or mutant strains have been artificially produced in other species 

 which differ from the normal parents in seemingly identical ways. 



COLOR MUTATIONS 



Mutations based upon the color of mature conidia are among the most 

 common and the most conspicuous types encountered. As a rule these 

 are either white (i.e., completely colorless) or show some tan to avellaneous 

 shade. White mutants have been isolated from Penicillium chrysogenum, 

 P. yioiaium, P. uriicae (Col. PI. I), and a strain formerly considered as 

 P. sartoryi Thom but now regarded as a variant of P. citrinum. Tan 



