USE OF THE MANUAL 115 



metulae, and sterigmata occurred in three's, four's, or other fixed numbers, 

 but no species of Penicillium is so precise in its pattern of development. 

 Specification of the usual number of elements in a verticil is very valuable, 

 but this number should not be dra\vn in too specific terms if it is to be repre- 

 sentative of a normal preparation rather than of a few selected fruiting 

 structures. 



Too great emphasis should not be placed upon limited changes in color, 

 amount of sporulation, or other features which may in part reflect changes 

 that are taking place in the culture substratum. Since the mold is usually 

 growing upon a mixture of several fermentable or decomposable materials, 

 the nature of the nutrient substratum may be expected to change from 

 day to day. For example, in the growth of Penicillia changes in pH are 

 common and may markedly aff'ect cultural characteristics such as amount 

 of sporulation, intensity of pigmentation, etc. 



Too great ex-pectations should not be placed in the continued stability 

 of individual strains in culture, or upon too exact duplication of new isolates 

 assigned to the same species. The existence of great series of related 

 strains, typified by such species as Penicillium expansum, P. chrysogenum, 

 P. roquejorti, P. terrestre, and others is evidence that mutability is the rule, 

 rather than the exception, throughout the genus. Strains and species 

 differ. Thom's type of P. camemberti (now maintained as NRRL 877, and 

 illustrated in fig. HOC and D) today presents a cultural picture indistin- 

 guishable from that developed when it was first isolated in 1904; his type 

 of P. oxalicum (now NRRL 787 and illustrated in fig. lOOE) isolated at 

 about the same time, lost the most striking characters emphasized in the 

 description within the first dozen transfers. It is today hardly recogniz- 

 able in culture as the same species, although the pattern and measurements 

 of its conidial structures confirm its identity (see p. 381). 



Biochemical reactions, or the elaboration of specific products, often 

 furnish valuable clues to relationships. However, extensive use of such 

 characters would necessitate a quantitative and qualitative studj^ of the 

 biochemical activities of each strain, hence add immeasurably to the task 

 of identification. Furthermore, the capacity of a given strain to produce a 

 specific reaction, or product, often diminishes and may even disappear un- 

 less the greatest care is exercised in culture maintenance. It is character- 

 istic of almost all members of the Penicillium chrysogenum series to form 

 penicillin, but the amount produced varies from very substantial to practi- 

 cally zero. At the same time, penicillin formation has been demonstrated 

 for P. avellaneum (in the Biverticillata-Symmetrica) and P. turhatum (in 

 the Monoverticillata) . The usefulness of biochemical identity as a de- 

 cisive criterion in the taxonomy of molds is open to serious question. 



With few exceptions the descriptions of species as presented in this Man- 



