114 A MANUAL OF THE PENICILLIA 



established, have to be centered around certain specified strains which are 

 noteworthy either for some historical reason, or are selected as representa- 

 tive of large but generally rather variable groups of strains. 



Throughout this study, the existence of limited groups, or series, of Peni- 

 cillia A\dth broad cultural and morphological characteristics in common has 

 become increasingly clear. As a rule the members of such series likewise 

 show the same general physiological and biochemical characteristics. 



Species 



What constitutes a species of Penicillium? No answer will satisfy all 

 requirements. From the standpoint of the mycologist working in a culture 

 laboratory, however, two or more organisms may be regarded as belonging 

 to the same species if they have the same morphology, including both cul- 

 tural and microscopic characteristics, and if they agree in such routine 

 reactions as are determinable without the introduction of elaborate quanti- 

 tative analytical methods. 



The extent of variation, to be tolerated, should be based upon repeated 

 parallel cultivation and observation of such organisms upon selected cul- 

 ture media and at different temperatures, to determine the effect of nutri- 

 tional and environmental factors. All observable information is theoreti- 

 cally valuable, but the physical limitations of the culture room, and the 

 time and energy of the investigator, dictates that comparative cultivation 

 and observation should be generally limited to two or three carefully se- 

 lected culture media. To undertake to define all possible responses, strain 

 by strain, can easily become an endless operation resulting in the accumula- 

 tion of vast amounts of data often possessing little real significance. 



Cultural appearance and morphology must be expected to vary within 

 a certain range, governed by the limits established for the species, and 

 these limits should represent the product of observation, experience, and 

 good judgment. 



Unusual or bizarre structures which are occasionally encountered should 

 not be over-emphasized. The response of a particular organism to a 

 particular environment must not be over-looked, but the unusual must be 

 scrutinized and shown to represent a consistent response to conditions be- 

 fore it warrants inclusion in either description or illustrations. In any 

 elaborate study of a particular species, the range of variation in structure is 

 important. Of greatest importance, however, is the type of organization 

 usually found in the penicillus, and the type of colony development and 

 color production usually encountered in the growing culture. 



Too great emphasis must not be placed upon exact measurements or 

 numbers. Ranges of measurements, if carefully arrived at, are far more 

 significant than specific figures. It would be most convenient if branches, 



