USE OF THE MANUAL 109 



Conidia: Asci: 



Form and markings : PoVm and dimensions : 



Dimensions, range: Origin, in chains or single: 



Arrangement of chains: 



Ascospores: 

 iyclerotia: Pattern and markings: 



Abundance and manner in which borne : Dimensions • 



Character, form, and measurements: 



Significance in nature and relation 

 to special substrata if known: 



In the preparation of any species diagnosis, or in the identification of any 

 given strain, some of the above information may not prove significant, hence 

 its inchision in a description may be unnecessary. The information should, 

 however, be routinely obtained, for only in this way can non-essentials be 

 omitted. 



With such a descriptive sheet before him, properly completed for a 

 particular strain, the worker should not experience too great difficulty in 

 locating the section of the genus and the general series to which his Peni- 

 cillimn belongs. Within the series, correct assignment to the proper species 

 may be reached through the series key and by carefully comparing the data 

 for his strain with that contained in the descriptions of recognized species. 

 Unfortunately, many of the separating characters which have to be used 

 in the genus Penicillium are not exact and clean-cut. Furthermore, the 

 genus is characterized by great variability and intergradation between 

 different strains and species. For these reasons, difficulties will be en- 

 countered. In the assignment of an unidentified strain to a certain series, 

 and especially to a particular species, it Avill often become necessary to 

 explore two or three possible assignments before reaching a decision as to 

 which represents the correct one. The placement of some species is un- 

 questionably arbitrary, but in these cases we have attempted, by cross- 

 keying and otherwise, to indicate other possible areas of relationship. 



Bases of Classification 



An ideal classification of the genus Penicillium should, theoretically, be 

 based upon the development of perithecia, since the occurrence and pattern 

 of a perfect stage is generally accepted as the deciding factor in determining 

 true relationships among the fungi. Furthermore, such a stage is, as a 

 rule, sufficiently conspicuous and distinctive to greatly facilitate identifica- 

 tion at all levels of classification. There can be little doubt but that if all 

 Penicillia produced an ascosporic stage we would be in position to present 

 a less arbitrary and probably a briefer, taxonomy of this difficult genus. 

 All of the Penicillia, however, do not develop ascospores. In fact, only a 

 relatively small percentage of the known species have a perfect stage. The 

 vast majority of the strains which are significant in agriculture or in in- 



