USE OF THE MANUAL 117 



strains, which differ from a well-recognized species in some marked char- 

 acteristic. For example, the usage Penicillium cydopium var. echinulatum 

 is introduced to include a limited number of cultures showing all of the 

 essential characteristics of P. cydopium except that they produce unusually 

 roughened conidiophores and globose rough conidia. The term variety 

 is only useful for recognizing a clearly defined variation in color, colony 

 habit, or structure within an otherwise homogenous species. 



Mutants 



The term mutant, is nowhere used in this Manual to designate a taxo- 

 nomic entity. It is, however, commonly employed as a designation for 

 strains of known origin which develop inherited characters differing sharply 

 from those of the parent strain and species. If the source of a mutant, or 

 mutation, were unknown, the taxonomist would probably recognize it as a 

 species or variety, depending upon the character and importance of the 

 change encountered. For this reason, studies in experimental evolution 

 may soon necessitate some form of taxonomic recognition of artificially 

 induced mutants. 



Strains 



The term sfram is commonly used in this text, and refers to a particular 

 isolate selected and maintained under a name, number, or laboratory sym- 

 bol which insures identity. Such an organism may be wholly typical of 

 some particular species, or it may represent a variant distinguished by 

 unique cultural or microscopic characteristics. It may be obtained by 

 direct isolation from some natural source, or it may originate as a sub-culture 

 from an organism significant in some current investigation. In any case 

 it represents as nearly as possible a continuation in culture of some selected 

 cultural entity. Complete identity of many strains may be encountered 

 when these arise from individual conidia of some heavily sporing and seem- 

 ingly stable parent. Minor or conspicuous differences may be expected 

 among strains representing isolates from widely separated or markedly 

 different sources. Differences may likewise be expected among strains 

 derived from parent cultures which show moderate to marked instability 

 in laboratory culture. 



An individual strain may be either worthless or very valuable. If a 

 strain is important because of its biochemical potentialities, or because of 

 research that may have been based upon it, that strain should be main- 

 tained with punctilious care. It should be maintained as free from varia- 

 tion or loss of vitality as possible since valuable stocks can seldom be re- 

 isolated or reidentified with certainty from new materials. Other suitable 

 strains may be found or developed, but this usually requires much time 



