40 A MANUAL OF THE PENICILLIA 



but recognition of their existence is essential to the establishment of a 

 reliable nomenclature. 



STERILE OVERGROWTHS 



Areas of sterile, or essentially sterile, overgrowth may at times develop 

 in otherwise typical strains of almost any species of Penicillium. Such 

 overgrowths appear to be particularly characteristic of certain species, 

 hence may introduce problems in identification and assignment. For 

 example, Penicillium solitum Westling not uncommonly exhibits such 

 developments in central colony areas after 10 to 14 days, and colonies of 

 P. italiciim, P. cyclopium, and other species often show developments of 

 flocculent, non-sporulating mycelium which may materially alter the 

 cultural aspect of strains unless special precautions are taken to eliminate 

 such variant growth. In the case of P. schneggii Boas, the occurrence of 

 sterile mycelial tufts was long recognized as a specific and diagnostic char- 

 acter; minimizing the significance of such tufts, we have in the present 

 Manual assigned this species with P. granulatum Bainier with which species 

 it is otherwise closely similar. Quite commonly, sterile overgrowths can 

 be isolated and continued in separate culture where they exhibit little if 

 any of the characteristics of the parent strains. 



Miscellaneous Types 



Whereas the above are among the types must commonly encountered, 

 many other kinds of variants and mutants have been observed among the 

 Penicillia. During the search for better penicillin producing molds, 

 conducted at the Northern Regional Research Laboratory and elsewhere, 

 variant strains showing the most diverse cultural characteristics were 

 isolated and studied (fig. 30). It is outside the scope of the present 

 Manual to discuss these in detail or to list the various types observed, but 

 it is important that the student of the Penicillia should be cognizant of 

 the range of variations which may be secured when a few selected strains 

 are investigated intensively. For detailed discussions of variation, 

 natural and induced, in members of the Penicillium chrysogenum series, 

 the reader is referred to papers by Raper, Alexander, and Coghill (1944), 

 Hansen and Snyder (1944), Pontecorvo and Gemmell (1944a and 1944b), 

 Raper and Alexander (194ob), Hanson, et al. (1945), Sansome (1946), 

 Raper and Fennell (1946), Whiff en and Savage (1947), Bonner (1947), 

 and others. There is little doubt but that an equal range of variant types 

 could be obtained from almost any species of Penicillium selected for 

 equally intensive study. 



