76 A MANUAL OF THE PENICILLIA 



sterile cover glass held in place by vaseline which was omitted along about 

 one-fourth of the line of contact, thus insuring ventilation while remaining 

 tight enough to exclude contamination and protect against evaporation. 

 We have used similar apparatus in studying the germination of conidia, 

 and specific types of morphology, but discarded it many years ago for the 

 study of mature penicilli because of the atypical character of the structures 

 produced as the colony developed beyond the germination stage. While 

 growth is not as a rule quite so restricted, the same general objections apply 

 to shde cultures as used by Henrici (1930, p. 30, figs. 25 and 26). The 

 latter type of culture could, however, have limited application in photo- 

 micrography. 



Incubation 

 temperature 



Cosmopolitan saprophytes such as most species of Penicillium will 

 grow well over a wide range of temperature. In general work, therefore, 

 it is not necessary to provide special incubators. Thorn, in 1910, reported 

 that a range of from 15 to 25°C. produced differences only in the rate of 

 development, not in the basic character of colonies. In our present study 

 we have extended this range to 30°C. for many species and find that in the 

 majority of cases the character of the colony still remains unaltered but 

 that growth is oftentimes more rapid and sporulation often somewhat 

 heavier. Less frequently, growth is reduced and sporulation decreased. 

 Zaleski incubated his cultures at 16 to 20°C., whereas Biourge generally 

 employed temperatures in the neighborhood of 18-20°C. For the present 

 study we have used incubation temperatures of 23 to 25°C. except where 

 otherwise noted. While this certainly does not represent the optimum 

 temperature for individual species in all cases, we consider it to be a favor- 

 able range for most forms. We feel that a uniform incubation temperature 

 is desirable for comparative purposes. 



Certain species of Penicillium represent marked exceptions and behave 

 very differently at temperatures above or below the level of 23-25°. 

 McCulloch and Thom (1928) found Penicillium gladioli to produce abun- 

 dant blue-green conidial structures when incubated at 14-15°C. The same 

 species, when incubated at 22°C. or higher, produced comparatively few 

 conidial structures but abundant hard sclerotia. Penicillium dwponti 

 represents a thermophilic species which grows luxuriantly upon most 

 natural substrata at temperatures of 40-47°C, (fig. 19B). The same spe- 

 cies makes a very limited growth at temperatures below this range and 

 fails to grow at room temperature (fig. 19A). Strains of Paecilomyces, 

 a genus generally regarded as related to Penicillium, grow quite well at 



