30 A MANUAL OF THE PENICILLIA 



diffuse into the substratum. In some species the substratum is quickly 

 discolored and follows the color changes of the mycelium. In other species 

 the color is retained within the hyphal cells. Such contrasting conditions 

 may occasionally occur in species believed to be closely related. For 

 this reason, differences in color are often believed to reflect minor changes 

 in biochemical reactions rather than differences of fundamental impor- 

 tance. 



The part played by oxidation in successive color changes has not been 

 worked out systematically but is indicated by isolated observations. 

 For example, in cutting open a fresh Roquefort cheese containing a pure 

 culture of Penicillium roqueforti the mold is sometimes first seen as yellow 

 but changes to deep green within a very few minutes after being exposed to 

 air. 



Within recent years Raistrick and others have initiated a study of the 

 pigments produced by the Penicillia which may in time help to clarify 

 relationships within this genus. Several of the pigments described act as 

 indicators, changing color when the reaction is shifted from acid to alka- 

 line and vice versa. 



APPEARANCE AND TEXTURE OF COLONIES 



By examination with the naked eye, the hand-lens, and the low powers 

 of the compound microscope, it is possible to establish four fairly well- 

 defined colony types, namely: velvety, floccose or lanose, funiculose, 

 and fasciculate or coremiform. 



Velvety (fig. 3A) : Colonies are regarded as velvety or velutinous if all, 

 or nearly all, of the vegetative hyphae are submerged in the nutrient sub- 

 stratum, and if the conidiophores rise above the surface in a fairly dense 

 and even stand. Such colonies give the appearance of a surface of velvet, 

 or of a field of grain in miniature. Colonies of this type are characteristi- 

 cally produced in such species as Penicillium frequentans Westling, P. 

 oxalicum Thom, P. roqueforti Thom, and P. chrysogenum Thom (fig. 3A). 

 Velvety colonies are, as a rule, comparatively thin but may, upon occa- 

 sion, range up to 0.5 to 1.0 mm. or more in depth, 



Floccose or lanose (fig. 3B): Other species, for example Penicillium 

 camemherti Thom and P. caseicolum Bainier (fig. SB) produce a cottony 

 mass of branching and interlacing hyphae evenly or unevenly over the 

 surface of the nutrient medium. At a characteristic period in their de- 

 velopment, conidiophores appear as branches of these aerial hyphae. 

 Conidial areas usually appear first in the center of the colony and gradually 

 extend toward the margin. Throughout the growing period a sterile 

 white, or in some cases yellowish, margin surrounds the fruiting area, but 



