16 CULTURAL STUDIES OF SPECIES OF PENICILLIUM. 



conidial fruit is usually a well-marked character and is often found 

 very useful in the separation of forms found growing together. 



Color. — Color and color changes are difficult to describe on account 

 of the deficiencies in the standards of color, but they form the first 

 character noticed. The variety of greens, blue greens, gray greens, 

 yellow greens, and shades of brown in the genus Penicillium baffles 

 one seeking descriptive terms. The shades of color peculiar to each 

 species under oft-repeated conditions are easily recognized and are 

 quite reliable. The alterations in the color of spores due to changing 

 the composition of the medium, as shown in the recent work of Stoll 24 

 and in this paper for species of this genus, and by others (Milburn, 15 

 Bessey 1 ) for other genera, emphasize again the necessity of uniformity 

 in and careful description of the culture medium. But in spite of 

 the admitted difficulties in color description, the careful observation 

 and record of the color of the colony at every stage of its development 

 is very necessary to identification. This is complicated by the 

 changes which occur at different ages of the colonies, so that it would 

 be easily possible to place certain species in at least two of the color 

 groups as designated by the older authors if we simply make our 

 observations a few days apart. 



The color of the mass of mycelium as observed from below (" re- 

 verse" as designated by Dierckx 5 ) in cultures gives striking con- 

 trasts. This must not be confused with discoloration of the sub- 

 stratum which may or may not be produced by the same species in a 

 given medium. The mycelium itself viewed in reverse has character- 

 istic colors in certain species which are useful in diagnosis and are 

 entirely independent of discolorations of the substratum. A colony 

 colorless itself may color the medium brightly, while a colony bright 

 colored itself may make no change in the color of the medium. 



Surface. — As a convenient term " surface" may be used to desig- 

 nate the general appearance or the texture of the aerial portion of 

 the colony. Perhaps the word "habit" would be in some measure 

 more accurate, but that would apply % also to the submerged myce- 

 lium. Comparison of the surface appearances of many cultures 

 shows that this is one of the most stable characters when the same 

 medium and conditions are used. For species of Penicillium and 

 allied genera two types of surface will include most of the species 

 met with. 



In the one type all or a large majority of the conidiophores in the 

 rapidly growing colony arise directly from strictly vegetative hyphse 

 which may be submerged in the substratum or lie upon its surface 

 or be alternately prostrate and submerged. Each conidiophore 

 stands separately, therefore, and usually all are found to be so nearly 

 of the same length that the surface appears to be velvety. Such a 

 surface may be called either velvety or "strict." A strict surface 

 may be called "closely strict" when the conidiophores are so short 



