34 A ]VL\NUAL OF THE PENICILLIA 



Various authors have discussed the phenomenon of coremium formation 

 in Penicillium and related genera (Wachter, 1910; Blochwitz, 1925; and 

 others). Some investigators hold that coremium formation may be in- 

 duced in any Penicillium by proper culture methods, while others maintain 

 that coremium formation is an inherent characteristic of certain species 

 only, and becomes evident Avhenever certain favorable conditions prevail. 

 Our OAvn observations indicate that coremia may arise in connection vnih. 

 two forms of antecedent structure: (1) the development in the mycelium 

 of trailing or ascending ropes of hyphae, or (2) the development of all or 

 part of the conidiophores into clusters or fasicles. An accentuation of 

 either tendency leads directly to structures described as coremia. 



When grown in laboratory culture a few species, such as Penicillium 

 claviforme, retain a strictly coremiform habit. Other species, including 

 P. expansum Link, normally produce conspicuous coremia in their natural 

 habitats, but tend to lose this characteristic when groA\Ti on the usual 

 substrata of the laboratory. 



COLONY MARGIN 



Correct information concerning the relation between submerged myce- 

 lium, aerial hyphae, and conidiophore origin can be best established at the 

 margin of the growing colony. The period of satisfactory observation 

 begins after three or four days in rapidly growing species and ends as a 

 rule with the cessation of growth during the second week. For slower 

 growing species the period for critical observation necessarily begins later 

 and lasts longer. In some species the mass of mycelium is sufficiently 

 dense and the growth sufficiently abrupt to interfere with satisfactory use 

 of the compound microscope at the colony margin. Usually, however, by 

 selecting petri dishes in which two or more colonies are present, essential 

 observations can be made in the inter-colony area where the rate of growth 

 is reduced and where the process of fruiting is accelerated. It is primarily 

 with this end in mind that our cultures are usually planted with three 

 colonies to the culture plate. 



COLONY GROWTH 



At the initiation of colony growth the germ tubes quickly elongate, 

 divide into cells by septation and branch extensively to form closely woven 

 networks or felts of mycelia. In a favorable nutrient evenly distributed as in 

 a petri dish the resulting colonies are usually approximately circular, al- 

 though marginal areas may be characteristically even, notched or stellate, 

 or crenulate because of the establishment of main radiating hyphae from 



