88 CULTURAL STUDIES OF SPECIES OF PENICILLIUM. 



A color determination is therefore dependent for its value upon 

 knowledge of the form of carbon presented by the medium. These 

 same species grown with lactose as a source of carbon respond exactly 

 as they do in potato or bean agar or in gelatin free from sugar. 

 Another series of forms are little if at all changed in color by changing 

 the form of carbon presented, provided only that they are able to 

 grow readily in the medium as presented. It seems therefore certain 

 that very many widely distributed species are capable of assimilating 

 carbon in very widely different chemical forms and to produce in 

 such cases normal and typical growth and colors, whereas other 

 species are entirely dependent for normal growth and color upon 

 the presence of particular chemical combinations or groups of com- 

 binations. Study of the forms so responding likewise shows that 

 the ubiquitous species are capable of assimilating carbon in the most 

 varied combinations, whereas the forms lacking this power are 

 mostly less common and more specialized. 



EFFECT OF CONCENTRATED MEDIA. 



Similarly great differences in the mass of the growth produced 

 are directly attributable to the presence of nutrients easily available 

 or in greater concentration. The formulas usually recommended con- 

 tain the nutrient used, in extremely dilute proportions. The deter- 

 minations already given with different percentages of cane sugar 

 show that these species are able to assimilate sugar in widely different 

 concentrations. With the majority of forms studied there seems to 

 be no deleterious effect from increasing the concentration of the nutri- 

 ents offered until the solution has attained an osmotic pressure suffi- 

 cient to inhibit growth by plasmolyzing the cells, or until the reduced 

 percentage of water gradually reduced the rate of fungous growth. In 

 one experiment nine species were inoculated into a medium contain- 

 ing 5 per cent cane sugar, 10 per cent Witte's peptone, and 5 per cent 

 Liebig's extract. All species grew luxuriously. The mass of myce- 

 lium produced was much in excess of the results with ordinary culture 

 media. One form normally producing sclerotia produced a very rich 

 growth of mycelium, but no sclerotia. In media of higher concen- 

 trations the amount of growth — the mass of mycelium and conidia — 

 is, however, so greatly increased as to render the discrimination of 

 the character of the species more difficult by mechanical interference 

 due to the quantity of material. For purposes of stud}', therefore, 

 the usual formulae really produce the more satisfactory growth in 

 nearly every species, although from the standpoint of fungus devel- 

 opment such media must be recognized as far below the optimum 

 concentration for the species of this genus. 



It must further be noted that in cultures containing cane sugar 

 many species continue to produce conidia for a much longer period 



