22 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



some one of the higher nitrogenous compounds, but many species refuse 

 to produce typically colored fruit without some form of starch or sugar 

 in addition to ordinary peptone and beef-extract. Very few species 

 grow well in alkaline media, but most species are tolerant of organic 

 acids at the concentrations found in fruits and vegetables. 



Some Common Species. Penicillium roqueforti, Thorn, is a green form 

 constantly found in pure culture in Roquefort cheese, frequently also in 

 ensilage. It is widely distributed and grows under many sets of conditions. 



Penicillium camemberti, Thorn, is the chief organic agent in ripening 

 Camembert cheese. Cultures of this species are floccose or cottony, at 

 first white, later gray-green. 



Penicillium expansum, Link, is a green form, always obtainable from 

 apples decaying in storage, upon which it frequently produces large 

 coremia. It is one of the most abundant species of the genus, widely 

 distributed in different countries. In cultures, colonies produce a 

 characteristic odor, suggestive of its common habitat, decaying apples. 



Penicillium brevicaule, Saccardo, is a form with rough or spiny brown 

 spores which has been used physiologically to detect the presence of arsenic 

 by its ability to set free arsine from such substrata. Except species 

 associated with particular processes or substrata, the identification of 

 the green species of Penicillium requires special methods and greater 

 care than is possible aside from special study of the group. 



ASPERGILLUS (AND STERIGMATOCYSTIS). The genus Aspergillus in- 

 cludes numerous species which develop under widely different conditions. 

 Many of these forms reach their typical development under drier condi- 

 tions than Penicillium and Mucor, such as stored grain, herbarium speci- 

 mens, dried flesh, or foods containing concentrated sugars, such as jams, 

 jellies, etc. Some excite processes of fermentation, and a few are asso- 

 ciated with diseases. 



Characters. The vegetative hyphae are creeping, submerged in the sub- 

 stratum or sometimes aerial also, loose, floccose, branched, septate, usually 

 colorless, and sometimes bright colored. Conidiophores or fertile hyphae 

 are erect, unseptate, or few-septate, usually much larger in diameter 

 than the vegetative hyphae, and gradually enlarged upward, ending in a 

 more or less abrupt dilation or head which bears closely packed colum- 

 nar sterigmata or conidiifierous cells over the whole or a large part of its 

 surface (Fig. 6). Each of these cells bears, in one group of species, a 

 single chain of conidia, in other species (called by some authorities 



