MOLDS. 15 



enous cells and the asci produced by them, about which a more or less 

 completely closed sac or wall has been formed, by the development of the 

 sterile cells adjacent to the fruiting ones. 



BASIDIOMYCETES. In the Basidiomycetes there is still further re- 

 duction of the evidences of sexuality. In one border group, the rusts, 

 sexual processes have been shown to be more or less developed. In the 

 typical Basidiomycetes sexuality is as yet undescribed if present at all. 

 The typical structure is the basidium, a 'spore-bearing cell characteristic- 

 ally producing at its apex four protuberances called sterigmata (singular, 

 sterigma), each bearing a single spore. These basidia are grouped into 

 many kinds of fruit bodies, from occurrence here and there upon a loose 

 web of hyphae to dense columnar areas covering the gills of the mushrooms 

 or lining the cavities of the puffballs. Very few of these species occur in 

 bacteriological studies. 



IMPERFECT FUNGI. A very large number of species are known which 

 have never been seen to produce the characteristic fruits of the great 

 groups. These are brought together and described as form-genera by 

 their method of asexual spore formation. From the lack of the organs 

 used in classifying the other groups, these are called the imperfect fungi 

 and their grouping regarded only as temporary, a convenience for the 

 identification of materials. These include many forms of economic im- 

 portance, and many of the species most frequently met in bacteriological 

 work. Sometimes one species of a large group produces a perfect form 

 while no other species can be induced to do so. Some of these species 

 undoubtedly represent stages of perfect fungi whose perfect forms 

 simply are not recognized as connected with these; others reproduce for 

 an indefinite number of generations by conidia. Such cases do not 

 appear to need the perfect form and hence apparently have, in some 

 cases, lost the power to produce it. 



As found in nature all these forms are parasitic, saprophytic, or 

 capable of both modes of life. All depend more or less completely 

 upon organic matter for nourishment. Great diversity exists, how- 

 ever, in their adaptation to environment. Many of them are not only 

 parasitic but so closely adapted or parasitizing particular host-species 

 as not to be found elsewhere. Others attack several or many species, 

 usually related. Even among saprophytes many species are found only 

 upon particular forms of decaying animal or vegetable matter. The 

 great economic importance of these parasitic and closely adapted sapro- 



