40 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



IMPERFECT FUNGI. A very large number of species are known 

 which have never been seen to produce sexual fruits or fruits character- 

 istic 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 importance, and many of the species most frequently 

 met in bacteriological work. Sometimes one or a few species of a large 

 group produce the perfect form while very many species cannot 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 as in the more common species of Peni- 

 cillium reproduce for an indefinite number of generations by conidia. 

 Such species do not appear to need the perfect form and hence ap- 

 parently have, in some cases, lost the power to produce it. Genera 

 consisting entirely of such species are very properly retained as form- 

 genera in the Imperfect group. 



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 to 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- 

 phytic species has been recognized by the development in recent years 

 of the literature of plant pathology (phytopathology). These cannot 

 be considered in this work. 



CYTOLOGY OF MOLDS* 



GENERAL STRUCTURE OF MOLDS. Three kinds of cell-structure 

 formation are found in molds: 



i. Some, belonging to the Phycomycetes, show no cross- walls; they 

 have a much branched, felted mycelium, but in the early stages there 



* Prepared by A. Guilliermond. 



