PHYCOMYCETES 



27 



classes in a natural system which expresses their phylogenetic rela- 

 tionship, whereas the class Fungi Imperfecti is, to some extent at 

 least, an artificial classification of coniclial stages of natural groups. 

 They consist of "form species," "form genera," "form families," "form 

 orders." From time to time the perfect forms of these are discovered 

 and then the species are removed from the class of Fungi Imperfecti 

 and placed in their proper place in 

 the Ascomycetes. This leads to 

 some confusion. Thus the large 

 genus Aspergillus is placed in the 

 Fungi Imperfecti because most of 

 the knowTi species do not form asco- 

 spores; but such sexual spores have 

 been found in some species, which 

 are therefore included by some au- 

 thors in another genus, Eurotium, 

 of the Ascomycetes. It might be 

 argued that finding ascospores in 

 one species of a genus of imperfect 

 fungi would warrant transferring 

 the whole genus to the Ascoraj'cetes. 

 But this cannot be safely done, 

 since the genera of Fungi Imper- 

 fecti are based upon the mode of 

 formation of their conidia and it is 

 known that diverse types of Asco- 

 mycetes may produce the same sorts 

 of conidia. In a few instances, fungi 

 formerly classified in the Fungi Im- 

 perfecti were found on further study 

 to belong in the Basidiomycetes or 

 Phycomycetes. It is probable, however, that most of the Fungi Im- 

 perfecti are conidial stages of Ascomycetes. See page 94- Unfor- 

 tunately, it is the imperfect fungi which are of least interest to the 

 mycologist and have therefore had, in general, less intensive study, 

 and at the same time they are of most interest to the practical bac- 

 teriologist. Most of the fungi pathogenic for man must be placed 

 in this group, and likewise most industrial molds (not yeasts) be- 

 long to the Fungi Imperfecti. 



Phycomycetes. The Phycomycetes are the most primitive class 

 of the fungi. Some of them reproduce by both sexual spores and non- 

 sexual spores. There are three subclasses which are so diverse that 



Fig. 21. Stained section through 

 a peritheciura of Claviceps pur- 

 purea showing the elongated asci. 

 Each of these contains eight 

 needle-like ascospores. 



