MORPHOLOGY AND CLASSIFICATION 139 



acterized by the occurrence of a spore-sac called the ascus which 

 usually contains eight spores. The common aspergilli belong here. 

 The basidiomycetes are characterized by the occurrence of a 

 spore-bearing cell, the basidium, which bears four protuberances 

 called sterigmata (singular sterigma) upon each of which is a 

 single spore. Mushrooms and puff-balls belong to this group. 

 Besides these three well-defined families, there are many kinds 

 of molds and fungi concerning which definite knowledge's still 



F-Vy 



FIG. 60. Yeast cells stained with fuchsin. (Xiooo.) 



too incomplete for them to be finally placed. The common 

 oidium and penicillium and many parasitic molds are included 

 here. The molds 1 are especially important as causes of disease 

 in plants. Relatively few diseases of man or other animals have 

 been shown to be due to them, although the first diseases proven 

 to be due to micro-organisms were those caused by certain molds. 

 The molds possess the general morphological features of plants 

 except for the absence of chlorophyll. 



1 For fuller discussion of molds in general see Marshall, Microbiology, pp. 12-27, 

 article by Thorn. 



