38 MORPHOLOGY AND CULTUILE OF MICROORGANISMS 



by a very large group. Between the typical forms are many gradations 

 resulting in many families whose relationship to one or the other 

 group is difficult to determine. Probably the ancestral history 

 (phylog^ny) of the fungi, if known, would show several or many-lines 

 of descent rather than one. Certain of these groups may be presented 

 briefly. 



Bacteria. — In the scheme of plant grouping presented (page 77), 

 which is only one of many attempts to show relationships, the bacteria 

 are placed with a group of single-celled green or blue-green forms as 

 Schizophyta or fission-plants because of reproduction only by the 

 division of the cells. 



Phycomycetes. — The Phycomycetes are called algal fungi because 

 they resemble certain groups of green filamentous forms in many 

 particulars. In this group two general t3^es of sexual reproduction 

 are met with — zygospore formation and oospore formation. The 

 first, found in the Zygomycetes represented by the common mucors, con- 

 sists of the fusion of terminal cells of branches of the myceHum similar 

 in appearance but differentiated in sex. As a result of this fertilization 

 large thick-walled resting cells are produced, called zygospores, from a 

 Greek root meaning yoked (Fig. 32). In oospore formation, found 

 in the Oomycetes, the conjugating cells differ in appearance as well as 

 in function. The oospore is large and is rich in food materials; the , 

 antheridium is much smaller, penetrates and fertilizes the egg, which 

 afterward develops into a thick-walled resting spore. The very de- 

 structive downy mildews belong to this group. 



AscoMYCETES.— In this great group sexuality was denied until recent 

 years, but has been proved in cases enough to establish a presumption of 

 more general occurrence. The characteristic structure of the group is 

 the ascus, a sac containing, when ripe, typically eight spores, some- 

 times a less number by the failure of some to develop, sometimes a larger 

 number, usually some multiple of eight. The ascus when sexuality is 

 known is developed subsequent to fertilization, not directly from an 

 egg cell. The group presents a great variety of fruiting masses pro- 

 duced in connection with the asci. The simplest forms are loose webs 

 of hyphas enmeshing a few asci; other forms show clubs, cups, flask 

 forms, crusted areas, the type of mass in each case being characteristic 

 of the family, genus and species represented. Only a few of many 

 thousands of these forms are encountered in bacteriological work. 



