14 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



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 10), 

 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 types of sexual reproduction are met with, 

 zygospore formation and oospore formation. The first, found in the 

 Zygomycetes represented by the common mucors, consists of the fusion 

 of terminal cells of branches of the mycelium 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 mean- 

 ing yoked (see Fig. 2). 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 destructive 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 where sexuality is 

 known is developed subsequent to fertilization, not directly from an 

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

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

 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. One genus is, however, 

 constantly found. The commonest species of Aspergillus produces bright 

 yellow, globose fruiting bodies, called perithecia filled with asci. These 

 are borne upon the surface of the substratum and often give a yellow color 

 to the colony by their abundance. Such perithecia consist of the ascog- 



