THE FUNGI 243 



closely resemble these plants in their structure and reproduc- 

 tion. They have nevertheless become profoundly modified by 

 their parasitic and saprophytic habits and are therefore usually 

 classified into plant groups distinct from the green algse. 



The following simple classification will serve to illustrate 

 something of the great variation in form, structure, and repro- 

 duction represented among the fungi. 



1. Bacteria, including the simplest of the fungi, which occur 

 in the form of single cells or in loose colonies (Fig. 134). 



2. Algal fungi, including the common black molds (Figs. 139 

 and 140) and water molds infesting dead fish and flies in ponds, 

 lakes, and streams. These fungi resemble closely the green 

 algse, from which they have sprung, in both structure and 

 reproduction. 



3. Sac fungi, including the yeasts, the common blue and green 

 molds (Figs. 143-145), and the blights found on the leaves of 

 trees and various plants. These, with the exception of the yeasts, 

 are moldlike fungi, which form spores in large, saclike cells 

 called asci. These spore sacs, or asci, are usually inclosed in 

 characteristic fruit bodies. 



4. Higher fungi, including mushrooms, puffballs, rusts, and 

 smuts of cereal grains (Figs. 145-155). These higher fungi are 

 characterized by the production of conspicuous fruit bodies for 

 the production and scattering of asexual spores. 



YEAST 



The origin and relationship of the yeast plants to the other 

 fungi is not definitely known, although they are usually classi- 

 fied with the sac fungi. They are for the most part single-celled 

 plants resembling spores and are widely distributed in nature, 

 since they reproduce rapidly and are easily blown about in the 

 open by the wind or by currents of air in dwellings. They there- 

 fore occur on almost all exposed surfaces, particularly on those 

 which are moist and contain sugar or acids, so that grapes, 

 apples, and other fruits, when exposed to the air, are sure to have 

 some of these little plants lodged on the skin. 



