THE ALGA-LIKE FUNGI 239 



of yeasts that are cultivated for the purpose. The yeasts that are 

 distributed indiscriminately by the air are called wild yeasts, to 

 distinguish them from those which are cultivated for the pur- 

 poses of brewing and bread making. The wild yeasts some- 

 times become established in cheeses and other dairy products, 

 and also in breweries, where they set up fermentations that 

 render the food or drink unfit for use. 



The raising of bread results from the fermentation by yeast 

 of sugar that is present in the dough. 1 The cavities, or holes, in 

 the dough are formed by bubbles of carbon dioxide- which, with 

 the small percentage of alcohol developed, is driven off in the 

 baking. Compressed yeast is made in certain distilleries from 

 cultures in large vats, whose yeast scum is removed and pressed 

 into the yeast cakes that are sold for domestic use. 



CLASS VII. THE ALGA-LIKE FUNGI, OR 

 PHYCOMYCETES 



260. The alga-like fungi. The Pliycomycetcs (meaning alga- 

 fungi) comprise a large number of forms which resemble the 

 alga3 in their structure and methods of reproduction. Some of 

 them are one-celled and microscopic, but others are very con- 

 spicuous mold forms, and certain types are destructive parasites 

 that cause some very serious plant diseases. The interesting 

 fungus (Empusa) which kills the house flies, that are frequently 

 found attached by their mouth parts to window panes and 

 woodwork, is in a special group of this assemblage. We shall 

 only be able to consider representatives of the following three 

 orders of this interesting class of the fungi: (1) the molds, 

 (2) the water molds, and (3) the blights and rots. 



261. The molds. The molds (order Mucorales) form very ex- 

 tensive and conspicuous shining cobweb-like growths (Fig. 210) 

 through and upon the material of manure heaps and other 



1 See the paper by Helen W. Atwater, " Bread and the Principles of Bread 

 Making," United States Department of Agriculture, Farmer's Bulletin 112, 

 1900. 



