252 Platit Biology 



Certain fungi kill insects which are harmful to man. Certain species 

 of bacteria attack insects and may produce illness and cause death, al- 

 though their specific pathogenicity has not been definitely proved. Bac- 

 teria and fungi aid in insect decay after death, thus returning their 

 chemical constituents to the soil to be used by future organisms. A few 

 species of slime molds (phylum Myxomycophyta) are parasites on living 

 seed plants (Fig. 35). 



There are two principal groups of yeasts: (1) the Saccharomyces 

 (sugar fungi), which are harmless (Fig. 37) and (2) the Blastomyces 

 (germ fungi), which are pathogenic. The harmless yeasts are of great 

 importance in connection with the manufacture of wines and beers and 

 in other industries which depend upon the fermentation of sugars and 

 similar substances. The most common fermentation which yeasts are 

 able to produce is the so-called alcoholic fermentation in which sugars 

 are attacked, with the formation of ethyl alcohol and carbon dioxide. 

 The various species of saccharomyces are able to ferment various sugars 

 and related substances, forming a large number of end products, many 

 of which are useful in industrial processes. The familiar "yeast cake" 

 is composed of yeast cells mixed with a small quantity of starch. When 

 harmless yeasts are added to bread dough, the cells multiply rapidly (if 

 proper temperature exists) and ferment the sugars, thus giving ofT car- 

 bon dioxide. This harmless gas escapes through the dough and causes 

 it to "rise." The gas leaves countless small holes which make the bread 

 porous and light. 



Certain fungi cause great economic losses by producing diseases of 

 higher plants. A few representative examples are the white rust of 

 radish, mustard, cress, and related plants; the chestnut blight, a fungus 

 disease which has exterminated practically all our chestnut trees; corn 

 smut; wheat rust; potato scab, which renders the skin of potatoes rough 

 and unsightly; the ergot of rye, in which the fungus, Ergot (Claviceps 

 purpurea) parasitizes the rye, resulting in poisonous, hypertrophied 

 grains. Epidemics of ergotism have been frequent in the past, but mod- 

 ern methods of cleaning have eliminated it to a great extent. The ergot 

 is a high-priced drug of high medicinal value. 



Certain pathogenic fungi cause diseases in man and other animals. 

 The following are rather common, representative types: (A) The patho- 

 genic yeastlike fungus (Blastomyces dermatitidis) (Fig. 74, A) produces 

 a chronic infection known as North American blastomycosis (Gilchrist' si 



