FUNGI AND FUNGOUS DISEASES OF PLANTS 247 
are enlarged, and at the same time the alcohol is evaporated. 
By former methods of bread-making pure cultures of yeast 
were less likely to be secured, wild yeasts very frequently ap- 
pearing. By modern methods, quite similar to those used in 
bacteriology, pure cultures may be obtained, and it is there- 
fore possible to secure the exact kind of fermentation of the 
dough that is desired.! 
The processes of fermentation by yeasts are used in the 
manufacture of alcohol, wine, beer, and other liquors which 
contain alcohol. Certain definite kinds of yeast produce cer- 
tain kinds of alcoholic fermentation, and it is necessary for the 
brewer to keep pure cultures of the desired yeasts in order to 
insure the particular quality of his product. In 1856 the great 
French scientist, Louis Pasteur, succeeded in devising methods 
of pure culture by isolating single yeast plants and growing 
a colony from each. Thus the particular result to be secured 
could be determined by the kind of yeast selected for use in 
fermentation. It was this method of pure culture which opened 
the way for many of the modern bacteriological investigations.? 
235. The lichens. This is a group of plants of peculiar 
habits and structures. Often members of the group may be 
seen adhering closely to the bark of trees (fig. 193) or to the 
surface of rocks. They also appear upon the soil in great 
abundance. Sometimes they hang from branches of trees in 
ragged gray masses (fig. 194) that look somewhat like the 
common Southern hanging seed plant known as Florida moss. 
1 An especially interesting paper is '‘ Bread and the Principles of Bread 
Making,’’ by Helen W. Atwater, Farmers’ Bulletin 112, U.S. Dept. Agr., 
1910. 
2 The following citations will indicate a few of the many plant diseases 
that are caused by sac fungi. 
** Root Rot of Tobacco,’’ Annual Report, Conn. Agr. Exp. Sta., 1906. 
‘* Peach Mildew,’’ Bulletin 107, Colo. Agr. Exp. Sta., 1906. 
‘* Wilt Disease of Cotton, Watermelon, and Cowpea,’’ Bulletin 17, Divi- 
sion of Vegetable Pathology, U.S. Dept. Agr., 1899. 
'*Black Knot,’’ Bulletin 81, Cornell University Agr. Exp. Sta., 1894. 
‘*The Bitter Rot of Apples,’? Bulletin 44, Bureau of Plant Industry, 
U.S. Dept. Agr., 1908. 
