ECONOMIC IMPORTANCE OF FUNGI 
317 
and in part by the action of an enzyme (galactase) inherent 
in the milk from which the cheese is made. 
296. Vinegar. Cider vinegar is in every respect a 
plant product. It is formed from the juice of apples, and 
ripens or "ages" under the influence of bacteria. These 
bacteria produce an enzyme which causes "acetic acid 
FIG. 228. Roots of soy bean plants. A, from seeds that were soaked 
in soy bean bacterial culture over night; B, roots of check plant (not inocu- 
lated). (After Carman and Didlake. Courtesy of Kentucky Agr. Exp. 
Station.) 
fermentation." This is the acid to which vinegar owes 
its sourness. There is not room here to explain the 
process of vinegar making in detail. 
297. The Secret of Clover. Farmers have known for 
many years that a given piece of land produces a larger 
yield if the same crop is not grown continuously, season 
