210 INTRODUCTION TO BOTANY 
vessels containing the milk are placed in water and brought 
to a temperature of 150° to 155° F., and then cooled and 
kept cool until used. This method kills most of the bacteria 
in milk and makes less change otherwise than does boiling.’ 
Bacteria are important in connection with many other in- 
dustries. The formation of acetic acid (the acid of vinegar) 
is due to the growth of several kinds of bacteria. Part or 
all of the processes of curing tobacco, tanning leather, pre- 
paring plant fibers (as flax and hemp), and making butter 
and cheese depend upon the growth processes of different 
kinds of bacteria. 
196. Bacterial diseases of plants and animals. Although 
many bacteria grow constantly upon and within other plants 
and animals, these are usually not disease-producing bacteria. 
Sometimes, however, malconditions in the host plant or ani- 
mal are produced by bacteria, and disease and death of the 
host may result.? 
Brief reference to a few kinds of bacterial disease will af- 
ford illustrations of some of the effects produced upon plants 
and animals (fig. 170). In the case of black rot of cabbage 
the bacteria enter the cabbage leaf through the leaf pores. 
Once within the leaf, they grow rapidly, and brown or black 
spots appear on the leaf as outward evidence of the inward 
ravages of the parasites. These leaves may become shriveled. 
The disease may spread throughout the plant and result in 
destruction of the entire head of cabbage. 
“Pear blight” is a disease which often seriously affects the 
leaves, young twigs, and fruit of the pear and apple. The 
bacteria cannot live under long exposure to direct sunlight 
1 “Care of Milk on the Farm,’ Farmers’ Bulletin 63, U.S. Dept. Agr., 1906. 
“Bacteria in Milk,” Farmers’ Bulletin 348, U.S. Dept. Agr., 1909. 
‘Sources of Bacteria in Milk,” Bulletin 51, Storrs Agr. Exp. Sta., Storrs, 
Conn., 1908. 
“Milk and its Products as Carriers of Tuberculosis Infection,” Bul- 
letin 143, Bureau of Animal Industry, U.S. Dept. Agr., 1909. 
2 A few diseases are caused by small animal parasites, but since these are 
not bacterial diseases, they are not included in this discussion. 
