THE BACTERIA (SCHIZOMYCETES) 171 



157. Relation to diseases of plants and animals. Growth of 

 bacteria within other plants and in animals often results in 

 disease of the host organism. Types of plant diseases that are 

 caused by bacteria are the crown galls produced upon roots of 

 apples, peaches, and pears; also cucumber wilt, and soft rot 

 of cabbage. IMany diseases of animals are caused by bacteria, 

 as hog cholera, tuberculosis of cattle, and anthrax or splenic 

 fever. It was in connection with the study of animal dis- 

 eases that the causal relation of bacteria was first conclusively 

 proved. Bacteria were discovered almost two hundred years 

 before it was proved that they cause diseases. Indeed, a good 

 deal was known about the nature of these diseases before it 

 was known that they are caused by bacteria. Means were 

 finally devised for isolating and growing alone many kinds of 

 bacteria, and then by the introduction of certain kinds into 

 susceptible animals definite kinds of disease resulted. 



Very brief reference to two kinds of bacterial disease will 

 afford illustrations of some effects produced upon plants and 

 animals by disease-producing bacteria. In the case of black 

 rot of cabbage the bacteria enter the cabbage leaf through 

 the leaf pores. Once a\ ithin the leaf, they grow rapidly, and 

 brown or black spots appear on the leaf as outward evidence 

 of the inward ravages of the bacteria. These leaves may 

 become shriveled. The disease may spread throughout the 

 plant and result in destruction of the entire head of cabbage. 

 The organisms of decay may follow the bacteria which pro- 

 duce the disease. 



In the disease known as anthrax, already referred to in 

 Sect. 152, when active anthrax bacteria or their spores are 

 introduced into the alimentary tract of cattle or sheep, they 

 find their way into blood vessels, where they grow with sur- 

 prising rapidity. In most cases the death of the host animal 

 occurs within a few hours to a few days from the time of 

 infection. If the dead body decays in an exposed place, the 

 anthrax bacteria or their spores may soon become a means of 

 new infection and the death of other animals. 



