39^ General Botany 



Furthermore, persons rarely contract bacterial diseases unless 

 they come in contact with another person carrying the disease. 

 With the exception of lockjaw and wound infections, diseases 

 are rarely spread by clothing, dust, or other objects. Apparent 

 exceptions to this statement are typhoid and diphtheria, carried 

 by water, milk, and other foods when handled and contaminated 

 by a diseased person. Typhoid may also be carried by flies 

 that have visited infected matter. 



Natural barriers to disease. The natural means of defense 

 against disease are somewhat similar in the higher plants and in 

 animals. The plant, in addition to protective chemical sub- 

 stances within its cells, has an epidermis which renders the 

 entrance of bacteria diihcult. Bacteria are able to enter, how- 

 ever, if the epidermis is bruised or broken. Plants probably 

 suffer from bacterial diseases as much as do animals. Most of 

 the well-known plant diseases, however, are produced by fungi. 

 Of the bacterial diseases of plants, the twig blight of pear and 

 apple, the cucumber wilt, and the crown gall of various plants 

 are perhaps best known. Some of these diseases are transported 

 from one plant to another by insects. 



Bacteria in the dairy. Milk is an ideal medium for the growth 

 of bacteria. This makes necessary the most careful handhng of 

 milk, especially when it is used directly as food. The bacteria 

 get into the milk from the cow, from the stable, from the vessels 

 into which the milk is put, and from the persons who handle it. 

 Evidently the cows should be kept clean, and the stable should 

 be as clean and free from dust as possible. The vessels with 

 which the milk comes in contact should be sterile. The dairy- 

 men should have clean hands and clothes, and above all they 

 should be free from infectious diseases. Because bacteria multiply 

 very rapidly at high temperatures, the milk should be chilled 

 at once and kept on ice. To make butter and cheese of fine 

 flavor, pure cultures of the proper bacteria are added to the milk 

 and allowed to develop for a time. 



