256 Science of Plant Life 



which possessed the other desired qualities but which was not 

 immune. Such desirable combinations are possible because 

 most of the qualities of plants are inherited independently of 

 one another. For example, the watermelon wilt threatened 

 to make the profitable growing of watermelons in certain 

 regions along the Atlantic coast impossible. It was dis- 

 covered, however, that a closely related plant, a small African 

 citron, was immune to the disease. The problem then was 

 to cross the watermelon with the citron and out of the hybrid 

 progeny to select those plants that possessed all the good 

 qualities of the watermelon and the immunity of the citron. 

 This was accomplished, and hybrid watermelons could then 

 be grown in the infected fields. 



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

 growth of bacteria. This makes necessary the most careful 

 handling 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 dairymen 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 desirable bac- 

 teria are added to the milk and allowed to develop for a time. 



In order to avoid the danger that lies in the use of milk 

 contaminated with disease germs, milk that is shipped into 

 the larger cities is usually pasteurized before being sold. This 

 treatment kills most of the bacteria, destroying all the kinds 



