THE DISEASES OF PLANTS 



101 



-b 



Later, other kinds of plants equally small will grow and 

 change the alcohol into an acid, which you will recognize 

 by its sour taste and peculiar odor. Thus vinegar is made 

 by the action of two different kinds of little living plants 

 in the cider. That these are living beings you can prove 

 by heating the cider and keeping it tightly sealed so that 

 nothing can enter the can. You will find that, the living 

 germs being killed by the heat, the cider will not ferment 

 or sour as it did before. The germs could of course be 

 killed by poisons, but then 

 the cider would be unfit for 

 use. It is also this same 

 little yeast plant that causes 

 bread to rise. 



When you see any decay- 

 ing matter, you may know 

 that in it minute plants much 

 like the yeast plant are at 

 work. Since decay is due 

 to them, we take advantage 

 of the fact that they cannot grow in strong brine or smoke, 

 and thus prepare meat for keeping by salting it or by 

 smoking it or by both of these methods. 



You see that some of the yeast plants and bacteria, as 

 many of these forms are called, are very friendly to us, 

 while others do us great harm. 



Some bacteria grow within the body of man and other 

 animals or in plants. When they do so, they may pro- 

 duce disease. Typhoid fever, diphtheria, consumption, 

 and many other serious diseases are caused by bacteria. 

 Fig. 88, e y shows the bacterium that causes typhoid fever. 



FIG. 87. YEAST PLANTS 



A, a single plant ; B, group of two 

 budding cells 



