352 



THE SMALLEST PLANTS (BACTERIA) 



from the air for plants, and aiding in the making of linen 

 and sponges. Some bacteria are harmful and cause dis- 

 eases in plants and animals. Bacteria are spherical, 

 spiral, or rod-shaped. They are found everywhere, un- 

 less special pains have been taken to remove them. If 

 they have plenty of food, air, moisture, and warmth, they 



multiply rapidly, and 

 they go into the resting 

 state, in which they can 

 remain for a long time 

 if any or all of the 

 necessary conditions of 

 growth are lacking. 

 The harmful bacteria 

 by their growth secrete 

 a poisonous substance. 

 When there are enough 

 bacteria present to make 

 a large quantity of toxin, 

 the animal or plant host 

 is made ill. Some bac- 

 teria, especially in the 

 resting state, can bear 

 freezing or boiling with- 

 out being killed. In order to make anything " keep," it 

 is necessary either to kill all the bacteria by making the 

 substance sterile or aseptic, or we must put into it a 

 preservative in which the bacteria cannot grow. We 

 should exercise great care to avoid the bacteria known to 

 produce disease. 



Milk, one of the most important articles of food, is a 

 possible source of danger from harmful bacteria which may 

 get into it in various ways. Milk should be kept cold, 

 and should be used before it is too old. The harmless 



Figure 373. — Bad and Good Bottling. 



The metal cap keeps out dirt which 

 can get by the paper stopper. 



