CHAPTER XIX 



BACTERIA AND DISEASES 



190. Bacteria. We have already learned something 

 of the deadly work of bacteria and other microorgan- 

 isms in previous chapters, but shall now study them a 

 little more closely to learn their relation to us in health 

 and in disease. Bacteria are among the smallest and 

 simplest forms of plant life 

 known at present. They are 

 so very small that it has 

 been difficult to study them. 

 But now many scientists are 

 devoting their whole time to 

 a careful investigation of mi- 

 croorganisms with the best 

 and most expensive micro- 

 scopes and other appliances FIG. 152. The three kinds of 



,, , 1^-1 bacteria (Pratt). 



that can be obtained. 



The largest bacteria are only a few hundred-thou- 

 sandths of an inch in diameter. They are either spheri- 

 cal, rod-shaped, or screw like, and are known as the 

 cocctfSj bacillus, or spirillum respectively. Any of these 

 forms may occur singly, or in pairs, and some of them 

 even in long chains. They multiply and grow so rap- 



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