CHAPTER XXV. 

 BACTERIA. 



391. General characters. The bacteria are very minute 

 plants, some of them being the smallest organisms known. An 

 idea of the size of very small ones can be obtained from the fact 

 that if placed side by side it would take 5000 to make a line i mm. 

 long, or 125,000 to make a line one inch long. Like the fungi they 

 are devoid of chlorophyll, not being able to make their own 

 carbohydrate food. A few can fix carbon from the air (see para- 

 graph 200). They are dependent on green plants for carbohy- 

 drate food, and obtain this from the sugar or starch, etc., in other 

 plants upon which they grow as saprophytes or parasites (see 

 Chapter XV), or from organic matter either of plant or animal 

 origin which was primarily obtained from green plants. They 

 are in the form of rods, or spheres, or screws. The outer layer of 

 the wall is slightly gelatinous, and this peculiarity is made use of 

 in fixing them to glass slips in mounting them for study, by the 

 use of heat. Some of the bacteria are non-motile, while others 

 are motile. The motion is usually a jerky irregular rotary 

 motion, but the spiral forms dart rapidly along like a forward- 

 moving screw. The motile ones are provided with delicate cilia 

 (fig. 101), which cannot be made visible except by special treat- 

 ment with mordants and stains. They multiply by simple fission 

 as in the blue-green algae. Resting spores are formed in many 

 species by the condensation of the protoplasm into a small shining 

 body within the cell which is able to resist greater extremes of 

 heat, cold, dryness, etc. 



392. The principal forms of bacteria and their methods 

 of multiplication. Many of the rod-like bacteria belong to the 

 genus Bacillus. In multiplication this rod divides into two short 

 rods which increase in length to the size of the parent rod. In 



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