CHAPTER XXVII. 

 BACTERIA 



405. 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 202). 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 XVI), or from or- 

 ganic 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. 218), which cannot be made visible except by special treat- 

 ment with mordants and stains. They multiply by simple fis- 

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



266 



B 



Fig. 218. 



Bacteria. A, Bacillus subtilis. Spores in 

 threads, unstained rods, and stained rods show- 

 ing cilia; B, Bacillus tetani, the tetanus or lock- 

 jaw bacillus, found in garden soil and on old 

 rusty nails. Spores in clubshaped ends. C, 

 Micrococcus; D, Sarcina; E, Streptococcus; F, 

 Spirillum. (After Migula.) 



