CHAPTER XX 

 BACTERIA 



Bacteria are the smallest of all plants and in fact some of 

 tlieni are the smallest known organisms. It would require 

 125,000 of the smaller ones, placed side by side, to make a line 

 one inch long. They are like the fungi in that they do not con- 

 tain chlorophvll and cannot perform the work of photosynthesis. 

 Therefore, they are either saprophytic or parasitic. Some are 

 motile and others non-motile. They multiply by simple cell 

 division and many species form resting spores which enable them 

 to resist extremes of temperature and humidity. 



Abundance. — Bacteria are more abundant than other forms 

 of life. They float in the air that we breathe, are in the water 

 that WQ drink and the food that we eat. They are to be found 

 in our mouths, lungs, stomach, and intestines. They cause the 

 souring of milk, the fermentation of various liquids, the decay 

 of fruits, vegetables and meats. Fortunately the great major- 

 ity are harmless and many are beneficial. Among the most 

 interesting of the beneficial species are those which are involved 

 in the fixation of nitrogen. (Page 117.) 



Causes of Disease. — However, many bacteria are the causes 

 of diseases of both plants and animals. Among the most serious 

 of plant diseases caused by bacteria are the fire blight of the 

 pear and apple, the black rot of cabbage and related plants and 

 the root or crown gall of our fruit trees and many other plants. 

 ]\rany serious animal diseases are caused by bacteria such as 

 tuberculosis or consumption, diphtheria, tetanus or lockjaw, 

 grippe, anthrax, cholera and many others which attack man 

 and the lower animals. 



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