CHAPTER IX 



PLANTS WHICH CAUSE DECAY, FERMENTATION 

 AND DISEASE 



What is the cause of decay ? Formerly it was sup- 

 posed to be due to some property residing in the 

 organism itself ; our present knowledge is that it is 

 due to plants called bacteria. Although too minute to 

 be seen except with the microscope, they can neverthe- 

 less be studied with the simplest apparatus. 



To begin with, we may get an abundant growth of 

 bacteria by putting a little hay in water and allowing 

 it to stand a few days, when the bacteria will form a 

 gelatinous film on the surface. On mounting a drop of 

 this in water on a slide and examining it under the 

 microscope, the bacteria appear as very small, glisten- 

 ing bodies approaching in shape some of the forms 

 shown in Fig. 205; very probably some of these will 

 be in motion. This motion may be merely mechan- 

 ical, i. e., a dancing motion such as any small particles 

 (e. g., India ink or vermilion) show in a liquid, or it 

 may be due to the activity of the bacteria, which move 

 by means of whip-like protoplasmic projections (cilia). 

 Pour off some of the liquid in which the bacteria are 



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