34 PROTOPLASM AND THE CELL 



processes of assimilation and the upbuilding of yeast proto- 

 plasm, are as obscure as elsewhere in the living world, but, 

 largely through the study of yeast cells, some good working 

 hypotheses have been formulated and many vital activities 

 have been traced to unstable chemical compounds termed 

 enzymes or ferments. 



t 



B. BACTERIA 



Bacteria is a term used to designate a great group of minute 

 forms of life intermediate, like yeast, between chlorophyll- 

 bearing plants and animals. Bacteria occur almost everywhere; 

 abundantly in the atmosphere accompanying dust particles; 

 frequently in fresh and salt water; abundantly in the digestive 

 tract of all kinds of animals. They abound in the upper layers 

 of the soil and in exposed fluids containing dead animal or 

 vegetable matter. Some types produce disease in man and 

 other animals, whence they are popularly known as germs or 

 microbes or parasites. Many of them, on the other hand, are 

 positively useful physiologically, in the functional activities of 

 higher animals, and economically and commercially in trans- 

 forming organic matter into simple salts (nitrites, nitrates, etc.) 

 or in the manufacture of various food stuffs (vinegar, butter, 

 cheese, etc.). 



Morphology of Bacteria. Bacteria are the smallest of the 

 known organisms. Some types placed end to end would 

 require 25,000 to cover a linear inch, and the line would be too 

 fine to be seen; 50,000 such lines side by side would cover a 

 square inch. Other types are larger, varying from 2ju to 6oju 

 in length. 1 



While small, the bacteria nevertheless have fairly definite 

 forms which may be grouped for convenience under three main 

 types: i. the bacillus or rod; 2. the coccus or ball; and 3. the 

 spiral or corkscrew. They are frequently united in chains or 

 filaments, in plate form (sarcina), or embedded in a gelatinous 

 matrix which they secrete (zoogloea) (Fig. 16). 



1 A /x (micron) equal 1-25,000 of an inch. 



