80 ELEMENTARY BIOLOGY 



114. Digestion universal. The process of digestion seems 

 to go on in nearly all living things. In the case of the ameba, 

 which consists of a single unit of naked protoplasm (see p. 24), 

 a solid particle of food can be swallowed by the naked proto- 

 plasm and then digested inside the cell. Among the bacteria, 

 which are the smallest living things known, each individual is 

 a single cell consisting of protoplasm and cell wall. These 

 tiny plants can get food only in a liquid state ; yet many of 

 them live on solid food that is not soluble in water. Under 

 suitable external conditions each cell throws out through the 



FIG. 27. Digestion by bacteria 



The organism a lying on a solid l>, which may serve as food, secretes an enzym, or 



ferment, which passes out of the cell e and changes the material to a liquid /. This is 



absorbed into the cell by osmosis, / 



cell wall, by osmosis, a liquid containing a ferment capable of 

 digesting the solid or insoluble food material. The liquid re- 

 sulting from the digestion is absorbed by osmosis. This may 

 account for the fact that when meat or cheese rots, it becomes 

 fluid. The rotting in such cases is the work of the ferments 

 contained in the digestive juices secreted by the bacteria 

 (Fig. 27). 



In higher animals like ourselves a similar process of diges- 

 tion takes place. But instead of every cell pouring out diges- 

 tive juices into its immediate neighborhood, only certain portions 

 of the body produce and throw out such juices. 



