LIVING MATTER 263 



292. The Importance of the Cell. From what has been 

 said of cells, it will be understood that the work of an organ is 

 really the result of the combined work of all the cells which 

 make up its tissue. The contraction of a muscle is in reality 

 due to the contraction of every cell of which the muscle is 

 composed. The cell may therefore be regarded as the work- 

 ing unit, as well as the unit of structure. 



293. Division of Labor. Some very tiny plants and 

 animals have bodies consisting of but one cell. Since this 

 cell must perform all the physiological functions essential to 

 life, each function must be performed in a very simple manner. 

 But the bodies of all the larger organisms consist of many 

 cells, among which the work of living is divided. Certain 

 cells are concerned with irritability alone, certain others with 

 the taking of food alone, and each has a shape fitted for the 

 work it has to perform in order to support life. 



The single-celled animal or plant may be compared to a 

 hermit, who must provide with his own hands his shelter, 

 food, and clothing. A man living in a large city, on the other 

 hand, may purchase all these necessities from different people, 

 each of whom has done one kind of work. Meanwhile the 

 city dweller has perhaps been making tools, or designing im- 

 proved machinery for the other workers. The assignment of 

 the necessary work of life among different individuals, whether 

 cells or complete organisms, is called division of labor 



294. Division of Cells. Most cells contain a structure 

 of denser protoplasm, known as the nucleus. A cell never 

 grows beyond a certain maximum size. When this is reached, 

 the cell may divide into two new cells through an equal divi- 

 sion of the nucleus and the rest of the protoplasm. Then 

 each resulting cell may grow to the maximum size of its kind, 

 when division may take place again. Thus, from one cell 

 many cells may result by repeated cell division. This process 

 is the usual method of growth observed in the tissues of plants 

 and animals. 



