CHAPTER THREE 



THE CELL AND ITS ACTIVITIES 



I. LIVING creatures are either single cells, or are made The cell 

 up of aggregations of cells. The word "cell" is rather 

 misleading ; it was given many years ago to those 

 plant cells which take the form of a little compartment 

 or box, containing a fluid. Such cells are rigid, some- 

 times large enough to be seen with a hand lens without 

 difficulty. We now know that the hard wall, the box, 

 is composed of cellulose, which is not part of the living 

 material, and that the essential thing is the protoplasm 

 which it incloses. Not only are all plants cells or 

 groups of cells, but the same is true of animals. In 

 animals, however, there is no stiff wall of cellulose, 

 though there is commonly a thin membrane, and the 

 distinctness of the cells is much less evident on in- 

 spection. Since we recognize the fundamental simi- 

 larity of plant and animal cells, we use the same word 

 for both, and think of the living unit rather than 

 anything inclosing it. Our definition is thus entirely 

 changed, and comes to be : a cell is a particle or unit of 

 protoplasmic material, which exhibits all the essential 

 phenomena of life. It consists, of course, of innumer- 

 able molecules which, taken by themselves, would 

 not function as living things. Such a cell may exist 

 apart from others, as in the Protozoa or one-celled 

 animals. There is a relation between the nature of 

 the cell wall and the activities of the organism ; the 

 essentially stationary plant has stiff cell-walls, but in 

 the mobile animal most of the cells are necessarily 

 flexible. The function of a muscle cell, for example, 

 is connected with its ability to change its shape. 



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