FUNCTIONS OF LIVING THINGS 



51 



Stages in the division of one cell to form two. 

 Which part of the cell diAddes first ? What seems 

 to become of the chromosomes ? 



formed from the old cell. In this way the matter in the chromosomes 

 is divided equally between the two new cells. Then the rest of 

 the protoplasm separates, and two new cells are formed. This 

 process is known as fis- 

 sion. It is the usual 

 method of growth found 

 in the tissues of plants 

 and animals. 



Cells of Various Sizes 

 and Shapes. — Plant 

 cells and animal cells are 

 of very diverse shapes 

 and sizes. There are 

 cells so large that they 

 can easily be seen with 

 the unaided eye ; for 

 example, the root hairs 

 of plants and eggs of some animals. On the other hand, cells 

 may be so minute, as in the case of the plant cells named bacteria, 

 that several million might be present in a few drops of milk. The 

 forms of cells may be extremely varied in different tissues ; they 

 may assume the form of cubes, columns, spheres, flat plates, or 

 may be extremely irregular in shape. One kind of tissue cell, 

 found in man, has a body so small as to be quite invisible to the 

 naked eye, although it has a prolongation several feet in length. 

 Such are some of the cells of the nervous system of man and other 

 large animals, as the ox, elephant, and whale. 



Varying Sizes of Living Things. — Plant cells and animal cells 

 may live alone, or they may form collections of cells. Some 

 plants are so simple in structure as to be formed of only one kind 

 of fcells. Usually living organisms are composed of several groups 

 of different kinds of cells. It is only necessary to call attention 

 to the fact that such collections of cells may form organisms so 

 tiny as to be barely visible to the eye ; as, for instance, some of the 

 small flowerless plants or many of the tiny animals living in fresli 

 water or salt water. On the other hand, among animals, the bulk 

 of the elephant and whale, and among plants the big trees of Cali- 



