CELLS OF VARIOUS SIZES AND SHAPES 49 



in both plant and animal cells. You will note from the diagram 

 that the division results in the placing of an equal number of 

 chromosomes in each of the two new cells formed. 



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, certain cells, like the bacteria, are so minute that several 

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

 cells are extremely varied in different tissues ; they may have the 

 shape of cubes, columns, spheres, or flat plates, or may be extremely 

 irregular in outline. 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 large animals such 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 cells. 

 Usually living organisms are composed of many groups of different 

 kinds of cells. 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 fresh 

 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- 

 fornia, stand out as notable examples. The large plants and ani- 

 mals are made up of more, not necessarily larger, cells. 



Summary. — This chapter has shown us that the units of build- 

 ing material in living things are called cells. These structures vary 

 greatly in size, shape, and number ; but the size of an individual 

 has little or no bearing on the size of the cells of which it is made. 



Animal cells are simply tiny bits of protoplasm, each containing 

 a nucleus and surrounded by a delicate living covering called a 

 membrane. Plant cells as a rule have a cellulose (woody) wall out- 

 side the membrane. This is not alive, but is made by the activity 

 of the protoplasm of the cell. Plant cells also contain large vacu- 

 oles and, if green, chloroplasts (chlorophyll bodies). 



The nucleus is evidently the center of activity in the cell. When, 



