50 FUNCTIONS OF LIVING THINGS 



in size and shape. Such a collection of cells is called a tissue. 

 Examples of tissues are the cells covering the outside of the human 

 body, the muscle cells, which collectively allow of movement, bony 

 tissues which form the framework to which the muscles are at- 

 tached, and many others. 



Cells. — A cell may be defined as a tiny mass of living matter 

 containing a nucleus, either living alone or forming a unit of 



the building material of a living thing. The 

 living matter of which all cells are formed is 

 known as protoplasrro (formed from two Greek 

 words meaning first form). If we examine 

 under a compound microscope a small bit of 

 the water plant Elodea, we see a number of 

 structures resembling bricks in a wall. Each 

 " brick," however, is really a plant cell 



-CltA. 



iT'V:.-v. ■ • "(iv- :•; ••••':K 

 ivX'afe-" .■•-■••.':■; •.■':'.-'V-.J 



A ceu. ch., chromo- bounded by a thin Wall. If we look carefully, 



somes; c.«\, celJ wall ; "^ ... . 



n., nucleus ; p., proto- we Can See that the material inside of this wall 

 ^ ^^' is slowly moving and is carrying around in its 



substance a number of little green bodies. This moving substance 

 is living matter, the protoplasm of the cell. The green bodies 

 (the chlorophyll bodies) we shall learn more about later ; they are 

 found only in plant cells. All plant and animal cells appear 

 to be alike in the fact that every living cell possesses a structure 

 known as the nucleus (pi. nuclei), which is found within the body 

 of the cell. This nucleus is not easy to find in the cells of Elodea. 

 Within the nucleus of all cells are found certain bodies called 

 chromosomes. These chromosomes in a given plant or animal are 

 always constant in number. These chromosomes are supposed to 

 be the bearers of the qualities which we believe can be handed 

 down from plant to plant and from animal to animal, in other 

 words, the inheritable qualities which make the offspring like its 

 parents. 



How Cells form Others. — Cells grow to a certain size and then 

 split into two new cells. In this process, which is of very great 

 importance in the growth of both plants and animals, the nucleus 

 divides first. The chromosomes also divide, each splitting length- 

 wise and the parts going in equal numbers to each of the two cells 



