LIVING MATTER 9 



anything to make it move. The streams of protoplasm 

 seem to flow from a spot of thickened protoplasm called 

 the nucleus. This spot is always a center of activity. 

 The transparent space around the protoplasm is called 

 the vacuole; the enclosing wall, the cell-wall; and the 

 entire structure, the cell. 



Meaning of the word Cell. — The name cell was given 

 to the structure when microscopes were very poor. In- 

 vestigators saw only the walls and attached undue im- 

 portance to them. The name now refers not to the 

 walls but to the living substance which we know to 

 be infinitely more important. A cell is a little mass 

 of protoplasm which contains a nucleus. It may, or 

 may not, be surrounded by a definite wall, and it may, 

 or may not, contain other things. 



Importance of the Cell. — A simple cell, or protoplasm 

 in its simplest form, is the starting point of every plant 

 and every animal. Some plants and animals never get 

 beyond the starting point ; that is, they remain all their 

 lives in the one-celled stage. Such single cells are able 

 to do everything necessary to the life of an independent 

 organism. By studying these one-celled forms, then, it 

 is possible to discover the wonderful powers that charac- 

 terize living substance and all living organisms. 



Irritability. — If a drop of stagnant water is placed on 

 a glass slide and examined with a compound microscope, 

 minute, one-celled, transparent animals may be seen 

 moving swiftly across the field. If one of them is quiet 

 long enough, we may see in its body what we have 

 already seen in the hair cell. The protoplasm is mov- 

 ing. But as we look, the animal scurries away. 



