CELLS AND PROTOPLASM 



73 



alive, whatever its nature may be, it is called protoplasm. 

 Whether in plants or in animals, whether active, as in a 

 growing leaf, or dormant, as in the embryo of a seed, whether 

 in a loose mass on decaying wood or at work in the brain 

 of the wisest man, this life stuff is called protoplasm. If 

 we knew its secrets, we should know the secrets of physical 

 life. All other parts of living bodies serve it; it manu- 

 factures them and uses them for its various purposes. 

 The cell walls which you see in Figure 24 were all manu- 

 factured by protoplasm within them. 



We can understand cells and protoplasm best by con- 

 sidering a single cell and its contents. 

 Look at Figure 25. It shows the 

 principal parts of a living cell. That 

 portion of protoplasm which is con- 

 tained in a single cell is called a 

 protoplast. You may think of pro- 

 toplasts as the units of function of 

 plants. It is by their action together 

 that everything which a plant does 

 is accomplished. The protoplast is 

 the essential part of a living cell, 

 but it is not easily seen. The other 

 parts, which it manufactures, are 

 more easily seen. They are usually 

 opaque and have definite form, while 

 the protoplast is transparent and 

 does not have definite form. 



Protoplasm has the consistency 

 of very thin jelly. In studying its 

 structure stains are used. The dots which you see inside 

 the wall in Figure 25 crudely represent the appearance of 



Fig. 25 



Diagram of a cell 

 from the inside of a leaf; 

 cy, cytoplasm ; n, nucleus ; 

 w, wall ; ch, chloroplast ; », 

 vacuole. 



