PHTSIOLOGT 



In order to understand how materials are ab- 

 sorbed and given out, we may consider first of 

 all the structure of the cell. A typical plant cell 

 consists of a cell wall (of non-living material) 

 within which is a layer of living protoplasm, 

 often so thin as to be little more than a delicate 

 membrane. Within this is a large central vacuole 

 filled with cell sap, and in many cases plastids 

 (such as the chloroplasts) are present. A simple 

 animal cell differs in several respects: of especial 

 significance for our present purpose is the ab- 

 sence of the cell wall and of the central vacuole. 



When a plant cell absorbs water the proto- 

 plasm expands and the cell wall is stretched. If 

 water is withdrawn the cell wall shrinks j this 

 shrinking soon stops but the protoplasm may 

 continue to contract until it is drawn away from 

 the wall. If it is allowed to reabsorb water it 

 expands, stretching the cell wall with consider- 

 able force (strikingly manifested in growth, 

 when mushrooms raise flagstones, or ferns push 

 up through concrete sidewalks^. 



The manner in which this force is generated 

 may be illustrated by tying a membrane to a 



