16 PROTOPLASM 



of life to the plant. In order to distinguish more clearly between 

 the dead cell of cellulose and the protoplasm which fills it, the 

 term 'protoplast was coined to indicate the living contents of a 

 cell considered as a unit. It is not used in the same sense as 

 protoplasm, for the latter is the living substance in any form, 

 while the protoplast is the complete, organized unit, that is to 

 say, the living cell without its (heavy) wall. 



That the living plant cell is really a protoplast — an individual 

 organized droplet of protoplasm housed in a cellulose box — is 

 easily proved by the process known as plasmolysis. If plant 

 tissue is put in a salt or sugar solution which is of higher concen- 



FiG. 9.— Drawing of a group of plant cells. {From F. T. Lewis.) 



tration than the salt content of the cell (about 5 per cent potas- 

 sium chloride or 18 per cent sugar), the cell loses water to the 

 outside solution, and the protoplast shrinks away from the cell 

 wall (Fig. 8). This takes place because the internal pressure, 

 the turgidity of the cell, is reduced. The protoplast shrinks as 

 does a toy balloon from which the air is slowly escaping. We 

 learn from this experiment that the protoplast is an entity in 

 itself and that the plant cell is not simply a cellulose container 

 with fluid poured into it like water into a box. In addition to a 

 wall of cellulose plant cells possess a protoplasmic membrane. 

 Such a membrane exists at the surface of all protoplasts but 

 is usually not distinguishable from the protoplasm within. In 

 normal plant cells, the membrane is pressed tight against the 

 inner surface of the cellulose wall. In plasmolyzed cells, the 

 membrane pulls away from the wall with the protoplasm (Fig. 8). 

 In most animal cells, the protoplasmic membrane is the outermost 

 layer, or "wall," of the cell. 



