38 



SOME PROPERTIES OF CELLS 



sugar, pass rapidly through membranes, while jelly-like sub- 

 stances, like white of egg, can hardly pass through them 

 at all. 



50. Study of osmotic action of living protoplasm ; plasmol- 

 ysis. The obvious parts of most living and growing plant cells 

 are a cell wall, which is a skin or inclosure made of cellulose, and 

 the living, active cell contents, or protoplasm (Sec. 44). Every 

 one is familiar with cellulose in various forms, one of the best 

 examples being that afforded by clean cotton. It is a tough, 

 white, or colorless substance, and chemically rather inactive. 



Often, in living cells, 

 the spaces between 

 ' J strands and protoplas- 

 p mic lining are filled 

 s with a watery liquid 

 called the cell sap. 



The action of living 

 protoplasm in control- 

 ling osmosis is well 

 shown by the process 

 known as plasmolysis. 

 If thin-walled cells 



A B 



FIG. 29. Cells from root of Indian corn 



A, in natural condition ; D, plasmolyzed in 5 per , 



cent solution of potassium nitrate; w, cell wall; il( l u m > 



p, denser part of protoplasm ; s, cell sap. Much such as tllOSC of O116 

 magnified. After Pfeffer . ,, -, 



or the pond scums, are 



put into a salt solution, the cell contents will shrink away from 

 the cell wall (Fig. 168, B) because the direction of flow, toward 

 the denser liquid, draws water out of the cell. Repeating the 

 experiment with a cell which has been killed by a few minutes' 

 immersion in a poisonous solution (e.g. of chromic acid) shows 

 no plasmolysis. 



So, too, slices of a red beet impart little color to water in which 

 they are placed, but after the cells are killed by boiling the color 

 comes out freely. 



