ABSORPTION FROM THE ENVIRONMENT 41 



there is always a double current : some materials are always 

 passing out of a live cell and other substances are passing in. 

 In this way protoplasm receives from the outside its supply of 

 water, salts, and food. And it is by this process that materials 

 in the cell pass out. Gases as well as liquids diffuse through 

 the cell walls. 



64. Osmosis in living things. The cell wall of a root cell 

 is seen to separate the protoplasm from the surrounding soil 

 water. Income through the root hair is therefore by diffusion 

 through the cell wall, or by osmosis. But the protoplasm 

 within the cell wall is not a uniform mass of substance. The 

 surface layer of protoplasm, the "protoplasmic membrane," 

 also offers obstacles to the free diffusion of liquids and gases 

 in solution, so that osmosis takes place here also. Indeed, 

 there are many substances that can pass through ordinary cell 

 walls of plants, but that cannot pass through the protoplasmic 

 membrane. Common sugar is an example of such a substance. 



Some substances diffuse in water more easily than others. 

 Some of the solids with which we are acquainted do not dis- 

 solve at all. Of the substances that dissolve in water and can 

 diffuse, some will diffuse more quickly through cell walls than 

 others — and some may not pass through at all. Of the sub- 

 stances that can diffuse through cellulose, some can diffuse 

 through protoplasmic membranes more quickly than others — 

 and some cannot diffuse through such membranes at all. As 

 a result of these differeoces, cells exposed to the same material 

 surroundings may not be equally affected. 



Not only do living things absorb materials from the outside 

 world by osmosis, but within the body of every plant and every 

 animal consisting of many cells, materials may pass from cell 

 to cell, or between cells and various body juices, by this process. 



