HOW FOOD IS TAKEN IN 127 



solve or diffuse. Substances that dissolve in water can thus dif- 

 fuse through a cell wall as long as the cellulose is full of water. 



101. Diffusion through a membrane. When solutions of two 

 different substances are separated by a layer of cellulose or gel- 

 atin, they may diffuse through the water contained in the sep- 

 arating membrane. In this way they may diffuse through the 

 membrane itself. Such diffusion is called osmosis. This proc- 

 ess takes place in the walls of cells, since the watery liquids on 

 the two sides of such a membrane are not the same. Thus there 

 is a double current : protoplasm receives from the outside its 

 supply of water, salts, and food, and materials in the cell pass 

 out. Gases as well as liquids diffuse through the wet cell wall. 



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

 separates the protoplasm from the surrounding soil water. In- 

 come 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, and diffusion takes 

 place between one part and another. 



Some substances dissolve in water more easily than others. 

 Some of the common solids do not dissolve at all. Of those that 

 can dissolve in water and diffuse, some will diffuse more quickly 

 through cell walls than others, and some will not pass through 

 at all. Of the substances 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 differences, cells exposed to the same mate- 

 rial surroundings may not be equally affected. 



Living things absorb materials from the outside world by 

 osmosis. W^ithin the body of every plant and every animal con- 

 sisting of many cells, materials also pass from cell to cell, or 

 between cells and various body juices, by this process. 



103. Plant income. Green plants, as we know, manufacture 

 their food out of raw material— water, carbon dioxid, and 

 various salts. Water is readily absorbed by cellulose. Through 

 the cell walls saturated with water, carbon dioxid from the air 

 and salts from the soil pass by diffusion, or osmosis. Within the 



