CHAPTER X 

 ABSORPTION FROM THE ENVIRONMENT , 



61. All cells absorb. The surface of a young root is made 

 up of cells packed so close together that even with the most 

 powerful microscopes we are unable to see any breaks through 

 which water can pass. Yet it cannot be doubted that water 

 does pass through, and we may be sure that materials pass 

 through the walls of the cells. 



62. Diffusion. Illuminating gas and the vapors of odorous 

 substances spread through the air very rapidly, by a process 

 called diffusion. Diffusion takes place also in liquids. Dif- 

 fusion represents a form of energy, since it is capable of 

 overcoming gravity, as we can see in the fact that sugar or 

 salt diffusing in water is actually lifted from the bottom of a 

 vessel and distributed to all parts. This attraction between 

 water and certain kinds of soluble substances helps us to 

 understand what happens in roots as well as in other parts of 

 living things. 



The substance of which the root's cell walls are made up is 

 called cellulose. This substance cannot dissolve in water, but 

 it can absorb water in much the same way as glue or gelatin 

 does. Now water can diffuse through cellulose, although the 

 cellulose cannot dissolve or diffuse in water. Substances that 

 can dissolve in water can thus diffuse through the cell wall. 



63. Diffusion through a membrane. When different sub- 

 stances dissolved in water are separated by a layer of cellulose 

 or gelatin, they may diffuse through the separating membrane. 

 Such diffusion is called osmosis. This process takes place in 

 the walls of cells, since the watery liquid on one side of such 

 a membrane is not the same as that on the other side. Thus 



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