CHAPTER XI 



HOW FOOD IS TAKEN IN 



Questions. 1. How does a cell distinguish between what it may take 

 in and what may be injurious ? 2. How do roots select the soil materials 

 that they can use ? 3. Why does not the substance in a root hair or other 

 living cell go out into the soil or water ? 4. How can hving things with- 

 out mouths take in food ? 



99. 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. Salt or 

 sugar left in the bottom of a vessel of water is gradually dis- 

 solved, lifted from the bottom, and distributed to all parts of 

 the liquid, overcoming gravity. Diffusion therefore represents 

 work, or the expenditure of energy. This attraction between 

 water and certain kinds of substances helps us to understand 

 what happens in roots and in other parts of living things. 



100. All cells absorb. In many one-celled animals, like 

 Ameba, the protoplasm is said to be naked, since there is no 

 permanent cell covering or wall. In most plants and animals 

 the protoplasm of each cell is more or less completely inclosed 

 by a membrane of non-living material. We know that the root 

 is capable of absorbing material from its surroundings and that 

 the many cells inside every plant or animal, away from the 

 surface, all absorb their water, food, and oxygen through the 

 cell wall, yet a powerful microscope fails to show any openings 

 through the cell walls, or even between adjoining cells. The 

 substance which makes up the cell walls in most plants is called 

 cellulose. This substance cannot dissolve in water, but it can 

 absorb water in the same way as glue or gelatin. Now water 

 can diffuse through cellulose, although the cellulose cannot dis- 



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