IMPORTANCE OF DIFFUSION AND OSMOSIS 73^ 



Physiological Importance of Diffusion and Osmosis. — The 



processes of diffusion and osmosis are of great importance not only 

 to a plant, but also to an animal. Foods are digested in the food 

 tube of an animal ; that is, they are changed into a soluble form 

 so that they may pass through the walls of the food tube and 

 become part of the blood. The inner lining of part of the food 

 tube is thrown into millions of little fingerlike projections called 

 villi, which look somewhat, in size at least, like root hairs. These 

 fingerlike processes are (unlike a root hair) made up of many cells, 

 but they serve the same purpose as the root hairs, for they absorb 

 liquid food into the blood. This process of absorption is not 

 entirely understood, but is largely by diffusion and osmosis. With- 

 out these processes we should be unable to use most of the food 

 we eat. 



Summary. — This chapter has first shown us that rocks, the 

 original earth material, have been broken down into the fragments 

 we call inorganic soil. To this soil have been added, through the 

 process of decay, the bodies of plants and animals which once 

 covered the earth. Plants take out of the soil, water and soluble 

 salts which are used by the plant in making food and eventually 

 living matter. 



The structures by means of which the soil water is absorbed are 

 called root hairs. These are elongated projections from cells of 

 the outer covering of the root. 



The methods by which the fluids are taken into the root hairs 

 and circulated through the cells of the root are known as diffusion 

 and osmosis. Since the membrane of the root hair and other 

 cells is semi-permeable, allowing the passage of some substances 

 but not of others, a flow of soil water is established toward the in- 

 side of the cell, because the membrane prevents the substances in 

 the cell sap from flowing out. Thus osmotic pressure is established 

 and roots are able to take in large amounts of water and soil salts. 



Problem Questions 



1. What are the chief differences between "poor" and "rich" soil? 



2. How is soil able to hold water? 



3. How are roots adapted to do their work? 



4. What part of the root is most sensitive to gravity? Prove your answer 

 by experiment. 



