ABSORPTION AND OSMOSIS 



149 



After reading the text, explain what has happened 

 in the right-hand tube and beaker. 



osmotic pressure. But if the walls of the vessel are less rigid, as 

 in the egg membrane, the osmotic pressure will cause the mem- 

 brane to swell and distend until it eventually may burst. 



Why root hairs absorb water and soil salts. The wall of the 

 root hair readily takes in water and dissolved soil salts by imbi- 

 bition. The membrane sur- 

 rounding the protoplasm of 

 every living cell is a semi- 

 permeable membrane, which, 

 while allowing water and 

 mineral salts in solution to 

 pass or diffuse toward the in- 

 side, will also allow some dif- 

 fusion outward of the water 

 and soluble materials within 

 the cell. But the inward flow 

 is much greater than the out- 

 ward flow. As soon as the 

 outer cells have increased their 



holdings of soil water, an osmosis inward from cell to cell is started 

 because the water tends to flow from the place of its greater con- 

 centration to the place of lesser concentration. Mineral salts in 

 solution are carried along with the water so that the needed soil 

 substances are carried along from cell to cell, until they reach the 

 small tubes of the central cylinder. The osmotic pressure in the 

 root hairs is sufficient to cause enough force in these tubes to raise 

 a column of water to a considerable height in the stem. This is 

 known as root pressure. 



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 (small intestines) is composed of millions of small fingerlike 

 projections called villi, which look somewhat, in size at least, like 

 root hairs. These villi are (unlike a root hair) made up of many 

 h. bio — 11 



