Plants in Relation to Man 461 



filled with, water reaching to the level of the solution in the tube. Let 

 this stand undisturbed for several hours. 



A more rapid reaction can be obtained by using olive oil instead of 

 molasses, and sulfuric ether instead of water. 



Arrange a similar apparatus, but fill both bulb and bottle with 

 clear water. Compare the results of the two. Does the water pass 

 through the membrane in both cases ? What facts support your 

 answer to this question? 



In the first experiment does the molasses pass through the mem- 

 brane into the water in the bottle? Test this point. In what direc- 

 tion does the larger amount of the liquid move, toward the denser 

 solution in the tube or toward the water in the bottle ? Does the fact 

 that the liquids in the second experiment are equal in density account 

 for the facts observed ? Explain. 



Osmosis. Water passes from the bottle through the mem- 

 brane into the bulb and tube, and may eventually cause the 

 solution in the tube to overflow. A small percentage of the 

 sugar sirup passes through the membrane into the bottle, as 

 your tests may show. The same fact may be observed when 

 dried fruits, such as raisins and prunes with intact skins, or 

 dried vegetables, like beans and peas, are soaked in water. This 

 passing of solutions through permeable, but not porous mem- 

 branes is known as osmosis. 



A similar interchange of solutions is taking place through 

 the membrane of root hairs and the adjacent parts of roots 

 of plants. Soil water containing in solution food substances 

 needed by plants is taken by osmotic absorption through the 

 root hairs and then passed in circulation through the plant. 

 At the same time certain substances in solution, such as carbon 

 dioxid, are excreted through the root hairs and attack and 

 dissolve certain materials, as limestone, in the soil, rendering 

 them capable of absorption. That this is done may be verified 

 by putting several small, smooth pieces of marble into soil 

 where the roots of plants will reach them. After a lapse of 

 several months examine them. 



The root hairs are exceedingly well adapted to their work. 

 They are so fine that they can penetrate the smallest inter- 



