EXCHANGE AT THE ROOT SURFACE 



9 



Osmosis. Liquids pass through cell walls and other mem- 

 branes by a trading process called osmosis. It is a -force or process 

 almost as universal as gravitation itself. Among the cells of all 

 living creatures exchange of liquids is taking place by an osmostic 

 process. It may be defined as an exchange 

 of liquids or gases of different density, 

 through a membrane. 



Tie a piece of bladder from a butcher 

 shop very tightly over the mouth of a fun- 

 nel tube (Fig. 5). Fill the funnel either 

 before or after with a very strong sugar 

 syrup as dense as it can be made when it 

 is hot. Then support the funnel as shown 

 in the figure in a vessel of water. The liquids 

 will immediately begin to exchange places 

 through the membrane. The greatest move- 

 ment of the liquids is always toward the 

 denser. But both liquids will pass through 

 the membrane to some extent. There is 

 an exchange going on. After a few hours 

 some sugar may be proved to be in the outer 

 vessel. The rise of liquid in the funnel tube 

 will prove that much of the water from the 

 outer vessel has gone into the funnel through 

 the membrane. If conditions are favorable 

 more glass tubes may be fastened with a piece 

 of rubber at the the top of the funnel and the 

 water will come to a height of several feet. 



It is by this process that water is forced 

 through the cells of the plant to the height 

 of the leaves. 



Exchange at the Root Surface. Growing 

 plants must have in them sap which is denser 

 than the moisture found in the films about 

 the grains of soil. If the soil water were very 

 heavily saturated with plant food in solution, 

 and if the sap in the roots were not so dense, 

 the plant would have to give off more water to the soil than it 

 would absorb. This would cause the plant to wilt. 



This may be illustrated by putting a fresh plant of lettuce 

 with its roots and lower part in a glass containing a dense syrup; 



Fio. 5. Osmosis with 

 a bladder membrane tied 

 on mouth of funnel tube. 

 After the bladder is tied 

 in place, the tube is filled 

 with thick syrup or other 

 strong solution. Water is 

 in the large bottle. As the 

 osmosis takes place the 

 liquid may ascend the tube 

 many feet if it is extended 

 high enough. 



