The Green Plant 



495 



off liquids and gases through their membranes. They absorb food 

 materials in solution through their root membranes into the sap. 

 The digested food is absorbed from the sap through cell walls 

 into the cells. Through the membranes of certain leaf cells plants 

 absorb and give off oxygen and carbon dioxide. In every case. 

 a fluid, either a liquid or a gas, passes through a membrane into 

 another fluid. 



Strange as it may seem, even the most powerful microscopes 

 fail to reveal any holes or pores in plant or animal membranes. 

 Yet the fact that liquids and gases pass through them readily 

 proves that openings of some sort must be there, since it is impos- 

 sible to explain this passage of liquid or gas in any other way. 

 This process of absorption or diffusion of a fluid substance, liquid 

 or gas, through a membrane without visible pores into another 

 fluid substance is called osmosis. 



Osmosis, then, may be defined as the mixing of liquids or 

 of gases, separated by an ani- 

 mal or a plant membrane, by 

 passing through the membrane. 

 The greater flow is always 

 from the less dense to the more 

 dense substance. 



Experiments to Show Prin- 

 ciple of Osmosis. Tie a piece 

 of animal membrane, taken 

 from the bladder or wall of the 

 intestine of an animal, over one 

 end of a glass tube from one to 

 three feet long. Partly fill the 

 tube with molasses and place it 

 on a frame so that the covered 

 end stands in a dish of pure 

 water. After leaving the appa- 

 ratus for some time, it will be 

 found that the liquid in the tube 

 has risen, while the water in the 



EXPERIMENT TO SHOW OSMOSIS 



dish has lowered. Water must The water pass e f s 



the membrane 



