[Chap. XXII THE MOVEMENT OF MATERIALS IiN PLANTS 203 



is the concentration of water. This fact may be demonstrated with a 

 simple physical apparatus and also with living plant cells. Membranes 

 such as filter paper and cellulose cell walls are very permeable to both 

 water and the solutes in water. This is only another way of saying that 

 water and solutes can diffuse readily through these membranes. Such 

 membranes are of no importance in osmosis in plant cells. 



There are, however, certain kinds of membranes through which water 

 can diffuse readily, but through which certain solutes in the water do 

 not diffuse so readily. These are the membranes that are important in 

 osmosis in plant cells. Protoplasm is an excellent example of this type of 

 membrane. It differs in its peiTneability to different substances. It is very 

 pemieable to water, but under most conditions it is practically im- 

 permeable to the anthocyanin and some of the other substances in solu- 

 tion in the water in the vacuole. Membranes of this type are said to be 

 differentially 'permeable} 



There is a second type of differentially permeable membrane which 

 is more pemieable to certain solutes than to water. Sheets of rubber and 

 cutinized cell walls are more permeable to carbon dioxide than they are 

 to water. Such membranes are of no importance in osmosis in plant 

 cells, but they are useful in demonstrating certain principles of osmosis 

 and permeability. 



Molecules of oxygen and nitrogen are smaller than those of carbon 

 dioxide, but sheets of rubber are much more permeable to carbon di- 

 oxide than to the other two gases. Consequently, if one ties a knot in the 

 neck of a rubber balloon filled with ordinary air and then places it in a 

 bottle of carbon dioxide," the balloon will gradually increase in size. 

 Outside the balloon there is pure carbon dioxide. In the air inside the 

 balloon there are about 3 parts of carbon dioxide in 10,000. At the be- 

 ginning of the demonstration, therefore, there must be about 3000 

 molecules of carbon dioxide entering the balloon for every one that 

 leaves it. Obviously both the swelling of the balloon and the pressure 

 inside that stretches it result from the entrance of carbon dioxide. The 

 energy of molecular motion is responsible for both the entrance of the 

 carbon dioxide and the pressure on the walls of the balloon. 



For a demonstration of the osmosis of water in a non-living system one 



^ The less appropriate terms, semipermeable and selecti\'ely permeable, are sometimes 

 used to indicate this type of membrane. 



- Place the balloon in a bottle of water. After inxerting the bottle replace the water with 

 carbon dioxide. 



