148 



GREEN PLANTS AS POOD MAKERS 



Evidently the egg membrane will permit the passage of water 

 but not of protein. Such a membrane is said to be semi-permeable 

 (pur'me-d-b'l). It is this kind of membrane that surrounds plant 

 and animal cells. It will permit certain substances such as water 

 to pass through it readily in either direction, and it will permit 

 certain substances in solution to pass less readily, while still other 

 substances will not be permitted to pass through at all. 



Demonstration 7. Fill the lower end of a thistle tube with a solution 

 of grape sugar and water. Tie tightly over it an animal membrane 

 (as a pig's bladder or skin of frog's leg), and place the tube in water, 

 as shown in the diagram. After a short time observe your apparatus. 

 What has happened? Why? At the end of an hour, test the water in 

 the beaker with Fehling's solution. Explain your result. 



If we could see the separate particles, or molecules, of the water 



and of the solution of water and sugar, they would be found to 



arrange themselves on each side of the membrane so as to cover 



it completely. But since the water mole- 



Icules pass easily through the membrane 

 and the sugar molecules do not pass so 

 easily, it will be seen that the inner side of 

 the membrane does not present so much 

 space for the diffusion of water particles as 

 does the other side. Hence the flow of 

 water into the tube is more rapid than the 

 flow out of the tube, and the water gradually 

 rises in the thistle tube. This passage of 

 water through a semipermeable membrane 

 is known as osmosis (os-mo'sis). It will be 

 seen that the greater flow of water mole- 

 cules is always from the point of greater 

 concentration of water to the point of lesser 

 concentration of water. If the solution is 

 completely inclosed in a vessel with rigid 

 walls, the entrance of more water will cause 

 osmosis in an egg. Ex- a pressure by the solution within these 

 plain, with reference to the c i ose d walls and will prevent the entrance 



text, why water will rise in r 



the tube. of any more water. This is known as 



.gkrss tube. 



■4-mernbrone 



