96 



INTRODUCTION TO EVOLUTION 



^ 



differentially 

 permeable 

 membrane 



r 



FRESH WATER 



/ 



■ SALT water;: 



illJ 



FIG. 5.3. Effects of osmotic pressure on two closed bags composed of difFerentially 

 permeable membrane: left, a bag containing salt water, immersed in fresh water; right, 

 a bag containing fresh water, immersed in salt water. 



on the other, the pure water will pass through the membrane more read- 

 ily than will the salt solution. Suppose we take a bag of differentially per- 

 meable membrane, fill it with salt solution, close it tightly, and then im- 

 merse it in pure water. We shall find that water passes into the bag through 

 the membrane more rapidly than salt solution passes out of the bag 

 through the membrane. The net result will be an increase in volume of 

 the solution in the bag. If, as we have stipulated, the bag is tightly closed, 

 an increase in volume of its contents will result in its becoming distended 

 (Fig. 5.3). In other words, a pressure will develop inside the bag. This 

 is known as osmotic pressure, and under some circumstances it may de- 

 velop considerable strength — sufficient to burst our bag, perhaps. If we re- 

 verse our experiment, placing pure water in the bag and immersing the 

 bag in salt solution, we shall find the greater movement of material in 

 the opposite direction: the contents of the bag will become less and less, 

 as it loses its water to the outer salt solution (Fig. 5.3). Note that in 

 both cases water passes through the membrane ]rom a region in which 

 there is much water but little or no salt to a region in which there is less 

 water in proportion to the concentration of salt. In other words, the water 



