SALT AND WATER IN LIFE AND GROWTH 11!) 



the flushing of extensive wounds, particularly in surgical 

 operations, a liquid is needed which will not damage the 

 (issues. Water will not do for this purpose; it would break 

 up the blood cells and cause a swelling of the other body 

 cells. Rapid injections into the veins of large amounts of 

 distilled water are deadly, because of the shock produced 

 by wholesale destruction of blood and other body cells. A 

 solution, known as Ringer's solution is used; it contains all 

 the salts ordinarily present in the blood, including those 

 present in small amounts. 



5. THE AMAZING SIZE OF THE OSMOTIC PRESSURE AND ITS 



MEASUREMENT 



The expanding force of growing plants is very great; the 

 delicate root tips of growing plants are strong enough to 

 burst heavy iron pipes. The thin tips of the growing root 

 first project into one or more of the crevices of the iron pipe. 

 As these root tips expand by growing, the heavy tube cracks 

 in two. 



Can we explain such a force as a result of the "osmotic 

 pressure"? How great is this pressure? Can it, like the 

 steam pressure of a boiler, be measured by a manometer? 



Such a measurement is indeed possible, and actually 

 shows that a heavy pressure is exerted by osmosis. Many 

 technical difficulties, however, must be overcome. It would 

 appear that a set-up with an artificial cell — such as de- 

 scribed in Figure 32 — is not suitable for this purpose. In- 

 stead of stretching the semi-permeable membrane we must 

 now use the expanding force of the osmotic pressure to work 

 upon a pressure gauge. To this end the membrane must 

 somehow be reinforced. Nature itself shows us that the 

 very delicate semi-permeable membrane can be rendered 

 more resistant by imbedding it in a supporting framework 

 as in the case of the Tradescantia cell (Fig. 33). 



For the purpose of measurements, we imbed the semi- 



