52 THE MECHANISM OF LIFE 



osmotic pressure of the blood was hardly decreased even after 

 an immersion of 72 hours. In the case of persons or animals 

 drowned in sea water, a similar alteration of the point of 

 congelation is observed, but in the reverse direction. In this 

 case the osmotic pressure is raised considerably in those who are 

 drowned, whereas no such rise is observed in those who are 

 thrown into the sea after death. 



The circulation of the sap in plants and trees is also in 

 great part due to osmotic pressure. The aspiration of the 

 water from the soil is due to the intracellular osmotic 

 pressure in the roots, which causes the sap to rise in the stem 

 of a plant as it would in the tube of a manometer. From a 

 knowledge of the osmotic pressure of the intracellular liquid 

 of the roots, we may calculate the height to which the sap can 

 be raised in the trunk of a tree, i.e. the maximum height to 

 which the tree can possibly grow. Suppose, for instance, the 

 plasma of the rootlets has an osmotic pressure of six atmo- 

 spheres, corresponding to that of a 9 per cent, solution of 

 sugar. A pressure of six atmospheres is equal to the weight 

 of a column of water G X '76 x 13596 = 61'95 metres high. 

 This, then, is the maximum height to which this osmotic 

 pressure is able to lift the sap. That is to say, a tree whose 

 rootlets contain a solution of sugar of 9 per cent, concentration, 

 or its equivalent, can grow to a height of 6'°. metres. 



Cryoscopy is also of great use in practical medicine, more 



especially for the examination of the urine. The freezing point 



of urine varies from — 1 "26° C. to — 2:3.">°. Koryani has studied 



the ratio of the point of congelation of urine to that of a 



solution containing an equal quantity of chloride of sodium. 



tt n j ii xi I- freezing point of urine . . 



He finds that the ratio ^ . " r . . „ K , „. increases when 



freezing point ot ,\aCl 



the circulation through the tubules of the kidney is diminished. 



Hans Koeppe has shown that the hydrochloric acid of the 



gastric juice is produced by the osmotic exchanges between the 



blood and the gastric contents. The ion Na of the salt in 



+ 

 the stomach contents exchanges with an ion II of the mono- 

 basic salts of the blood- NaHGO.+ NaCl = HCl + Na.,('()... 



