290 NATURE AND PROPERTIES OF SOILS 



tion of cane-sugar. The walls of such a bag are semi-permea- 

 ble, that is, certain materials will pass through readily while 

 others will pass but slowly. For example, the sugar mole- 

 cules penetrate with difficulty, while the water finds the walls 

 of the bag but a slight obstacle. 



If this collodion bag with its sugar solution is attached to a 

 capillary tube and immersed in pure water, it at once becomes 

 distended and the liquid will rise in the capillary tube, indi- 

 cating an unequal pressure within the system. The pressure 

 develops because of the separation of the pure water and the 

 sugar solution by a membrane that is penetrated at different 

 rates by the molecules and ions in contact with it. A tendency 

 towards equalization of course occurs and, as the water moves 

 in faster than the sugar moves out, a pressure is developed 

 within the bag which becomes apparent by the rise of the 

 liquid in the capillary tube. Such a phenomenon is called 

 osmosis and the pressure osmotic pressure. Such force prob- 

 ably has much to do with the movement of plant saps and 

 fluids. Under such conditions as those maintained in the ex- 

 periment, the water tends to move from the dilute solution to 

 the more concentrated one. 



Suppose the collodion bag be considered as typical of the 

 cells, which form the feeding surface of an active rootlet, and 

 the sugar solution the relatively concentrated and partially 

 colloidal cell contents. The water outside the bag will, of 

 course, represent the. dilute soil solution which bathes the roots. 

 With such substitutions it can readily be seen why the plant 

 exerts an osmotic "pull" and how the water moves through 

 the cell-wall. Such a transfer will continue until the move- 

 ment of the water in the soil becomes too slow for normal 

 plant activities. Wilting then occurs. (See Fig. 51.) 



In alkali soils, where the soil solution becomes very concen- 

 trated, the process above described may be reversed. Out- 

 ward osmosis then occurs and plasmolysis 1 may result. 



1 Plasmolysis is a separation of the plasma from the cell-wall due to a 



