OSMOSIS 



67 



were on both sides of the membrane, it would diffuse 

 equally both ways and there would be no rise of liquid. 

 The presence of salt in N diminishes the amount of fluid 

 passing out, and more water comes in than water and 

 salt go out; hence there is pressure in the tube. 



140. The cell -sap of the root -hair absorbs water from the 

 soil by osmotic action. The above experiment enables 

 us to understand how the countless 



little root-hairs act, each one like 

 the tube N, if only the whole surface 

 of the tube were a bladder membrane, 

 or something acting similarly. The 

 soil water does not contain much of 

 the land's fertility : that is, it is a 

 very weak solution. The active little 

 root-hair, on the other hand, is always 

 filled with cell -sap, a more concen- 

 trated solution : hence soil water must 

 come in, and along with it come also 

 small quantities of dissolved food 

 materials. Some of these materials 

 may be fertilizers which have been A^ 

 applied to the land. 



141. The plant absorbs these solu- 

 tions as long as they are used for 

 the growth of the plant. The salts 

 which are dissolved in the soil water 



diffuse themselves through the tiny membrane of the 

 root -hairs, each ingredient tending independently to be- 

 come as abundant inside the root-hair as outside in the 

 soil water. Once inside the root-hair, these absorbed 

 solutions pass on to root and stem and leaf, to be 

 utilized in growth. As long as they are used, how- 

 ever, more must come into the root -hairs, in order to 

 restore the equilibrium. Thus those substances which are 



106. To illustrate osmosis. 



