OSMOSIS 



67 



were on both sides of the membrane, it would diffuse 

 equally both wuys 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 wafer 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 

 tilled 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 h 

 materials. Some of these materials 

 may be fertilizers which have been 

 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 ^^^- t° illustrate osmosis 

 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 equilibriuni. Thus those snhsiance.s which are 



A- 



