1 84 OSMOSIS : ABSORPTION OF WATER 



or destroyed, so long as the other young parts of the roots are 

 kept in contact with water. 



It has been experimentally proved that it is only through the 

 root-hairs and the youngest parts in the immediate neighbour- 

 hood of the root-hairs that the absorption of water occurs : 

 through the older parts on which the root-hairs have shrivelled 

 and which have become covered with a tissue of cork-cells water 

 is unable to penetrate. 



The walls of the root-hairs consist of ordinary uncutinized 

 cellulose through which water readily passes, and it is on 

 account of the existence of osmotic substances in the cell-sap 

 within the hairs that water with which they come in contact is 

 attracted into them. 



After carrying on their work for a short time they wither and 

 die, but before this occurs a new set of hairs arises on the 

 extending rootlet. 



The greatest development of root-hairs occurs upon roots 

 which are allowed to grow in damp air or in a moderately dry 

 soil. When roots are immersed altogether in water, root-hairs 

 are generally absent; the necessary absorption in such roots 

 is carried on by the unextended superficial cells of the piliferous 

 layer, there being no need for the extension of these cells into 

 long hairs. 



In very dry soils the development of root-hairs is feeble or 

 entirely checked. 



On account of the delicate nature of the root-hairs it is not 

 possible to remove a plant from the soil without breaking the 

 connection of the hairs with the fine particles of earth and 

 permanently destroying many of them ; transplanted plants, 

 therefore, always suffer for want of water until new hairs are 

 formed on the rootlets. Among certain plants new roots and 

 root-hairs do not form readily and such plants cannot be trans- 

 planted. When trees or other plants are removed, it is advisable 

 to specially preserve the youngest rootlets from which fresh 



