84 PLANT LIFE 



molecules of the water are free to move within 

 the column. 



Of course, this condition of stress is propa- 

 gated throughout the water system from the 

 leaf back to the root. Water thus tends to 

 be withdrawn from the outer cells of the 

 root which abut on the ends of the conducting 

 tissues within, and in this way a continuous 

 flow is maintained from the root-hairs, which 

 in their turn are replenished from the supplies 

 of water contained in the soil. It comes to 

 be a balance of forces represented on the one 

 hand by those leading to the escape of water 

 vapour from the leaf, and on the other the 

 forces which tend to cause the water to be 

 retained by the soil plus the effects of friction, 

 etc., within the plant itself. 



But, as a matter of fact, although the short 

 description given above probably represents 

 in a general way what goes on in connection 

 with the translocation of water in a plant, there 

 are other factors which are involved and may 

 affect the process. 



The living parenchymatous cells of the 

 roots are not merely passive agents in the 

 matter, for the water absorbed from the 

 soil is in many plants (and perhaps in all) 

 forcibly pressed or excreted from these living 

 cells into the conducting channels. It is to 

 this active propulsion of water within the 

 plant that the phenomenon of " bleeding " is 

 due. When trees are felled in spring, sap 

 may continue for a long time to flow forcibly 



