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be withdrawn from the outer cells of the 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, asa 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 ina 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 
