TRANSPIRATION AND MOVEMENT OF WATER 241 



afternoon hours, at a period when the water columns are undoubtedly under 

 their maximum tension. While under tension the water columns are stretched 

 and decrease in diameter. Due to the enormous adhesive force between water 

 and the walls of the vessels, this results in a slight contraction in the diameter 

 of the vessels. Such diurnal changes in the diameter of a tree trunk are due 

 to alternate contraction of the vessels when the water in them is under tension 

 follow^ed by their dilation when the tension is slackened. 



Vessels and tracheids normally contain WMter at the time of their differ- 

 entiation and remain filled with water for varying periods of time thereafter. 

 Ultimately most of the water columns in a plant break, but this does not hap- 

 pen to all of them at any one time except under such extreme conditions as 

 prolonged drought. Breaking of water columns occurs principally at times 

 when they are subjected to high tensions. 



The Relation of Transpiration to the Movement of Water through 

 the Plant. — In most discussions of the cohesion of water theory of the rise 

 of water in plants, the relation of transpiration to this process is so greatly 

 emphasized that it has come to be almost indelibly associated with this con- 

 cept. Transpiration is not, how^ever, the fundamental cause of the upward 

 movement of w^ater in plants. Water ascends in the xylem ducts because of 

 and only when a diffusion pressure deficit has been created in the water of 

 the vacuole or the cell walls of the mesophyll cells. Since evaporation of 

 water from the walls of the mesophyll cells is the most frequent cause of such 

 diffusion pressure deficits, the process of transpiration has been generally linked 

 in discussions with the mechanism of the ascent of sap. It is only because of 

 its effect in increasing the diffusion pressure deficit of the water in the 

 mesophyll cells that transpiration sets in motion the entire train of water 

 through the plant. Any other process which results in an increase in the 

 diffusion pressure deficit in the cells at the terminus of any plant axis may 

 also induce the translocation of water. Upward movement of water often 

 continues during the night hours after transpiration has virtually ceased. 

 This lag is due to the residual high diffusion pressure deficit of the leaf cells 

 at the end of the daylight period. Water will continue to enter these cells 

 until they reattain the maximum turgidit}^ of which they are capable under 

 the conditions prevailing. Similarly movement of water will occur into any 

 rapidly growing stem tip because the binding up of water in certain phases 

 of the growth process creates a diffusion pressure deficit in the cells at the top 

 of the growing axis, thus inducing the migration of water towards such centers 

 of growth activity. 



At the present time most plant physiologists are agreed that the cohesion 

 theory is a correct representation of the principal mechanism by which water 



