THE PATHWAY OF WATER MOVEMENT ACROSS 



THE ROOT CORTEX AND LEAF MESOPHYLL OF 



TRANSPIRING PLANTS 



P. E. Weatherley 

 Botany Department, University of Aberdeen 



I. INTRODUCTION 



In its passage from the soil to the atmosphere, water of the transpiration 

 stream must traverse two cellular barriers: that between the soil and the 

 xylem elements of the root, conveniently designated root cortex, and that 

 between the xylem of the leaf vein and the transpiring cell, designated leaf 

 mesophyll. Transpirational loss of water from the mesophyU cell causes a 

 gradient of water potential through the leaf tissue in response to which 

 water moves from the vein. This results in a reduction of hydrostatic 

 pressure in the xylem in response to which water moves across the root 

 cortex from the soil. 



Of course such cellular barriers present an extremely complex pathway. 

 The question before us in this paper is whether the water moves from 

 vacuole to vacuole crossing the cell walls and membranes of each cell in 

 turn or moves roimd the cells keeping within the cell walls (or perhaps in 

 addition witliin the outer region of the cells as well) thus bypassing the 

 vacuoles. We have in a sense two pathways in parallel. Water will move 

 through both, but if there is a large difference in resistance between the 

 two, water wiU move largely through the one with the lower resistance. 



The older view was certainly that in the leaf, movement was from 

 vacuole to vacuole and this seems to persist. Evidence that movement is in 

 fact through the mesophyll cell walls was obtained by Strugger (1938-39) 

 using fluorescent dyes although Hiilsbruch (1956) has cast doubt on the 

 validity of this evidence. Levitt (1956) on the basis of purely thoeretical 

 considerations concludes that movement through the mesophyll cannot be 

 diffusional and is prseumably mass flow in the ceU walls. But there seems 

 to be a dearth of positive experimental evidence one way or the other. In 

 the furst part of this paper a new experimental approach is described which 

 strongly points to a superficial pathway round the mesophyll cells. 



The root has been the subject of more extensive study but the position is 

 more complex than that in the leaf and even less well understood. The 

 complexity arises because the root is the seat of vigorous secretory processes 



