THE PATHWAY OF WATER MOVEMENT 99 



If water moves so readily through the mesophyll walls the same might 

 well be expected of parenchymatous cells generally and the root cortex in 

 particular. How then is this reconciled with at least 75 % of the flux through 

 the root being due to osmotic diffusion and its extreme sensitivity to cyanide 

 and low temperatures? As was suggested in the introduction the answer 

 probably lies in the root endodermis. Here, as is well known, flow through 

 the walls is prevented by the casparian strips and the transpiration stream 

 must be canalised through the cells. Thus the root behaves Hke an osmometer 

 albeit one possessing a membrane endowed with solute secreting proper- 

 ties, sensitive to pressure gradients and needing constant metabohc activity 

 to maintain its permeabiHty to water. 



As for the difference between k^ and kp this may be regarded as due to 

 mass flow through the passage cells in the endodermis or through regions 

 in which the casparian strips are not suberised. In the case of culture-grown 

 tomato plants these together seem to allow about J of the flux through the 

 root as mass flow with pressure differences of about 2 atm. On this view 

 and assuming movement in the xylem is by mass flow, transpirational flux 

 is a purely mechanical flow between soil and air except at one point, the 

 endodermis, which constitutes the only Hving barrier in the whole complex 

 catena. Thus the flux from soil to air through the plant is a tensile stream 

 everywhere save through the inner space of the endodermis where the 

 water representing at least | of the total flux is transitorily under pressure. 

 The other cells of the root, cortex, stele and leaf, play little part as Dixon 

 recognised (1938), but merely act like manometers attached to the main 

 stream and adjusting their water content, perhaps somewhat tardily, to the 

 tension therein. No doubt they are also in a state of dynamic equilibrium 

 with the stream as regards ions and other solutes (Jackson and Weatherley, 

 1 961). But through the endodermis alone are water and solutes dispensed 

 through the hving cells. As was recognised long ago, it is a knowledge of the 

 physiology of this important tissue which may be crucial to our further 

 understanding of water and ion movement in the higher plant. 



REFERENCES 



Arisz, W.H., Helder, R.J. & Nie, R. van (195 i) Analysis of the exudation process in 



tomato plants. J. exp. Bot. 2, 257. 

 Barrs, H.D. & Weatherley, P. E. (1962) A re-examination of the relative turgidity 



technique for estimating v^^ater deficits in leaves. (In the press) 

 Bernstein, L. & Nieman, R.H. (i960) Apparent free space of plant roots. PI. Physiol. 



35, 589- 

 Brewig, a. (1936) Beobachtungen iiber den Einflusz der Sproszaugung auf die 

 StofFdurchlassigkeit der Wurzel. Ber. dtsch. Bot. Ges. 54, 80. 



