The Mechanism of Root Pressure. 
199 
assumption that solutes must enter the xylem vessel from the 
secreting cells in the root? 
The assumption and the factor are probably reconcilable, on the 
grounds that after excretion and during the subsequent upward 
passage in the xylem the ascending stream is always in contact 
through the xylem walls with the surrounding protoplasts. Under 
these circumstances, physical adsorption on the surfaces limiting the 
stream may well be followed by physiological absorption, whether 
the solutes in the ascending stream are organic or inorganic, sugars 
or acids. 
This suggestion must probably appear in any theory of root 
pressure, because the fundamental tenets of thermodynamics seem 
difficult to reconcile with any other assumption than the original 
excretion of solutes with the water into the xylem vessel. 
It is not claimed that this discussion is exhaustive, but it is 
submitted that the account of the mechanism of root pressure given 
above fairly states the difficulties and provides, with the minimum 
of assumption, a hypothesis, founded chiefly on observation and 
experiment, that is adequate to account for the facts. 
This hypothesis should be of interest because on the one hand 
it suggests the revision of anatomical data with reference to the 
endodermis to see if they will permit this interpretation of its 
function and on the other it points the way to a series of 
physiological investigations which may throw further light on the 
phenomena of permeability. 
It is hoped to give a more detailed account later of the 
experimental work arising out of the views stated above, and with 
such an account other names would be associated. In connexion 
with the experimental work briefly touched upon above, I have to 
acknowledge the help of Miss D. Armstead, Research Assistant in 
the Department, and of Miss Tupper Carey. I should also like to 
express my great obligation to Dr. S. A. Shorter, for his patient 
assistance in helping me to struggle towards enlightenment on the 
physical side of the problem. 
Department of Botany, Leeds University. 
REFERENCES. 
1. Atkins, W. R. G. Some recent researches in Plant Physiology 
Whittaker & Co., 1916. 
2. Bayliss, W. M. Principles of General Physiology, Longmans, Green 
& Co., 1915. 
3. De Rufz de Lavison, Jean. Du mode de penetration de quelques sels 
dans la plante vivante. Role de l’endoderme. Revue 
General de Botanique, Vol. 22, p. 225, 1910. 
