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PHYSIOLOGICAL STUDIES IN PLANT 
ANATOMY 
II. THE PHYSIOLOGICAL RELATION OF THE SURROUND¬ 
ING TISSUE TO THE XYLEM AND ITS CONTENTS 
By J. H. PRIESTLEY and DOROTHY ARMSTEAD 
CONTENTS 
PAGE 
Introduction ........ 62 
I. Experiments with Stains and Living Tissues . . 63 
II. Exudation Pressures in Stem and Leaves . . 64 
III. Solutes in the Xylem Sap ..... 68 
IV. Exchange of Sugar between the Xylem and the 
Surrounding Protoplasts ..... 73 
Discussion ........ 77 
Summary ........ 79 
Introduction 
an account has recently been given (Priestley (20)) of a theory of 
y^^the mechanism by which exudation pressure may be developed 
by a root system. In the present paper some fresh experimental 
evidence is presented which is relevant to this theory and the 
experimental investigation is extended to the study of such 
exudation pressures in stem and leaf. It is therefore necessary at 
the outset to re-state very briefly the essential features postulated 
for the root-pressure mechanism, viz. 
(1) A xylem strand, which contains rigid permeable tubes, pro¬ 
viding an open channel for the flow of sap. 
(2) A supply of solutes to the xylem which maintains the sap at 
higher osmotic concentration than the dilute solution bathing the 
protoplasts at the surface of the root. 
(3) The maintenance of this supply of solutes requires 
(a) The presence of a functional endodermis, that is an endo- 
dermis whose radial and transverse walls are so relatively imper¬ 
meable that the outward leakage of the solutes is prevented. 
(b) The constant diffusion of solutes into the xylem sap from 
relatively permeable protoplasts within the endodermal cylinder. 
The next paper in this series will examine the structure of the 
endodermis in relation to this mechanism, for the present we are 
