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V 
PHYSIOLOGICAL STUDIES IN PLANT 
ANATOMY 
IV. THE WATER RELATIONS OF THE PLANT 
GROWING POINT 
By J. H. PRIESTLEY and R. M. TUPPER-CAREY 
CONTENTS 
PAGE 
Introduction . . . . . . . . . .210 
I. Experimental Investigation of the Permeability of the Root Apex 211 
(1) Experiments with Water under Pressures . . . . 211 
(2) Experiments with Dyes and Salts in Solution . . . 212 
(3) Conductivity Experiments . . . . . . . 213 
II. A Comparison of the Water Relations of Root and Stem Apices . 215 
(1) The Relations of a Meristematic Tissue to Water . . 215 
(2) The Permeability of the Stem Apex . . . . .217 
(3) Composition of the Wall at the Apical Meristem of Stem 
and Root .......... 219 
(4) Morphological Consequences of the Different Permeabilities 
of the Apical Meristerns . . . . . . .222 
III. The Root Apex as an Absorptive Organ? ..... 225 
Summary ........... 227 
References ........... 228 
Introduction 
A recent investigation into the mechanism of exudation pres¬ 
sures exerted by the root (Priestley (20)) revealed the necessity 
for the presence of a tissue formation, throughout the root cylinder, 
which would be capable of supporting the pressure of the ascending 
water column, without allowing serious leakage back to the surface 
of the root. This necessary lateral supporting tissue has been found 
in the endodermis (Priestley and North (21)) or in the periderm which 
replaces the endodermis in many older roots. 
The endodermal or periderm layer forms a continuous sheath 
around the central cylinder except for a relatively short region at 
the root apex where it is replaced by the meristematic tissue of the 
growing point. 
The present investigation began as an attempt to test experi¬ 
mentally the capacity of this meristematic cap closing the endo¬ 
dermal cylinder to retain water held under pressure within the 
endodermis. Experiments are first described which supply con- 
