Physiological Studies in Plant Anatomy 217 
fact upon the relation of the plant growing point to water has seldom 
if ever been emphasized. 
It must be remembered, however, that this relative imperme¬ 
ability of the protoplast has to be reconciled with the need of 
the growing tissues for metabolic “Bausteine” from which the 
protoplasmic complex is constructed. The organic substances re¬ 
quired, constituents of the translocation stream, are presumably 
presented to the surface of the meristematic protoplast in the form 
of solutes in the cell sap and though the water is unable to enter the 
cell, the solute must be picked up by the plasma membrane, possibly 
by a chemical combination succeeding a preliminary adsorption. 
These conflicting requirements, coupled with certain funda¬ 
mental differences in the behaviour of the apices of root and stem 
to water, suggest possible reasons for the marked morphological 
differences between the two apices. We must first, however, deal 
with the experimental work which justifies the statement that the 
stem apex behaves very differently in its permeability to water and 
dissolved substances. 
(2) The Permeability of the Stem Apex 
One marked difference between the apex of stem and root is 
that the meristematic tissue of the stem does not form a cap to the 
endodermal cylinder. Mylius(i 9 ) has shown that the endodermis is 
never formed so close to the apex in the stem as in the root. In 
many stems it is never present as a functional layer, with Casparian 
strip (see Priestley and North (21)). In underground stems and stems 
growing in water the Casparian strip appears within a relatively 
short distance of the growing point, but it will be shown in a later 
paper in this series (Priestley and Ewing (6)) that when the stem 
apex develops under etiolation conditions the conditions at the 
apex approximate to those present at the apex of a root. On the 
other hand, when the stem is developing freely in the light the 
functional endodermis with Casparian strip is seldom detected 
in the proximity of the growing point. Mylius(i 9 ) points out that 
a secondary endodermis is frequently present in the aerial stem and 
that this secondary endodermis when the stem is actively growing 
can only be found at a considerable distance behind the growing 
point, but that in the winter its formation appears to overtake the 
slower activity of the meristem and it can be found very close to 
the growing point. This we can confirm, but in two cases that have 
been under very thorough investigation in this department, in 
Phyt. XXI. IV. 
15 
