6 o 
J. H. Priestley 
unremitting supply of such solutes to the meristem is to be found in 
the sap from the neighbouring vascular strand. These solutes cannot 
diffuse from vascular strand to meristem, however, across a func¬ 
tional endodermis (III). 
Consideration of later experimental results (II and V) suggests 
that the sap in the vascular strand is only likely to contain 
the necessary organic solutes when it is under pressure. It was 
the study of the development of such exudation pressures in the 
vascular strand (II) which first led to an investigation of their effect 
upon the growth of meristems. The most efficient organ in their pro¬ 
duction is the root, and reasons have been given (Priestley ( 3 )) for 
considering that the endodermis plays an important role in enabling 
the root to develop and maintain these exudation pressures. A fuller 
study has therefore been made of the endodermis, and although the 
developmental side of this problem is still far from clear, a con¬ 
sideration of the results of two later papers (III and VI) certainly 
suggests that the sequence of events in the development of the endo¬ 
dermis follows naturally from the composition of the apical meristem 
and the interaction of the tissues just behind the meristem with the 
external environment. 
A functional endodermis, by restraining within a limited area the 
supply of sap necessary for continuous merismatic growth, exerts a 
profound effect upon the future development of the plant. Thus the 
presence or absence of an endodermis seems to be responsible in part 
for the endogenous or exogenous origin of the lateral organs charac¬ 
teristic respectively of root and stem. 
Two anatomical questions have enabled this view as to the 
significance of a functional endodermis to be submitted to a fairly 
severe test, from which it appears to emerge with credit. 
(1) The occurrence of the meristem of periderm at various depths 
within the tissue of root and stem, as also its occurrence at leaf-fall 
or in response to injury, i$ analysed as fully as possible (V), and 
adequate causal mechanisms can be found accounting in all cases for 
the development of sap pressures prior to the origin of the phellogen. 
A striking correlation can be traced between the position of the 
phellogen and the presence or absence of a functional endodermis. 
(2) It is shown (VI) that the shoot apex of some vascular plants, 
when etiolated, that is, when grown under conditions normal for a 
root, develops structural features more characteristic of a root. The 
lateral organs, the leaves, are much suppressed, and within the stem 
a functional endodermis, absent in the normal plant, encloses the 
