124 J. H. Priestley and Edith E. North 
endodermis at an early stage in root development. In stems, on the 
other hand, except in submerged aquatic plants and in certain cases 
to be considered in later papers, notably the case of etiolated plants, 
the embryonic nature of the wall disappears in earlier stages at 
the growing point, its impregnation does not therefore occur, the 
fatty substances being presumably swept onward by the sap to the 
cuticle; as a consequence the primary endodermis with Casparian 
strip is never developed. In such stems a functional endodermis 
may never be present (Mylius(i8) shows how frequently this is the 
case in the axis of the flowering plant aboveground), or a secondary 
endodermis may arise later in development, e.g. in Escallonia, 
Camellia, etc. 
Summarising the results of the micro-chemical study of the 
primary endodermis, the cells at this stage possess a cellulose inner 
lamella, and an outer middle lamella soluble, after treatment with 
acids, in the usual pectin solvents. This wall is interrupted by a 
continuous band around its radial and transverse walls, the Casparian 
strip, which differs throughout its substance from the rest of the 
wall. The Casparian strip is laid down at an early stage of develop¬ 
ment before celullose can be detected in the endodermal cell walls. 
In this region the original lamellae are impregnated with complex 
substances which invariably contain derivatives of fatty substances, 
probably allied to the suberogenic and cutinogenic acids, and with 
substances giving reactions for lignin. The basal substance of the 
Casparian strip is therefore apparently closely allied to the normal 
wall of the merismatic cell; this basal substance contains some nitro¬ 
gen, is very resistant to acids and alkalis and dissolves in oxidising 
agents. 
(3) The Function of the Primary Endodermis. 
Earlier work upon this subject has been summarised elsewhere 
(Priestley (20)), when it was shown that the experiments of de Vries ( 30 ) 
and de Lavison(i 4 ) suggested that the passage of water and solutes 
through the endodermis was considerably impeded by the structure 
of the radial and transverse walls, and as a consequence took place 
mainly, if not entirely, through the protoplasts themselves. Further 
experiments in this laboratory have confirmed this conclusion. An 
extended investigation by Miss Helen Heaps, M.Sc. on the entry of 
iron salts into the plant has shown that, by the use of suitable micro¬ 
chemical methods, the iron can in certain cases be traced across the 
endodermis, but in this case it is always found within the protoplasts 
