Physiological Studies in Plant Anatomy 117 
final stage an endodermis in the secondary stage (Rumpf), similarly 
in the stem and frond, with the exception of Trichomanes and the 
storage rhizomes in the more- complex types (Basecke). A tertiary 
stage endodermis does not appear to occur in the Pteridophyta. All 
these workers on the Pteridophyta quoted above have met with 
cases where the suberin lamella is only deposited over the inner 
tangential surface of the cell, and in all these cases it covers com¬ 
pletely the whole surface up to the inner edge of the Casparian 
strip. 
Of the four stages distinguished by Kroemer it will only be 
necessary to consider two in this paper. In the embryonal stage the 
cells of the future endodermal cylinder possess the same relation to 
the sap contained within them as do the cells of the apical meristem, 
a problem which is dealt with in the next paper in this series 
(Priestley and Tupper-Carey( 23 )). 
The structure and function of the endodermis in its primary and 
secondary stages will now be considered at length. It will be clear 
after the secondary stage has been discussed that the tertiary stage 
requires no further attention; from our present standpoint these two 
stages of the endodermis may be regarded as functioning in a similar 
way. 
The Primary Endodermis 
(1) Its Micro-chemical Structure. 
At this stage the walls of the endodermal cells are so thin that it 
is difficult to distinguish anything beyond a thin cellulose wall, in 
which one band stretching around both radial and transverse walls, 
and frequently undulating, forms the characteristic Casparian strip 
(c.s. Text-fig. 1). 
Recourse to macerating agents which do not attack cellulose will 
show, however, that we must distinguish between the layer most 
external to the protoplast, which is the middle lamella ( ml . Text- 
fig. 1), and the cellulose lamella within this (c.l. Text-fig. 1). When 
sections of herbaceous stems or roots possessing an endodermis are 
macerated in dilute acids followed by dilute alkalies, or by prolonged 
standing in ammonium oxalate after dilute acids, so that the tissues 
disintegrate owing to the solution of the middle lamella (Mangin(i6)), 
then the endodermal layer is freed from the tissues adjacent to it 
owing to the solution of the middle lamella upon the tangential walls. 
The endodermal cells, however, do not separate from each other as 
the cementing middle lamella of the radial walls does not disappear. 
