Structure and Function of Endodermis in Abietinece 289 
large cells of four-sided prismatic form with large nuclei and plentiful 
protoplasmic contents. Starch grains were always present. 
Certain constant features are presented by the endodermis in the 
leaves, among these being the structure of the radial and transverse 
walls, and it is on this that the consideration of the importance of 
the endodermis as a physiological unit in the economy of the leaf is 
based. Caspary’s band was not observed to be present, but in every 
case the radial walls were found to consist of a core of lignocellulose 
with a layer of suberin superimposed on either side. Simple cellulose 
pits are always present, though the frequency with which these occur 
varies in the different genera from many in Pinus sylvestris and Picea 
excelsa to few in Abies pectinata and Larix europcea. The transverse 
walls consist likewise of a lignified core with surfaces of suberin, but 
these are never pitted. The continuity of the thickening of the radial 
walls is interrupted by passage cells in only a very few cases, these 
being observed in Tsuga canadensis, Pseudotsuga Douglasii and Larix 
europcea. 
The structure of the tangential walls varies considerably in the 
leaves described. In such species as Pinus sylvestris and Abies 
pectinata both tangential walls are usually lignified and have in 
addition patches of suberised tissue. The amount of suberin deposited 
seems to depend on the age of the leaf, more being found in older 
leaves. In other cases, the outer tangential walls only may be 
lignified, and it is very common to find many of the inner tangential 
walls wholly of cellulose where they abut on unlignified pericyclic 
tissue. This is the case in Pinus excelsa, Tsuga canadensis, Pseudo¬ 
tsuga Douglasii and Larix europcea. The outer tangential walls were 
in every case found to be more continuously lignified than the inner 
ones. Whatever be the nature of the tangential walls, the arrange¬ 
ment of the thickening is always such as to render them permeable 
to water, the suberised layer, when present, being incomplete. Pits 
or breaks in the suberin are always present in the most thickened 
walls. 
As has recently been emphasised by Priestley (12), the passage of 
water through the walls between the endodermal cells in roots is 
impossible when an impermeable strip is continuous all round the 
radial and transverse walls of the cell. This is the case only if at 
some one point the radial walls are completely suberised throughout 
their thickness. In all the leaves described the suberin was present 
in the form of a surface layer on each side of the radial walls, leaving 
a core of lignocellulose through which water could pass from the 
19—5 
