Structure and Function of Endodermis in Abietinece 275 
by the fact that some of the cells are left undissolved after treatment 
with concentrated sulphuric acid. 
The endodermal cells are all rich in protoplasmic contents and 
contain numerous starch grains and large nuclei. 
(d) Young leaves at the growing point of the shoot. 
In sections of young leaves the endodermis appears as a regular 
layer, the cells of which are full of contents and contain especially 
large nuclei, the walls being all composed of unthickened cellulose. 
No infoldings of the mesophyll walls can be seen (Fig. 4, a). 
Fig. 3. Pinus sylvestris L. Radial longitudinal section of leaf treated with eau 
de Javelle followed by dilute sulphuric acid. Simple pits ( s.p .) are shown 
in the inner tangential wall of the endodermis ( en .) and bordered pits ( b.p .) 
in the tracheidal pericycle tissue. ( x 240.) 
As the leaf grows older and the characteristic plicate arrange¬ 
ment of the mesophyll appears, each radial longitudinal wall of the 
endodermal cell becomes thickened with a band of lignocellulose 
extending over both surfaces of the wall and round the corners on to 
the tangential walls for a short distance. The tangential walls are 
unlignified and the transverse walls resemble the radial walls in 
structure, though the latter only are pitted. No sign of the usual 
wavy appearance due to Caspary’s band can be seen in the young 
radial walls, these being lignified over the whole surface and pitted 
at frequent and irregular intervals. 
At a later stage the radial walls become lignified throughout their 
thickness, the change appearing first in their outer comers. At the 
same time a thin layer of suberin is deposited over the surface 
