Epidermis of Mesembryanthemum pseudotruncatelhim . 1139 
In a median longitudinal section perpendicular to the vertical plane 
containing this slit the following features are seen (Fig. 2). The two older 
leaves surround a fleshy column composed of the two next youngest leaves, 
which are at this time almost completely united. The dividing slit is at 
right angles to the older one, the leaves being therefore decussate. 1 he 
two older leaves form a cap, which fits closely over the young shoot, the 
slit forming an aperture in the roof (Fig. 2). 
In both the older and younger leaves occur the cystolith structures de¬ 
scribed by Marloth and referred to above. In M. pseudotruncatellum these 
are almost spherical, and occur at intervals as a single layer and separated 
from the upper epidermis by two layers of hypoderm. From one another 
they are separated by the simply perforated cells of the water-storage paren¬ 
chyma. When young their walls are thin and the tannin content poor. 
Later,an increase of thickness is seen 
in the walls, while the tannin con¬ 
tent becomes considerable, so that 
an opaque sphere results. 
The assimilatory tissue, which 
is shaded in Fig. 2, forms a band 
under and concentric with the side 
epidermis. It dies out near the 
upper surface, where the tannin- 
containing structures appear. In 
this band the two outer cell layers 
contain chlorophyll only, but the 
next eighteen or twenty layers are 
crowded with both simple and com¬ 
pound starch grains. 
The whole of the exterior is strongly encrusted with calcium oxalate^ 
the outer walls of the epidermis being impregnated with it. It is often 
excreted in considerable quantity on the upper surface, where it is per¬ 
ceptible as white streaks. Over the assimilatory tissue the quantity of 
calcium oxalate is considerably less. It is easily removable with the help 
of dilute hydrochloric acid. The stomata are sunken and the depression is 
free from encrustation. 
The most characteristic feature is exhibited by the epidermis itself. 
Here two extreme types of cell are present, together with transitional forms 
between them. In section all were found to possess the form of lens-cells, 
and the possession of the lens function was afterwards confirmed as described 
below. 
The cells of the upper epidermis are in surface view regularly hexagonal, 
the side walls being strongly cutinized. The basal walls are flat, thin, and 
consist of pure cellulose, while the upper walls are slightly cutinized. On 
Fig. 2. Longitudinal section through the 
‘ corpusculum \ 
