588 BRITTON: VEGETATION OF THE 
surfaces. Stomata occur on both surfaces on the sides of the 
‘grooves between mestome bundles. Assimilating cells fill the 
entire space between the epidermal layers and between the 
bundles. : 
CENCHRUS TRIBULOIDES.— Over the keel are two or three layers 
of bulliform cells forming one half the thickness of the leaf. A 
single layer extends over the upper surface of the leaf, being par- 
tially interrupted over the stereome strands. This layer forms 
nearly one third of the thickness of the leaf. The lower surface 
has an epidermal layer of smaller colorless cells which undoubt- 
edly serve a similar purpose. The bundles are all enclosed in 
parenchyma sheaths. Large green cells surround the small mes- 
tome bundles, but these cells are considerably smaller in the 
sheaths of the larger bundles, The small bundles do not have 
the leptome and hadrome clearly differentiated. | Subepidermal 
strands of stereome occur on the keel and margins of the leaf, and 
above and below the larger mestome bundles. To some extent 
the leaf is furrowed on the upper surface, typical bulliform cells 
occurring in the furrows, and short spines or hairs which are modi- 
fications of the epidermal cells appear on the ridges. Stomata are 
found on the sides of these furrows, but are much more abundant 
on the lower surface, where they occur in rows between the mes- 
tome bundles. 
ANDROPOGON SCOPARIUS.— Over the midrib there are nine oF 
ten rows of very large bulliform cells, of pyriform shape in cross- 
section (shown on f/, 26, g). These are a continuation of the 
epidermis of the upper surface of the leaf which is composed of a 
layer of water-cells, this layer making fully one third of the thick- 
ness of the blade. Outer walls of cells are much thicker than the 
partition walls between the cells. Over the mestome bundles the 
cells are smaller and some are extended into stout pointed hairs. 
Keel formed mostly of stereome-cells. Subepidermal strands of 
stereome tissue are found above and below the larger mestome 
bundles. The large bundles are provided with a sheath of ster- 
eome-cells, and a row of similar thick-walled cells separating the 
leptome and hadrome. Small bundles have a sheath of rather 
thin-walled parenchyma-cells, and are not clearly differentiated 
into leptome and hadrome. Stomata do not occur on the upper 
