590 BRITTON: VEGETATION OF THE 
more frequently on the leptome side. In some cases a single 
large colorless cell completes the connection. Epidermal cells 
are very small over stereome tissue and some of them are extended 
into spiny or hair-like projections which overhang the grooves, 
protecting both the stomata and the bulliform cells, and occurring 
on both superior and inferior surfaces. In the grooves of the in- 
ferior surface there are curious unicellular projections showing 
cell-contents and resembling glandular hairs. The spiny projections 
of the epidermal cells appear to be empty. Stomata occur along 
the sides of the bulliform cells on the upper surface and in the 
grooves of the lower surface. Mesophy] borders directly on the 
larger cells of the sheath and also upon the bulliform cells. 
Poa compressa. — Leaf is conduplicate on drying. Sections 
examined have large bundle in keel and seven mestome bundles 
on each side. Subepidermal strands of stereome occur in the 
keel, in the margins of the leaf, and opposite most of the mestome 
bundles on both the upper and lower surfaces, though not arranged 
regularly. In some cases these stereome strands connect the 
bundles with the epidermis. Epidermal cells of the upper surface 
have their inner walls in the same plane, but the outer extremities 
are very irregular. On each side of the keel the epidermal cells 
are greatly enlarged, and probably serve for storing water. Epi- 
dermal cells of the upper surface have the outer walls much thick- 
ened, which is not the case with the cells of the lower surface. 
The cells of the lower epidermis are quite uniform on their outer 
surface and have thin walls. The small mestome bundles are 
each provided with a parenchyma sheath ; the larger bundles have 
a sheath of stereome inside the parenchyma sheath. Stomata 
are found on both surfaces ; in slight depressions on the lower Sur 
face and in modified depressions of the upper surface, forming 
grooves * of nearly the depth of the thickness of the epidermal 
layer of cells. This layer is connected with the guard-cells of the 
stomata by very small cells which are doubtless modifications of 
the epidermal cells. Stomata of the lower surface are much 
larger than those of the upper surface. Stout blunt hairs OF 
spines occur along the keel and the margins, and sharp-pointed 
ones are found on the upper surface opposite some of the mestome 
bundles. 
* O’Brien, Mss. (see fifth footnote on p. 585). 
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