64 EPIDERMIS [CH. 
are wide open in the dewy mornings in summer, close 
up as the air gets dry and hot; and any such leaf may 
be seen to roll up after plucking and can be reopened 
by moistening it. 
Fig. 22. Transverse section of left-half of leaf of Festuca elatior, var. 
pratensis (x about 50). The ridges are well marked and flattened 
above. The vascular bundles of two orders are girdered below, but 
only slightly above. There is no keel. There are well marked motor- 
cells—not shown in the figure—in each groove. 
The epidermis of grasses has been closely investigated 
by Grob, but unfortunately his results concern very few 
of our native species. The principal elements are ordinary 
elongated cells, with plane or sinuous walls, various kinds 
of short cells intercalated between the ends of these, several 
forms of papille, hairs, &c. and stomata. 
The epidermis over the parenchyma of Digraphis 
arundinacea consists of rectangular cells with plane walls. 
Series or bands of long cells only may alternate with 
other series where short cells intervene between the long 
ones—e.g. Nardus. 
Nardus has some of the bands devoid of stomata, 
but abounding in short cells, whereas others (above) have 
stomata throughout. 
In Nardus stricta, Glycerva fluitans, Sesleria, &c., there 
are two kinds of short cells, some siliceous, others cutinized 
only. | 
Nardus has closely appressed small 2-celled hairs bent 
