Fig. 22. Transverse section of left-half of leaf of _Festuca elatior_, var. _pratensis_ (× 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 papillæ, 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_, _Glyceria fluitans_, _Sesleria_, &c., there are two kinds of short cells, some siliceous, others cutinized only. _Nardus_ has closely appressed small 2-celled hairs bent at right-angles, and some epidermal and parenchyma cells--especially below the stomata--have solid masses of silica filling the lumina. [Illustration: Fig. 23. Part of transverse section of leaf of _Aira cæspitosa_ (× about 30). Ridges very high and acute, each tipped with sclerenchyma, and containing an isolated vascular bundle--sometimes one or more small ones also. Motor-cells well developed at the base of each groove. The bundles are not girdered, but numerous bands of sclerenchyma almost join into a continuous band below. The leaf rolls inwards.] Short cells occur in _Holcus lanatus_, _Hierochloe borealis_ and _Dactylis glomerata_ interspersed between plane-walled cells. They may be silicified and vary in shape--square, saddle-shaped, elliptical, irregular, &c.; or they may be replaced here and there by asperities--e.g. _Elymus_--or in rarer cases by stomata. Grob has attempted the classification of their distribution in different grasses, but the subject is too complex for treatment here. The epidermis of many grasses is studded with short two-celled hairs bent sharply at right-angles; so that the pointed or blunt, hollow or solid, apical portion is appressed to the surface. Grob says that these are absent from the Hordeæ, whereas 90% of the Panicoideæ and many species of all other groups have them. Examples of the sharply pointed form occur in _Nardus_, of blunt ones in _Cynodon_ &c. In _Nardus_ they occur on the leaf surface both between and above the veins, but in _Hierochloe_ &c. they are confined to the margins. The following grasses have no hairs of either type: