The principal anatomical features observed in the leaves of grasses--apart from finer histological details into which it is not my purpose to enter--concern the characters of the epidermis and distribution of the stomata and hairs, the arrangement of the chlorophyll-tissue, that of the mechanical tissue (sclerenchyma) and the vascular bundles to which the venation and ribbing of the leaves are due, and the presence or absence of those peculiar thin-walled cells (motor-cells) which bring about the infolding or inrolling of the lamina (see p. 25) as they lose water, and, finally, the presence or absence of conspicuous lacunæ or air-spaces so characteristic of aquatic species. Several observers have occupied themselves with these matters, and the researches of Schwendener, Duval Jouve, Pfitzer, Pée-Laby, and others have rendered it possible to group most of our grasses according to the microscopic characters of the leaves, somewhat as I have done in Chapter V. Reference has been made to the rolling and folding of leaves, due to the thin-walled cells on the upper surfaces capable of varying in turgescence (motor-cells). These are specially adapted epidermal cells found on the upper surfaces only. In the leaves of _Poa compressa_, _P. annua_ (Fig. 21), _P. nemoralis_, _P. alpina_, _Catabrosa_, _Sesleria_, &c., a row of these motor-cells, easily distinguished by their large size, thin walls and clear contents, is found on each side of the mid-rib; as they dry the leaf folds its two halves together (conduplicate), and on the re-absorption of water they flatten the two halves out again. In _Dactylis_ these flanking rows coalesce into one over the mid-rib. In other leaves, e.g. _Avena pratensis_, _Festuca elatior_ (Figs. 17, 22), _Melica_, _Elymus_ (Fig. 25), &c., there are in addition to these two flanking rows, other sets of motor-cells between the other ribs, and their combined action causes the halves of the lamina to inroll, usually one-half inside the other--convolute. [Illustration: Fig. 21. Transverse section of left-half of leaf of _Poa annua_ (× about 50) showing keel below, and two flanking lines of motor-cells (slightly shaded) above the median vascular bundle of the mid-rib. Hence the leaf folds. The half lamina has six smaller vascular bundles, only the stronger one girdered. Ridges practically obsolete and subtending bands of sclerenchyma slight: hence the leaf-surfaces are parallel.] It is easy to observe leaves of such grasses as _Festuca pratensis_ (Fig. 22), _Aira cæspitosa_ (Fig. 23), &c., which 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. [Illustration: