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.
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.
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.