Güntz points out that xerophilous grasses are apt to have upright, narrow (Figs. 26-28), grooved or folded leaves, with strong cuticle, and marked motor-cells when the leaves open. It is in grasses of this kind, especially such as inhabit dry sandy districts, that the subulate, solid or grooved leaves shown in Figures 18, 19 occur--e.g. _Festuca ovina_ and its varieties, _Aira flexuosa_, _Nardus stricta_, &c. The epidermal cell-walls are sinuous, the stomata protected--e.g. on the flanks of ribs and in grooves--and waxy or hairy coverings occur. Colourless water-storing cells are apt to occur between or around the vascular bundles, and the chlorophyll-tissues tend to be dense and well protected inside the leaf: strongly developed bast-sclerenchyma is also frequent (Fig. 18). In shade-grasses, on the other hand, and in hygrophilous species, the leaves are as a rule flat, with thin epidermal cell-walls, which have plane sides, free stomata, and no wax &c. Water-storing tissue (apart from tropical species) is sparse or absent, and the chlorophyll-tissues have well aerated lacunar spaces. Bast-sclerenchyma is in these cases feebly developed. In the following chapter I have brought together some of the principal anatomical features, in such form that the characters can be employed in checking other determinations of grass leaves. The results, which are based on the elaborate investigations of Duval Jouve, Schroeter, Pée-Laby and Grob, as well as on my own observations, are not complete in all respects, and much more should be done to extend the theme, but the account given will serve to show the student how such results may be employed. It is as yet impossible to decide how far these characters are constant--they are known to be fairly so in many cases--but several grasses cannot yet be distinguished by them alone. [Illustration: Fig. 28. Transverse section of subulate leaf of _Aira flexuosa_ (× about 50), the upper surface represented by a mere ridge with two flanking grooves each with but traces of motor-cells below. One large vascular bundle and four much smaller ones are seen. There are no girders, but slender bands of sclerenchyma at the lower surface nearly join into a continuous sub-epidermal sheath. This type is the extreme form of that in Fig. 26.] It should also be added that some grasses develope two types of leaves (heterophylly), solid or subulate below, flat or slightly inrolled above--e.g. _Festuca heterophylla_--and the following arrangement is intended to apply to the vegetative lower leaves and not to those on the upper parts of the flowering specimen. Moreover the sections should be cut from the basal third of the lamina, and not from the tip of the leaf. CHAPTER V. GRASSES CLASSIFIED ACCORDING TO THE ANATOMICAL CHARACTERS OF THE LEAF.