Fig. 15. Holcus lanatus. Transverse section of leaf-blade (× 10).
Fig. 16. Cynosurus cristatus. Transverse section of the leaf-blade (× 20). Stebler.
The ridges are almost always evident—Catabrosa, Poa, and Avena furnishing the chief exceptions—and are nearly invariably on the upper surface: they are below in Melica, however; and their relative numbers, heights and breadths, section—acute, rounded, or flattened—furnish valuable characters; as also does the coexistence or absence of hairs, asperities, &c.
Fig. 17. Transverse section of the leaf of Festuca elatior, var. pratensis (× 12).
Fig. 18. Ditto of the leaf of F. ovina (× 15).
Fig. 19. Ditto of the leaf of F. ovina, var. rubra (× 35).
Fig. 20. Festuca ovina, var. rubra. Transverse section of the blade of an upper leaf (× 35). Stebler.
A very interesting anatomical adaptation is met with in the leaves of many grasses which grow in dry situations (xerophytes) such as on sandy sea-shores, exposed mountains and so forth. When the air is moist, in wet weather or in the dews, and the sun’s rays not too powerful, the leaf is spread out with its upper surface flat or nearly so, but when the scorching sun and dry air or winds prevail, the leaves fold or roll up, with the upper sides apposed or overlapping inside the hollow cylinder thus made.