Читать книгу Grasses: A Handbook for use in the Field and Laboratory - H. Marshall Ward - Страница 5

Table of Contents

The leaves of all our grasses consist of the blade, which passes directly into the sheath, without any petiole or leaf-stalk (Fig. 1).

The sheath is usually obviously split, and so rolled round the internode that one edge overlaps the other, but in the following grasses the sheath is either quite entire, or only slit a short way down, the two edges being fused as it were for the greater part of its length.

Sheath more or less entire.

Glyceria aquatica and G. fluitans. Melica uniflora and M. nutans. Dactylis glomerata. Poa trivialis (Fig. 8), P. pratensis, P. alpina. Sesleria cærulea. Bromus (all the species). Briza media and B. minor.

In some cases—e.g. Arrhenatherum, Bromus asper, and Holcus lanatus—the sheath is marked with a more or less prominent ridge down its back, due to the continuation of the keel of the leaf. The sheath may also be glabrous or hairy, and grooved or not.

A few grasses are so apt to develope characteristic colours in their sheaths, especially below, that they may often be recognised in winter by this peculiarity.

Sheaths coloured.

Lolium—all red. Holcus—red with purple veins. Festuca elatior—red. Cynosurus—yellow. Alopecurus pratensis, and A. agrestis—violet-brown, &c. Festuca ovina, var. rubra—red.

Fig. 8. Poa trivialis. A, base of blade. B, ligule. C, sheath. D, culm (× about 3).	Fig. 9. Alopecurus pratensis. A, base of blade. B, ligule. C, sheath. Slightly magnified.	Fig. 10. Avena flavescens. Lettering as before (× 2). Note the split sheath, the hairs and ridges. Stebler.

At the junction of the blade with the sheath there is in most cases a delicate membranous upgrowth of the former, more or less appressed to the stem, and called the Ligule (Figs. 8−13). Its use is probably to facilitate the shedding of water which has run down the leaf; and so lessen the danger of rotting between the sheath and stem: possibly the shelves and ears commonly met with at the base of the lamina (Fig. 12) aid in the same process. This ligule may be long or short, acute or obtuse, toothed or entire, or it may be reduced to a mere line, or tuft of hairs, or even be obsolete, and is of considerable value in classification—e.g. the ligule is obsolete or wanting in Melica, Festuca ovina, F. Myurus, F. elatior, Kœleria and Panicum.

It is represented by a tuft of hairs in Molinia, Triodia and Arundo.

Fig. 11. Lolium perenne. A, base of lamina, B, ligule. C, sheath (× 3). Note the low ribs, and absence of hairs (glabrous).	Fig. 12. Festuca elatior, var. pratensis. A, base of lamina. B, the extremely short ligule, with pointed ears. C, sheath (× 3).	Fig. 13. Festuca ovina. A, base of lamina. B, ligular ears. C, sheath (× about 4). Stebler.

Our other ordinary grasses have a more or less well-developed membranous ligule (Fig. 8).

The leaf-blade is long or short, broad or narrow, but always of some elongated form such as linear, linear-lanceolate or linear-acuminate, or subulate, setaceous, &c., varying as to the degree of acuteness of the apex, and the tapering of the base.

In the following native grasses the form of the lamina affords a useful character.

The base tapers to the sheath below—i.e. the leaf is more or less linear-lanceolate—in Molinia, Brachypodium, Melica, Milium, Kœleria, and the very rare Hierochloe; less distinctly so in Bromus asper and species of Hordeum. The base is rounded in Arundo. In the following cases the leaves are setaceous, due to the very narrow blade remaining permanently folded or inrolled at its edges, and usually being thickened and hardened also (Figs. 13 and 18). The habitat of these moor-and heath-grasses suggests that these are no doubt adaptations to prevent excessive evaporation by the exposure of too large a surface—e.g. various species of Aira, Festuca ovina, F. Myurus and allies, Nardus, and several other species; whereas, conversely, the thin flat leaves of shade-grasses facilitate exposure to light and transpiration. In Avena pratensis and Agrostis canina some of the leaves are involute and subulate, and the thickened leaves of Poa maritima also are turned up at the edges, and are U-shaped in cross-section.

As we shall see later the degree of inrolling of many grass leaves varies with circumstances.

In most others the blades are either flat (Figs. 8−12), or more or less conduplicate on the mid-rib. The latter case occurs, for example, in grasses with flattened shoots, especially at the lower part of the blade—e.g. Lolium perenne, Dactylis, Glyceria, and some species of Poa, and the cross-section of the leaf below, just before it enters the sheath, is V-shaped. In Glyceria the leaf-bases may show yellow or brownish triangles.

Further characters of the leaves are derived from their texture, apex, margins, mid-ribs and venation, hairiness, and especially the presence and characters of the longitudinal ridges which run along the upper or lower surface in many cases.

The venation is parallel from base to apex in nearly all our grasses, but such is not always the case—e.g. in the exotic Panicum plicatum the mid-rib, which enters the leaf with several vascular bundles, gives off strong and weak veins below, which first diverge and then run in arches which converge upwards: this leaf is also remarkable in being plaited (plicate) in vernation. In Arundo Donax also the veins, though approximately parallel, do not all run to the apex of the tapering leaf; the outer ones end above in the margins and are shorter than the mid-rib.

As regards texture, the leaves of most grasses are thin and herbaceous; but in some they are dry and harsh to the touch. They are thin and dry in Agropyrum caninum, Hordeum pratense, H. murinum, Avena pratensis, &c., very hard and leathery (coriaceous) in Psamma, Nardus, species of Festuca, Aira, Agropyrum junceum, Elymus, &c. In aquatic grasses like Glyceria, the leaf is almost spongy owing to the large air-chambers developed in the tissues. These are easily visible with a lens.

The apex is in most cases slender and tapering—acuminate; but in some it is merely brought to a point (acute) as in Catabrosa, Glyceria and several species of Poa and Avena, &c., usually flat, but somewhat hooded or curved up in some Poas. In cases where the leaves are setaceous or subulate, the apex is like a thin tapering bristle, and even flatter leaves may be so inrolled at the tips as to have the apex prolonged into a sharp needle-like pungent or spinescent point—e.g. Hordeum pratense, Avena pratensis to a slight extent, and pronounced in Elymus, &c. In Sesleria the apex is rounded with a short, sharp, prickle-like median projection (mucronate).

The passage of blade into sheath has already been described, but the base of the blade may have its margins projecting as horizontal shelves, like a Byron collar, round the sides of the throat of the sheath, sometimes tinged with yellow or pink—e.g. Lolium, Holcus, Bromus inermis, Hordeum; the ends of these may project as auricles or ears—e.g. Festuca elatior, Elymus, Agropyrum, Anthoxanthum, Bromus asper, Hordeum, &c. In Festuca ovina the ears are short, stiff, and erect (Fig. 13).

The margin may be perfectly even, as in most grasses, or it is more or less scabrid or scaberulous, as in Aira cæspitosa, Poa maritima, Festuca elatior, Avena pratensis, Agrostis, Milium, Phleum, Briza, the minute teeth (serrulæ) pointing up or down.

The surface may be bright green, or glaucous, harsh, hairy or glabrous, and is not uncommonly also scabrid, like a file or emery-paper, and sometimes only when rubbed in one direction up or down, owing to the minute teeth being directed all one way. These teeth are developed on the ridges.

All our ordinary grass leaves are parallel-veined, and the vascular strands (the veins) can usually be seen on holding the leaf up to the light. In most cases the tissue is raised over the veins, as ridges or “ribs," and according to the height of these ridges the thinner parts between look like deep or shallow furrows (cf. Figs. 8−16 and Chapter IV.). If the leaf is held up to the light the ridges appear dark in proportion to their opacity—i.e. height or thickness—and the furrows light in proportion to the thinness of the tissues there. If the contrast is very great, as in Aira cæspitosa (Fig. 23), the furrows seem like transparent sharp lines, and when, as in Poa, which is practically devoid of ridges, the difference of thickness is small they appear merely as fine striæ. These characters must be determined on the fresh leaves, however, because the contraction in drying draws the ridges closer together and tends to obliterate the lines.