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.
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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.
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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.