A point of considerable classificatory value is the shape of the transverse section of the shoot, which is correlated with the mode of folding up of the young leaf-blades.
In most grasses the blades are convolute—i.e. rolled up like the paper of a cigarette, one edge over the other—and the section of the shoot is round (Fig. [7]). In some cases, however, the leaves are conduplicate—i.e. each half of the lamina is folded flat on the other, the upper sides being turned face to face inwards, with the mid-rib as the hinge—and in this case the shoots are more or less compressed (Fig. [6]).
In these latter cases the transverse section may be elliptical—e.g. Poa pratensis and P. alpina, Briza, &c., or more flattened and linear-oblong—e.g. Glyceria fluitans—with the flattened sides straight, or the section is oval but pointed more or less at each end owing to projecting keels and leaf-edges, and the form is naviculate—e.g. Glyceria aquatica, Dactylis (Fig. [6])—or, the sides being less flattened, more or less rhomboidal as in Poa trivialis. In Melica the leaves are convolute and the shoot-section quadrangular.
Flat, and usually sharp-edged shoots.
Dactylis glomerata (Fig. [6]).
Poa trivialis, P. annua, P. pratensis, P. compressa, P. maritima,
and P. alpina.
Glyceria aquatica and G. fluitans.
Avena pubescens.
Lolium perenne.
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Dactylis glomerata (Fig. [6]). Poa trivialis, P. annua, P. pratensis, P. compressa, P. maritima, and P. alpina. Glyceria aquatica and G. fluitans. Avena pubescens. Lolium perenne. |
| Fig. 6. Dactylis glomerata. Transverse section of a leaf-shoot (× 5). A, conduplicate leaf-blade. B, sheath. Stebler. | Fig. 7. Digraphis arundinacea. Transverse section of a leaf-shoot (× 5). A, sheath. B, convolute leaves. Compare Fig. [14]. Stebler. |