Grasses

by H. Marshall Ward

Fig. 25. Transverse section of part of leaf of Elymus arenarius, partly inrolled (× about 30), showing ridges of unequal height, of which the higher are flat above. Vascular bundles girdered, the stronger above and below. Motor-cells in each groove cause the inrolling of the lamina by their contraction.

As regards the vascular bundles constituting the venation, they are as is well known parallel from base to apex in our common grasses, with linear leaves, and are usually of four orders as regards strength. Those of the first (e.g. mid-rib) and second orders have conspicuous vessels, but those of the third and fourth orders may be practically devoid of vessels, though xylem and phloem elements are always present. Contrary to the general assumption, there are frequent though minute transverse bundles joining the parallel veins.

The rule is that one vascular bundle runs up each mid-rib or ridge, but exceptions occur—e.g. in Arundo several bundles run up the mid-rib, and in Aira cæspitosa (Fig. [23]) and others even the strong ribs may have two or three bundles.

Each vascular bundle has its own sclerenchyma sheath, and very often the stronger veins are accentuated owing to the vascular bundle having a girder-like band of sclerenchyma running conjointly with its sheath and joining the latter above and below—or below only—to the epidermis (Figs. 24 and 25). In many cases these lower girders spread out laterally below—fan-shaped in section—and nearly join the neighbouring girders.

In other cases the strands of sclerenchymatous supporting tissue do not join the bundles, but run parallel to them, above or below, as separate strands just beneath the epidermis.

Finally, these strands may separate from the bundles, and fuse below into a continuous layer under the epidermis; this occurs especially in leaves of xerophytes where the cuticle is well developed—e.g. in varieties of Festuca ovina (Fig. [18]), Aira flexuosa (Fig. [28]).

The distribution of the strands of isolated sclerenchyma affords good characters. While there are none in Mibora, we find one large strand at the ridge of the keel and one at each margin, in addition to smaller ones subtending each vascular bundle, in Avena pubescens, Sesleria, Poa annua (Fig. [21]), P. bulbosa, P. compressa and Dactylis glomerata. In Festuca ovina, F. rubra, F. heterophylla (Figs. [18], [27]) there are groups more or less pronounced at the keel and margins, or even a continuous band below, but none above the bundles.

Fig. 26. Transverse section of leaf of Nardus stricta (× about 50). The upper surface is represented by the four grooves and five ridges, each of the former with traces of motor-cells at its base. The deep shaded portions are sclerenchyma, strong girders of which join the vascular bundle of each ridge to the lower surface. This type is obviously derived from that in Fig. [19], and may be regarded as a permanently rolled leaf.

Fig. 27. Transverse section of leaf of Festuca ovina, var. duriuscula (× about 50), the type of a permanently folded leaf. Seven ridges and six intervening grooves are seen: each of the latter with traces of motor-cells below. In each ridge is an isolated vascular bundle, and a narrow sclerenchyma band below.

Many grasses have an isolated band above and below each primary bundle only—e.g. Panicum, Cynodon—or above and below each of the other bundles as well—e.g. Spartina, Arundo, Polypogon, Agrostis alba, Aira cæspitosa (Fig. [23]), Holcus lanatus, Glyceria aquatica, G. fluitans, Digraphis, Elymus (Fig. [25]), Agropyrum (Fig. [24]), Brachypodium, Nardus (Fig. [26]). In Psamma arenaria the lower bands join into a continuous layer.