Grasses

by H. Marshall Ward

In such leaves some of the upper epidermal cells, either next the mid-rib (Sesleria &c.) or between the other ribs (Festuca &c.) are large and very thin-walled, full of sap when distended, and so placed that as they lose water by evaporation they contract, and so draw together the two halves of the lamina (Sesleria) or each ribbed segment (Festuca), thus causing the infolding or inrolling (see Chapter [IV]). Not only from the structure and actions of these motor-cells, but also from the fact that the stomata are on the upper surfaces and thus protected, and that the lower surfaces which alone are exposed to the drought are defended by hard and impenetrable tissues, we must look upon these as adaptations to the xerophytic conditions.

Leaves prominently ridged.

Elymus. Psamma. Aira cæspitosa. Lolium. Cynosurus (Fig. [16]). Agrostis.

Alopecurus. Glyceria fluitans. Kœleria. Festuca elatior. Festuca Myurus (var. sciuroides). Melica has ridges on the lower surface.

Elymus. Psamma. Aira cæspitosa. Lolium. Cynosurus (Fig. [16]). Agrostis.

Alopecurus. Glyceria fluitans. Kœleria. Festuca elatior. Festuca Myurus (var. sciuroides). Melica has ridges on the lower surface.

Ridges are less prominent in Phleum pratense, Briza, Agropyrum, Triodia, Arrhenatherum avenaceum.

Leaves practically devoid of ridges.

Poa—all common species. Glyceria aquatica. Catabrosa aquatica. Avena pratensis.

Poa—all common species. Glyceria aquatica. Catabrosa aquatica. Avena pratensis.

In some grasses the tissue over the mid-rib is considerably raised and strengthened on the dorsal side of the blade as a “keel."

Keel more or less prominent.

Arrhenatherum (sheath keeled). Poa (all except P. maritima). Dactylis. Bromus. Bromus asper (sheath keeled, often a white line). Holcus lanatus (slight and decurrent) (Fig. [15]). Digraphis (Fig. [14]). Glyceria.

Arrhenatherum (sheath keeled). Poa (all except P. maritima). Dactylis. Bromus. Bromus asper (sheath keeled, often a white line). Holcus lanatus (slight and decurrent) (Fig. [15]). Digraphis (Fig. [14]). Glyceria.