Grasses: A Handbook for use in the Field and Laboratory

Fig.14. Digraphis arundinacea. Transverse section of mid-rib and half the leaf (×about6).

Fig.15. Holcus lanatus. Transverse section of leaf-blade (×10).

Fig.16. Cynosurus cristatus. Transverse section of the leaf-blade (×20). Stebler.

The ridges are almost always evident-Catabrosa, Poa, and Avena furnishing the chief exceptions-and are nearly invariably on the upper surface: they are below in Melica, however; and their relative numbers, heights and breadths, section-acute, rounded, or flattened-furnish valuable characters; as also does the coexistence or absence of hairs, asperities, &c.

Fig.17. Transverse section of the leaf of Festuca elatior, var. pratensis (× 12).

Fig.18. Ditto of the leaf of F. ovina (×15).

Fig.19. Ditto of the leaf of F. ovina, var. rubra (×35).

Fig.20. Festuca ovina, var. rubra. Transverse section of the blade of an upper leaf (×35). Stebler.

A very interesting anatomical adaptation is met with in the leaves of many grasses which grow in dry situations (xerophytes) such as on sandy sea-shores, exposed mountains and so forth. When the air is moist, in wet weather or in the dews, and the sun's rays not too powerful, the leaf is spread out with its upper surface flat or nearly so, but when the scorching sun and dry air or winds prevail, the leaves fold or roll up, with the upper sides apposed or overlapping inside the hollow cylinder thus made.

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.
Koleria.
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.

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.

Most grasses are glabrous, but there are a number in which hairs are nearly always a prominent feature. It must be remarked, however, that with grasses, as with other plants, the character of pubescence is apt to vary with the situation. In general it may be stated that a hairy grass tends to become more glabrous in a moist situation, and more pubescent in a dry one, but the rule is by no means absolute. In some cases,-e.g. Avena pubescens, A. flavescens, Agropyrum, the hairs are almost entirely confined to the crests of the ridges (Figs. 10, 15). The following is a list of hairy grasses.

Hairy Grasses.

Holcus (Fig.15).
Molinia cærulea.
Brachypodium sylvaticum.
Agropyrum (variable).
Bromus asper.
B. mollis. Hordeum.
Anthoxanthum.
Avena flavescens (Fig.10).
A. pubescens.
Triodia.
Koleria. To a less extent. Festuca sciuroides (on ribs). Melica.

Grasses as a rule are devoid of strong scents5 or tastes, but Anthoxanthum has a faint but distinct sweet odour, especially as it dries-it is one of the grasses which give the scent to new-mown hay-and a bitter flavour, and Milium, Hierochloe and Holcus are also more or less bitter. Spartina stricta emits a strong unpleasant odour.

The habitat of grasses is of great importance as an aid to determination. No one would expect to find a sea-shore grass growing in a beech-forest, or an aquatic grass on a dry chalk-down; but they are even more true to their habitats than this, and I append the following lists of habitats of British grasses as of use in determining them, though it is not pretended that the limits are absolute.

In the following list "pasture-grass" (P) means useful for grazing, and "meadow-grass" (M) one that is especially valuable for mowing-i.e. for hay. A "weed" (W) is used in its agricultural sense for a grass not useful and not wanted on cultivated land, though often found there.

Meadow- and Pasture-grasses.

(P and M) Dactylis glomerata (fields, &c.).
(P and M) Poa trivialis (meadow and pasture).
(W) Bromus arvensis (cultivated and waste places, meadow and pasture).
(W) B. sterilis (ruderal).
(P and M) Poa pratensis (meadow and pasture).
(W) Briza media (meadow and pasture).
(P) Avena pratensis (meadow and pasture, especially hilly).
(P) A. pubescens (var.)-dry.
(P and M) Lolium perenne (meadow, pasture and waste places).
(P and M) L. italicum (valuable culture grass).
(P) Cynosurus cristatus (downs).
(M and P) Festuca elatior (meadow and moist pasture, banks and river-sides).
(W) Agrostis alba and A. canina (pasture and waste places, wet or dry).
(P and M) Alopecurus pratensis (meadow and pasture).
(W) A. geniculatus (moist meadows and marshes).
(P and M) Phleum pratense (meadow and pasture).
(P) Arrhenatherum avenaceum (meadow, hedges and copse).
(P and M) Anthoxanthum odoratum (fields generally).
(W) Hordeum pratense (moist meadow and pasture).
(W) Holcus lanatus and H. mollis (meadow, pasture and waste).
(P and M) Avena flavescens (dry meadow and pasture).
(W) Avena fatua (corn-weed).
(P) Festuca ovina (light limestone pastures and chalk downs).

Shade-grasses.

Found in woods, copses, &c., under shade.

Melica uniflora (woods, &c.).
Bromus asper (hedges, thickets, and edges of woods).
B. giganteus (hedges and woods).
Aira cæspitosa (moist shade and damp hedges).
Poa nemoralis (woods, shady places and damp mountain rocks).
Milium effusum (moist woods, &c.).
Agropyrum caninum (woods and shady places).
Hordeum sylvaticum (woods and copse).
Brachypodium sylvaticum (woods, hedges and thickets).
Arrhenatherum avenaceum (meadows, hedges and copse).
Festuca sylvatica (mountain woods).

Aquatic and Semi-aquatic Grasses.

Found in wet ditches, ponds, and on marshes, river-banks, &c.

Glyceria fluitans (wet ditches and slow waters).
G. aquatica (wet ditches and shallow waters).
Alopecurus geniculatus (moist meadow and marsh lands).
Digraphis arundinacea (river-banks, marshes).
Arundo Phragmites (wet ditches, marshes and shallow waters).
Molinia cærulea (wet heaths and moors, woods and waste places).
Triodia decumbens, Agrostis alba, Catabrosa and Calamagrostis.

Moor-and Heath-grasses.

Downs and dry hill-pastures.

Nardus stricta (moors, heaths and hilly pastures).
Aira flexuosa (heaths and hill pastures).
Molinia cærulea (wet heathy moors, woods and waste places).
Koleria cristata (dry pasture).
Triodia decumbens (dry heathy and hilly pastures).
Festuca ovina (hilly pastures-especially dry and open-rarer in moist situations).
Agrostis vulgaris and A. canina.

Maritime or Seaside Grasses.

Poa maritima (maritime).
P. distans (sandy pastures and wastes near sea).
Elymus arenarius (coasts).
Psamma arenaria (coasts).
Poa bulbosa (waste places in S.E. of England).
Agropyrum...