CHAPTER V.

GRASSES CLASSIFIED ACCORDING TO THE ANATOMICAL CHARACTERS OF THE LEAF.

I. THE CHLOROPHYLL-TISSUE, ON TRANSVERSE SECTIONS, IS ARRANGED IN RINGS ROUND THE VASCULAR BUNDLES. THERE ARE MOTOR-CELLS BETWEEN THE RIBS, AND THE STOMATA ARE SUNK AND OCCUR ON BOTH FACES.

_Cynodon Dactylon._ The larger lateral nerves have as a rule three smaller ones between each pair, hardly projecting as ribs. Chlorophyll chiefly in a ring round the vascular bundle. Long hairs on lower surface, a few papillæ above. Motor-cells in each shallow furrow. Short cells occur between the long epidermal cells over the bast-region.

The Panicums also come here, and differ according to the disposition of the sclerenchyma sheaths around the bundles.

II. THE CHLOROPHYLL-TISSUE IS BETWEEN THE VASCULAR BUNDLES, AND NOT CONFINED TO RINGS SURROUNDING THEM.

+A. Conspicuous lacunæ between the vascular bundles. Stomata on both faces. Motor-cells occur.+

Lacunæ large and rectangular. Motor-cells confined to a flanking line on each side of the mid-rib.

_Glyceria aquatica._ Leaves folded and in section +V+-shaped, hardly keeled, with sclerenchyma at apex. Motor-cells each side of the mid-rib only. The large square or rectangular lacunæ bounded by stellate cells. Papillæ on epidermal cells. Vascular bundles midway between upper and lower surfaces.

_Glyceria fluitans._ Section +V+-shaped and keeled, the roof of each polygonal lacuna arched, hence the “ribs” on the upper surface are between the vascular bundles. The latter lie nearer the lower epidermis. The epidermis has papillæ.

_Catabrosa aquatica_ and _Hierochloe_ also come here, the former with small lacunæ, the latter with larger ones chiefly towards the upper surface of the leaf.

_Digraphis_ is also apt to have a few air cavities near the mid-rib.

+B. Lacunæ none, or inconspicuous, the chlorophyll-tissue filling up between the ribs.+

(_a_) Upper and lower leaf-surfaces parallel, or nearly so, and much alike, the ridges being very low or obsolete. Stomata equal or nearly so on both surfaces.

(1) Motor-cells absent; vascular bundles feeble and very few.

_Mibora verna._ The small leaves are flat, or nearly so, and have three isolated and very feebly developed bundles, devoid of girders or sclerenchyma bands.

(2) Motor-cells present, vascular bundles of various orders, with sclerenchyma bands or girders.

* _Leaf keeled, and folded--not inrolled. Motor-cells confined to the neighbourhood of the mid-rib. No hairs._

† _Motor-cells conspicuous and conjoined into a band above the mid-rib._

_Dactylis glomerata._ Keel pronounced, with one large vascular bundle and a sclerenchyma band occupying its crest. Motor-cells forming one conjoint band along the upper course of the mid-rib only. Stomata on both faces, but no hairs or thick cuticle. Ribs low, and all bundles have feeble girders. A little sclerenchyma at the margins. A few pale cells in the chlorophyll-tissue.

†† _Motor-cells inconspicuous and in two flanking lines, one on each side of the mid-rib._

_Poa trivialis._ Keel with sclerenchyma at its apex, and a small band of the same at the margins. Vascular bundles of three orders, isolated, without girders, but with a small band of sclerenchyma above and below. Ridges obsolete. Short hook-asperities above. No thickened cuticle.

Other species of Poa also come here: I cannot distinguish them by the leaf anatomy; but _P. annua_, _P. compressa_, _P. nemoralis_ and _P. pratensis_ are devoid of the hooked asperities; _P. nemoralis_ has a thicker lamina than the rest, and girders to the secondary bundles. _P. annua_ agrees in the latter point.

** _Leaf not keeled: rolling up. Motor-cells distributed between the ridges._

† _Hairs none or rare, or at most a few asperities._

≡ _Veins numerous, 30-40 on each half lamina. Motor-cells very large._

⊙ _All vascular bundles with girders above and below._

_Digraphis arundinacea._ No keel. Marginal sclerenchyma conspicuous. A few asperities below. Leaf thin, and all the bundles joined to the epidermis above and below by girders (Fig. 14). Stomata on both surfaces, fairly large: epidermal cells with plane walls. There may be a few irregular air cavities, especially near the mid-rib.

⊙⊙ _Only the principal bundles girdered._

_Arundo Phragmites._ Ridges very numerous and low. No keel. Marginal sclerenchyma strong. Vascular bundles with sheaths of large colourless cells, a few of the strongest girdered below, but most have only sclerenchyma bands above and below. Motor-cells particularly large, between all the bundles. There are no conspicuous lacunæ. Hairs very rare. Epidermal cells small, with sinuous walls: all the cell-walls contain silica. Stomata on both faces, sunk, small and more difficult to see than in _Digraphis_, where the epidermal cells are plane walled, or nearly so.

_Arundo Donax_ is very like _A. Phragmites_, but has larger bundles each with a horse-shoe shaped sclerenchymatous mass below, and larger lacunæ.

≡≡ _Veins not more than 10-20 in each half lamina._

⊙ _More or less conspicuously hairy. The smaller bundles isolated and devoid of girders._

_Bromus sterilis._ Girders to the stronger bundles only. Stiff hairs above and below. Motor-cells poorly developed between each pair of low ridges. No pronounced cuticle. A faint sclerenchyma-band at margin, and at apex of low rounded keel. Stomata on both faces.

_Bromus arvensis._ Similar to _B. sterilis_, with stiff hairs commoner below. Harsh in cutting.

_B. giganteus_ shows no hairs, but I cannot distinguish the Bromes generally by the leaf anatomy.

_Anthoxanthum odoratum._ No keel, ridges obsolete, the stronger bundles only with girders. Motor-cells conspicuous between all the ribs. Marginal sclerenchyma, and that above and below the bundles, poorly developed. A few coarse hairs both above and below, and stomata on both faces. Leaf thin and narrow.

_Hordeum murinum._ Few girdered bundles, and sclerenchyma at margins poor. Hairs sparse and coarse.

_Bromus asper_, _Brachypodium sylvaticum_ and _Lagurus_ also come here.

In all these grasses the epidermal cells are chiefly long, rectangular or slightly hexagonal, with thin and plane walls.

⊙⊙ _Hairs none or very rare on the sections._

_Phleum pratense._ Low rounded ribs with motor-cells between. The larger vascular bundles girdered. Stomata about equal on both faces. No hairs. No keel. Marginal sclerenchyma scanty.

_Arrhenatherum avenaceum._ Very rare hairs above: a few blunt asperities here and there. No keel. Ridges low. Girders to the primary bundles, but not very strong: marginal and other sclerenchyma faint, as is also the cuticle. Stomata on both faces. Motor-cells fairly developed between the ridges.

_Briza media._ No keel, and mere traces of marginal sclerenchyma. Ribs practically obsolete, but well developed motor-cells in furrows. Principal bundles girdered. Stomata on both sides. No hairs or thickened cuticle.

_Avena fatua_, _Molinia_ and _Leersia_ also come here.

(_b_) Upper and lower leaf-surfaces dissimilar, or at least not parallel, owing to the conspicuous ridges and grooves above.

(1) No stomata below.

* _Leaves flat or nearly so, or at least exhibit a conspicuous concave upper surface._

⊙ _Motor-cells between each pair of ribs: sclerenchyma not forming a continuous layer below._

≡ _Ridges at least 5-6 times as high as the leaf-thickness between._

_Aira cæspitosa._ Ridges high, 7-10 times as high as the breadth of leaf between, triangular, each with 1-3 vascular bundles devoid of girders, with an upper isolated band of sclerenchyma at the acute tip, and another below the principal bundle. Also small bands below each group of motor-cells. Small conical asperities on the ridges and below. No mid-rib. Stomata on flanks of ridges only, and few motor-cells between (Fig. 23).

Each vascular bundle has a sheath, but is isolated. Sclerenchyma at tips of the ridges dense: smaller bands below: strong at margins. Lower cuticle strong. Leaf rolls up.

The flat upper leaves of _Festuca rubra_ (Fig. 20) and _F. heterophylla_ are somewhat similar in type. They have stiff hairs on the ridges.

≡≡ _Ridges not more than 2-3 times as high as the tissue between; each furrow with motor-cells, and each vascular bundle joined to epidermis above and below by a sclerenchyma girder._

_Brachypodium pinnatum._ Smooth. Ridges rounded. Hairs rare. The strong sclerenchyma girders below almost continuous laterally. Epidermal cells with sinuous thick walls, and a few tooth-hairs.

Note the differences from _B. sylvaticum_, p. 76.

_Melica nutans_, _M. uniflora_, and _Calamagrostis Epigeios_ also come here.

⊙⊙ _Motor-cells confined to the innermost 2-4 furrows. Sclerenchyma in a continuous band just inside the thick cuticle below._

_Festuca duriuscula._ The ridges are only about half to one-third as high again as the thickness between, and the motor-cells in four series at the base of the three innermost ridges. Each ridge has only one isolated sheathed bundle, without girders. Stomata on the flanks of the ridges, and few in number. The sclerenchyma forms a thick band just inside the strong cuticle below. The leaf is conduplicate, not convolute.

This applies particularly to the more open leaves: the subulate leaves belong to the next type (see Fig. 27).

_Aira canescens_ and _Spartina stricta_ also come here.

_Psamma arenaria._ Inrolled. Smooth below and devoid of keel, with sub-epidermal band of sclerenchyma, and similar tissue at the margin. Ridges of three sizes, the largest twice or three times as high as the leaf-tissue between is thick, all rounded above, and very hairy. Stomata above only. Motor-cells in each sinus not large. Vascular bundles isolated, without girders or bands of sclerenchyma.

_Elymus_ is very like _Psamma_, but has a few stomata below and the sub-epidermal sclerenchyma is not continuous (see Fig. 25).

** _Leaves (subulate) not opening out, the upper surface represented by a groove or a few ridges above the angular or ovate solid section._

⊙ _Section pentagonal or angular-ovate: sclerenchyma below in a continuous band._

_Aira flexuosa._ Upper surface a depression, with one ridge flanked by two grooves at its base, the depression extending about one-fifth through the whole thickness of the nearly solid leaf. Vascular bundles about 3-5, isolated, sheathed. Sclerenchyma band extending all round the lower surface just inside the thick cuticle. Stomata very few, flanking the ridge; motor-cells in the furrows, poorly developed (Fig. 28).

⊙⊙ _Section elliptical or angular-ovate; sclerenchyma not always in a continuous band below._

_Festuca ovina._ Upper surface a deep fold, with three ridges and 2-4 grooves at its base. Vascular bundles several, with girders. Motor-cells in four series, in the grooves. The lower girders may not fuse laterally into a continuous band of sclerenchyma below (Fig. 18).

The folded lower leaves of _F. rubra_ and _F. heterophylla_ come here also. For the flatter leaves of _F. duriuscula_ see p. 78 and compare Fig. 27.

The epidermal cells in this series have sinuous thickened walls, and here and there small tooth-like hairs.

_Nardus_ also comes here (see Fig. 26).

(2) There are stomata below, but fewer than on the upper surface. Motor-cells usually conspicuous between the ridges.

* _Stronger bundles with girders of sclerenchyma joining them to the epidermis, at least below._

⊙ _Hairs sparse or none._

_Cynosurus cristatus._ Mid-rib obsolete, except the strong vascular bundle. Ridges low and rounded, with 2-4 flanking stomata, and well developed motor-cells in furrows. Secondary vascular bundles with strong girders below, the smaller bundles sheathed only and isolated. Each ridge with slight sclerenchyma above. A few stiff short hairs above, and the leaves are convolute. Ridges about twice the height of the leaf-thickness between (Fig. 16).

_Agropyrum repens._ Mid-rib and margin with strong sclerenchyma-groups: ridges unequal, low and rounded and each vascular bundle girdered. A few pointed hairs above, and motor-cells in all the grooves. A slight keel, stomata on both surfaces.

_Agropyrum caninum._ All the bundles have girders. Slight keel. Marginal sclerenchyma. Few, very short, hard, hooked asperities above and below. Ridges low, and motor-cells poorly developed between. Few stomata on lower surface. Very like _A. repens_, but the principal ridges are more prominent below and those nearer the mid-rib have asperities.

_A. junceum_ resembles _Psamma_, but the ridges are much lower, and there are a few stomata on the under surface (Fig. 24).

⊙⊙ _Leaf obviously hairy._

† _Hairs more especially above._

_Avena flavescens_ is very similar to _Cynosurus_, but is evidently hairy, and _A. pratensis_ also comes here.

†† _Hairs abundant on both surfaces._

_Holcus lanatus._ Very hairy above and below, and at the margins. Slight keel with sclerenchyma band: sclerenchyma at margin slight. Ridges rounded, about twice as high as thickness between. Stomata more abundant above. Cuticle very thin and leaf soft. All bundles except the mid-rib with girders. Motor-cells fairly well developed between the ridges (Fig. 15).

_Kœleria cristata._ Very hairy on both surfaces. Ridges irregular, the largest flat and high, the others rounded or triangular. Vascular bundles isolated, and the sclerenchyma reduced to a few cells in a single layer beneath the epidermis at the apex of each ridge and below the bundle. Motor-cells well developed in each furrow. Stomata more numerous above.

** _No girders to the vascular bundles._

_Lolium perenne._ Ridges numerous and unequal. Vascular bundles sheathed and isolated--i.e. devoid of girders: small patches of sclerenchyma at the apex of each stronger ridge, and on the opposite side below only. No hairs.

_Lolium temulentum_ is similar but is more apt to be convolute, whereas _L. perenne_ is more folded.

_Alopecurus pratensis._ Leaf thin and somewhat like _Phleum_, but the ridges somewhat higher and more rounded, and only the principal bundles girdered below. Stomata on both faces.

_Festuca elatior_, _Bromus giganteus_ and most species of _Agrostis_ come near _Lolium_. See Figs. 17, 22.