CHAPTER IV.

ANATOMY AND HISTOLOGY.

The principal anatomical features observed in the leaves of grasses--apart from finer histological details into which it is not my purpose to enter--concern the characters of the epidermis and distribution of the stomata and hairs, the arrangement of the chlorophyll-tissue, that of the mechanical tissue (sclerenchyma) and the vascular bundles to which the venation and ribbing of the leaves are due, and the presence or absence of those peculiar thin-walled cells (motor-cells) which bring about the infolding or inrolling of the lamina (see p. 25) as they lose water, and, finally, the presence or absence of conspicuous lacunæ or air-spaces so characteristic of aquatic species. Several observers have occupied themselves with these matters, and the researches of Schwendener, Duval Jouve, Pfitzer, Pée-Laby, and others have rendered it possible to group most of our grasses according to the microscopic characters of the leaves, somewhat as I have done in Chapter V.

Reference has been made to the rolling and folding of leaves, due to the thin-walled cells on the upper surfaces capable of varying in turgescence (motor-cells). These are specially adapted epidermal cells found on the upper surfaces only. In the leaves of _Poa compressa_, _P. annua_ (Fig. 21), _P. nemoralis_, _P. alpina_, _Catabrosa_, _Sesleria_, &c., a row of these motor-cells, easily distinguished by their large size, thin walls and clear contents, is found on each side of the mid-rib; as they dry the leaf folds its two halves together (conduplicate), and on the re-absorption of water they flatten the two halves out again. In _Dactylis_ these flanking rows coalesce into one over the mid-rib. In other leaves, e.g. _Avena pratensis_, _Festuca elatior_ (Figs. 17, 22), _Melica_, _Elymus_ (Fig. 25), &c., there are in addition to these two flanking rows, other sets of motor-cells between the other ribs, and their combined action causes the halves of the lamina to inroll, usually one-half inside the other--convolute.

It is easy to observe leaves of such grasses as _Festuca pratensis_ (Fig. 22), _Aira cæspitosa_ (Fig. 23), &c., which are wide open in the dewy mornings in summer, close up as the air gets dry and hot; and any such leaf may be seen to roll up after plucking and can be reopened by moistening it.

The epidermis of grasses has been closely investigated by Grob, but unfortunately his results concern very few of our native species. The principal elements are ordinary elongated cells, with plane or sinuous walls, various kinds of short cells intercalated between the ends of these, several forms of papillæ, hairs, &c. and stomata.

The epidermis over the parenchyma of _Digraphis arundinacea_ consists of rectangular cells with plane walls.

Series or bands of long cells only may alternate with other series where short cells intervene between the long ones--e.g. _Nardus_.

_Nardus_ has some of the bands devoid of stomata, but abounding in short cells, whereas others (above) have stomata throughout.

In _Nardus stricta_, _Glyceria fluitans_, _Sesleria_, &c., there are two kinds of short cells, some siliceous, others cutinized only.

_Nardus_ has closely appressed small 2-celled hairs bent at right-angles, and some epidermal and parenchyma cells--especially below the stomata--have solid masses of silica filling the lumina.

Short cells occur in _Holcus lanatus_, _Hierochloe borealis_ and _Dactylis glomerata_ interspersed between plane-walled cells. They may be silicified and vary in shape--square, saddle-shaped, elliptical, irregular, &c.; or they may be replaced here and there by asperities--e.g. _Elymus_--or in rarer cases by stomata. Grob has attempted the classification of their distribution in different grasses, but the subject is too complex for treatment here.

The epidermis of many grasses is studded with short two-celled hairs bent sharply at right-angles; so that the pointed or blunt, hollow or solid, apical portion is appressed to the surface. Grob says that these are absent from the Hordeæ, whereas 90% of the Panicoideæ and many species of all other groups have them. Examples of the sharply pointed form occur in _Nardus_, of blunt ones in _Cynodon_ &c.

In _Nardus_ they occur on the leaf surface both between and above the veins, but in _Hierochloe_ &c. they are confined to the margins.

The following grasses have no hairs of either type:

_Agrostis vulgaris_, _Calamagrostis lanceolata_, _Avena pratensis_, _Arrhenatherum avenaceum_, _Dactylis glomerata_, _Briza media_, _Arundo Phragmites_, _Glyceria fluitans_.

The sharp, hard prickle-hairs which give the pronounced roughness to many leaves of grasses are longer than the foregoing, and stand off more from the leaf. They occur both on the surface and at the margins, and may be isolated--e.g. _Avena pratensis_,--or mixed with the short cells--_Aira canescens_, _Elymus arenarius_. They are very abundant on _Kœleria cristata_.

_Leersia oryzoides_ has asperities at the margin of the leaf with their points directed upwards on the upper part of the leaf, downwards on the basal parts, and the direction of such minute marginal asperities often affords a useful distinctive character--e.g. _Phleum_, _Arrhenatherum_. The marginal asperities in _Nardus_ are siliceous.

Bristles--i.e. long, sharp, stiff hairs--are not very common. They occur on _Nardus_, _Anthoxanthum Puelii_, _Panicum_, _Cynodon_.

Papillæ occur on the leaves of _Glyceria_, _Nardus_, _Leersia_, &c.

_Poa pratensis_ has soft hairs on the upper epidermis.

The stomata of _Sesleria cærulea_ are depressed and six-celled, two guard-cells being overgrown by four accessory cells, but in most grasses they are of the ordinary type with two elongated guard-cells only.

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.

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.

In the following there is a band like a girder above and below each bundle, and contiguous with it, joining it to the epidermis above and below--_Leersia_, _Phleum pratense_, _Calamagrostis Epigeios_, _Bromus erectus_, &c.

Güntz points out that xerophilous grasses are apt to have upright, narrow (Figs. 26-28), grooved or folded leaves, with strong cuticle, and marked motor-cells when the leaves open. It is in grasses of this kind, especially such as inhabit dry sandy districts, that the subulate, solid or grooved leaves shown in Figures 18, 19 occur--e.g. _Festuca ovina_ and its varieties, _Aira flexuosa_, _Nardus stricta_, &c. The epidermal cell-walls are sinuous, the stomata protected--e.g. on the flanks of ribs and in grooves--and waxy or hairy coverings occur. Colourless water-storing cells are apt to occur between or around the vascular bundles, and the chlorophyll-tissues tend to be dense and well protected inside the leaf: strongly developed bast-sclerenchyma is also frequent (Fig. 18).

In shade-grasses, on the other hand, and in hygrophilous species, the leaves are as a rule flat, with thin epidermal cell-walls, which have plane sides, free stomata, and no wax &c. Water-storing tissue (apart from tropical species) is sparse or absent, and the chlorophyll-tissues have well aerated lacunar spaces. Bast-sclerenchyma is in these cases feebly developed.

In the following chapter I have brought together some of the principal anatomical features, in such form that the characters can be employed in checking other determinations of grass leaves. The results, which are based on the elaborate investigations of Duval Jouve, Schroeter, Pée-Laby and Grob, as well as on my own observations, are not complete in all respects, and much more should be done to extend the theme, but the account given will serve to show the student how such results may be employed. It is as yet impossible to decide how far these characters are constant--they are known to be fairly so in many cases--but several grasses cannot yet be distinguished by them alone.

It should also be added that some grasses develope two types of leaves (heterophylly), solid or subulate below, flat or slightly inrolled above--e.g. _Festuca heterophylla_--and the following arrangement is intended to apply to the vegetative lower leaves and not to those on the upper parts of the flowering specimen. Moreover the sections should be cut from the basal third of the lamina, and not from the tip of the leaf.