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.
[Illustration: Fig. 21. Transverse section of left-half of leaf of
_Poa annua_ (× about 50) showing keel below, and two flanking lines of
motor-cells (slightly shaded) above the median vascular bundle of the
mid-rib. Hence the leaf folds. The half lamina has six smaller vascular
bundles, only the stronger one girdered. Ridges practically obsolete
and subtending bands of sclerenchyma slight: hence the leaf-surfaces
are parallel.]
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.
[Illustration:
Fig. 22. Transverse section of left-half of leaf of _Festuca
elatior_, var. _pratensis_ (× about 50). The ridges are well marked
and flattened above. The vascular bundles of two orders are girdered
below, but only slightly above. There is no keel. There are well
marked motor-cells--not shown in the figure--in each groove.]
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.
[Illustration: Fig. 23. Part of transverse section of leaf of _Aira
cæspitosa_ (× about 30). Ridges very high and acute, each tipped with
sclerenchyma, and containing an isolated vascular bundle--sometimes
one or more small ones also. Motor-cells well developed at the base
of each groove. The bundles are not girdered, but numerous bands of
sclerenchyma almost join into a continuous band below. The leaf rolls
inwards.]
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.
[Illustration: Fig. 24. Transverse section of part of leaf of
_Agropyrum junceum_ (× about 40) partly inrolled; showing unequal
ridges. The principal vascular bundles are girdered below, the
sclerenchyma joining into a strong continuous sheath. Each ridge is
tipped with sclerenchyma, and each groove has motor-cells--not shown in
the figure--below.]
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.
[Illustration: 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.
[Illustration: 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.]
[Illustration: 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.
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.
[Illustration: Fig. 28. Transverse section of subulate leaf of _Aira
flexuosa_ (× about 50), the upper surface represented by a mere ridge
with two flanking grooves each with but traces of motor-cells below.
One large vascular bundle and four much smaller ones are seen. There
are no girders, but slender bands of sclerenchyma at the lower surface
nearly join into a continuous sub-epidermal sheath. This type is the
extreme form of that in Fig. 26.]
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.