THE VEGETATIVE ORGANS (_continued_).
The leaves of all our grasses consist of the _blade_, which passes
directly into the _sheath_, without any petiole or leaf-stalk (Fig. 1).
The sheath is usually obviously _split_, and so rolled round the
internode that one edge overlaps the other, but in the following
grasses the sheath is either quite _entire_, or only slit a short way
down, the two edges being fused as it were for the greater part of its
length.
Sheath more or less entire.
_Glyceria aquatica_ and _G. fluitans_.
_Melica uniflora_ and _M. nutans_.
_Dactylis glomerata._
_Poa trivialis_ (Fig. 8), _P. pratensis_, _P. alpina_.
_Sesleria cærulea._
_Bromus_ (all the species).
_Briza media_ and _B. minor_.
In some cases--e.g. _Arrhenatherum_, _Bromus asper_, and _Holcus
lanatus_--the sheath is marked with a more or less prominent ridge
down its back, due to the continuation of the _keel_ of the leaf. The
sheath may also be glabrous or hairy, and grooved or not.
A few grasses are so apt to develope characteristic colours in their
sheaths, especially below, that they may often be recognised in winter
by this peculiarity.
Sheaths coloured.
_Lolium_--all red.
_Holcus_--red with purple veins.
_Festuca elatior_--red.
_Cynosurus_--yellow.
_Alopecurus pratensis_, and
_A. agrestis_--violet-brown, &c.
_Festuca ovina_, var. _rubra_--red.
[Illustration: Fig. 8. _Poa trivialis. A_, base of blade. _B_,
ligule. _C_, sheath. _D_, culm (× about 3).]
[Illustration: Fig. 9. _Alopecurus pratensis. A_, base of blade. _B_,
ligule. _C_, sheath. Slightly magnified.]
[Illustration: Fig. 10. _Avena flavescens._ Lettering as before (× 2).
Note the split sheath, the hairs and ridges. Stebler.]
At the junction of the blade with the sheath there is in most cases a
delicate membranous upgrowth of the former, more or less appressed to
the stem, and called the _Ligule_ (Figs. 8-13). Its use is probably
to facilitate the shedding of water which has run down the leaf; and
so lessen the danger of rotting between the sheath and stem: possibly
the shelves and ears commonly met with at the base of the lamina (Fig.
12) aid in the same process. This ligule may be long or short, acute
or obtuse, toothed or entire, or it may be reduced to a mere line, or
tuft of hairs, or even be obsolete, and is of considerable value in
classification--e.g. the ligule is obsolete or wanting in _Melica_,
_Festuca ovina_, _F. Myurus_, _F. elatior_, _Kœleria_ and _Panicum_.
It is represented by a tuft of hairs in _Molinia_, _Triodia_ and
_Arundo_.
[Illustration: Fig. 11. _Lolium perenne. A_, base of lamina, _B_,
ligule. _C_, sheath (× 3). Note the low ribs, and absence of hairs
(glabrous).]
[Illustration: Fig. 12. _Festuca elatior_, var. _pratensis_. _A_, base
of lamina. _B_, the extremely short ligule, with pointed ears. _C_,
sheath (× 3).]
[Illustration: Fig. 13. _Festuca ovina. A_, base of lamina. _B_,
ligular ears. _C_, sheath (× about 4). Stebler.]
Our other ordinary grasses have a more or less well-developed
_membranous_ ligule (Fig. 8).
The leaf-blade is long or short, broad or narrow, but always of some
elongated form such as linear, linear-lanceolate or linear-acuminate,
or subulate, setaceous, &c., varying as to the degree of acuteness of
the apex, and the tapering of the base.
In the following native grasses the _form_ of the lamina affords a
useful character.
The base tapers to the sheath below--i.e. the leaf is more or less
linear-lanceolate--in _Molinia_, _Brachypodium_, _Melica_, _Milium_,
_Kœleria_, and the very rare _Hierochloe_; less distinctly so
in _Bromus asper_ and species of _Hordeum_. The base is rounded in
_Arundo_. In the following cases the leaves are setaceous, due to the
very narrow blade remaining permanently folded or inrolled at its
edges, and usually being thickened and hardened also (Figs. 13 and 18).
The habitat of these moor-and heath-grasses suggests that these are no
doubt adaptations to prevent excessive evaporation by the exposure of
too large a surface--e.g. various species of _Aira_, _Festuca ovina_,
_F. Myurus_ and allies, _Nardus_, and several other species; whereas,
conversely, the thin flat leaves of shade-grasses facilitate exposure
to light and transpiration. In _Avena pratensis_ and _Agrostis canina_
some of the leaves are involute and subulate, and the thickened leaves
of _Poa maritima_ also are turned up at the edges, and are +U+-shaped
in cross-section.
As we shall see later the degree of inrolling of many grass leaves
varies with circumstances.
In most others the blades are either flat (Figs. 8-12), or more or
less conduplicate on the mid-rib. The latter case occurs, for example,
in grasses with flattened shoots, especially at the lower part of the
blade--e.g. _Lolium perenne_, _Dactylis_, _Glyceria_, and some species
of _Poa_, and the cross-section of the leaf below, just before it
enters the sheath, is +V+-shaped. In _Glyceria_ the leaf-bases may show
yellow or brownish triangles.
Further characters of the leaves are derived from their texture, apex,
margins, mid-ribs and venation, hairiness, and especially the presence
and characters of the longitudinal ridges which run along the upper or
lower surface in many cases.
The venation is parallel from base to apex in nearly all our grasses,
but such is not always the case--e.g. in the exotic _Panicum plicatum_
the mid-rib, which enters the leaf with several vascular bundles,
gives off strong and weak veins below, which first diverge and then
run in arches which converge upwards: this leaf is also remarkable in
being _plaited_ (plicate) in vernation. In _Arundo Donax_ also the
veins, though approximately parallel, do not all run to the apex of the
tapering leaf; the outer ones end above in the margins and are shorter
than the mid-rib.
As regards _texture_, the leaves of most grasses are thin and
herbaceous; but in some they are dry and harsh to the touch. They are
thin and dry in _Agropyrum caninum_, _Hordeum pratense_, _H. murinum_,
_Avena pratensis_, &c., very hard and leathery (_coriaceous_) in
_Psamma_, _Nardus_, species of _Festuca_, _Aira_, _Agropyrum junceum_,
_Elymus_, &c. In aquatic grasses like _Glyceria_, the leaf is almost
spongy owing to the large air-chambers developed in the tissues. These
are easily visible with a lens.
The _apex_ is in most cases slender and tapering--_acuminate_; but
in some it is merely brought to a point (_acute_) as in _Catabrosa_,
_Glyceria_ and several species of _Poa_ and _Avena_, &c., usually
flat, but somewhat hooded or curved up in some Poas. In cases where
the leaves are _setaceous_ or _subulate_, the apex is like a thin
tapering bristle, and even flatter leaves may be so inrolled at the
tips as to have the apex prolonged into a sharp needle-like _pungent_
or _spinescent_ point--e.g. _Hordeum pratense_, _Avena pratensis_ to
a slight extent, and pronounced in _Elymus_, &c. In _Sesleria_ the
apex is rounded with a short, sharp, prickle-like median projection
(_mucronate_).
The passage of blade into sheath has already been described, but
the base of the blade may have its margins projecting as horizontal
shelves, like a Byron collar, round the sides of the throat of
the sheath, sometimes tinged with yellow or pink--e.g. _Lolium_,
_Holcus_, _Bromus inermis_, _Hordeum_; the ends of these may project
as _auricles_ or ears--e.g. _Festuca elatior_, _Elymus_, _Agropyrum_,
_Anthoxanthum_, _Bromus asper_, _Hordeum_, &c. In _Festuca ovina_ the
ears are short, stiff, and erect (Fig. 13).
The margin may be perfectly even, as in most grasses, or it is more
or less _scabrid_ or _scaberulous_, as in _Aira cæspitosa_, _Poa
maritima_, _Festuca elatior_, _Avena pratensis_, _Agrostis_, _Milium_,
_Phleum_, _Briza_, the minute teeth (_serrulæ_) pointing up or down.
The surface may be bright green, or glaucous, harsh, hairy or glabrous,
and is not uncommonly also _scabrid_, like a file or emery-paper, and
sometimes only when rubbed in one direction up or down, owing to the
minute teeth being directed all one way. These teeth are developed on
the ridges.
All our ordinary grass leaves are parallel-veined, and the vascular
strands (the veins) can usually be seen on holding the leaf up to the
light. In most cases the tissue is raised over the veins, as ridges or
“ribs,” and according to the height of these ridges the thinner parts
between look like deep or shallow furrows (cf. Figs. 8-16 and Chapter
IV.). If the leaf is held up to the light the ridges appear _dark_ in
proportion to their opacity--i.e. height or thickness--and the furrows
_light_ in proportion to the thinness of the tissues there. If the
contrast is very great, as in _Aira cæspitosa_ (Fig. 23), the furrows
seem like transparent sharp lines, and when, as in _Poa_, which is
practically devoid of ridges, the difference of thickness is small they
appear merely as fine striæ. These characters must be determined on
the fresh leaves, however, because the contraction in drying draws the
ridges closer together and tends to obliterate the lines.
[Illustration: Fig. 14. _Digraphis arundinacea._ Transverse section of
mid-rib and half the leaf (× about 6).]
[Illustration: Fig. 15. _Holcus lanatus._ Transverse section of
leaf-blade (× 10).]
[Illustration: 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.
[Illustration: Fig. 17. Transverse section of the leaf of _Festuca
elatior_, var. _pratensis_ (× 12).]
[Illustration: Fig. 18. Ditto of the leaf of _F. ovina_ (× 15).]
[Illustration: Fig. 19. Ditto of the leaf of _F. ovina_, var. _rubra_
(× 35).]
[Illustration: 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._
_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._
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._
_Kœleria._
To a less extent.
_Festuca sciuroides_ (on ribs).
_Melica._
Grasses as a rule are devoid of strong scents[5] 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).
_Kœleria 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 junceum_ (coasts).
_Hordeum maritimum_ (S. and E. coast).
_Phleum arenarium_ (coasts).
RUDERAL OR VAGABOND GRASSES.
Waste places, walls, road-sides and dry sandy situations.
_Molinia cærulea_ (wet, heathy moors, woods and waste places).
_Festuca Myurus_ (waste places, walls, road-sides).
_F. ovina_ (hilly pastures and especially dry, rarely moist situations).
_Aira caryophyllea_ (sandy and hilly pastures).
_Aira præcox_ (sandy and hilly pastures).
_Poa distans_ (sandy wastes near the sea).
_P. compressa_ (dry, barren, waste ground).
_P. annua_ (cultivated and waste lands and fields).
_Agropyrum repens_ (fields and waste places).
_Hordeum murinum_ (waste places and road-sides).
_Holcus lanatus_ (meadow, pasture, and waste lands).
_H. mollis_ (same--rarer).
_Alopecurus agrestis_ (waste lands and roads in S. of England).
_Lolium perenne_ (meadows, pastures and waste places).
_L. temulentum_ (fields and waste places, not common).
_Bromus sterilis_ (on way-sides, &c.).
_B. arvensis_ (cultivated and waste meadows and pastures).
_Poa rigida_ (dry, rocky places).
It is also often useful to know whether a grass is rare or local,
especially for the purpose we have in view, and I have therefore drawn
up the following list of rare, local or introduced foreign grasses
either not noticed at all, or only referred to incidentally in this
work.
In many cases these introduced foreign grasses have sprung up from
seeds brought over in cargoes of hay, wool, and other products and
packing materials, which in part accounts for their occurrence only
near certain sea-ports, manufacturing towns and so forth. Such
plants are frequently termed ballast plants. Foreign plants are also
introduced in seed, as mixtures or impurities, and frequently escape
from corn-fields &c.
_Leersia oryzoides_ (ditches of Hants., Sussex and Surrey).
_Panicum sanguinale_ (S. England).
_P. verticillatum_ (fields in S. and E.).
_P. glaucum_ (rarely introduced).
_Hierochloe borealis_ (Thurso only).
_Phleum alpinum_ (Highlands only).
_P. Bœhmeri_ (Eastern counties, rare).
_P. asperum_ ” ” ”
_Phalaris canariensis_ (rare weed).
_Alopecurus alpinus_ (Highlands).
_Mibora verna_ (Anglesea and Channel Islands).
_Lagurus ovatus_ (Suffolk coasts).
_Polypogon monspeliensis_ (rare, in S. England near sea).
_P. littoralis_ (salt marshes S. England).
_Agrostis setacea_ (dry heaths of S. Wales).
_A. Spica-venti_ (sandy fields of E. counties).
_Gastridium lendigerum_ (fields and waste places in S. Wales
and Norfolk).
_Calamagrostis Epigeios_ (moist glades &c. in Scotland).
_C. lanceolata_ (moist shades, scattered in England).
_C. stricta_ (bogs, &c., very rare).
_Cynodon Dactylon_ (waste and cultivated lands near sea in Scotland).
_Spartina stricta_ (salt marshes S. and E. coast).
_Lepturus incurvatus_ (scattered on shores).
_Bromus maximus_ (Jersey).
_B. madritensis_ (roads and waste, Scotland and Tipperary).
_B. inermis_ (introduced from Hungary).
_Lolium italicum_ (introduced from Lombardy).
_Festuca uniglumis_ (Irish and S.E. coast).
_Poa procumbens_ (waste ground near sea).
_P. loliacea_ (sandy sea-shores).
_P. laxa_ (Ben Nevis, &c.).
_P. alpina_ (Highlands and N.).
_Catabrosa aquatica_ (shallow pools and ditches, scattered).
Finally, a few words may be said on a subject still in its
infancy--that of Indicator-plants. In many cases certain plants are
found so confined to certain classes of soil, that foresters and
agriculturists have claimed to be able to infer from their presence
the presence or absence of certain chemical or other constituents of
soils: on the contrary we find other plants so universally distributed
without reference to the quality of the soil, that they are not
indicative. The latter are often termed _ruderal_ or _vagabonds_ (see
p. 29). Without attempting too rigid a classification of Grasses in
this connection--which would be premature in this early state of our
knowledge--the following remarks are at least generally true.
A few grasses are Indicators of chalk and limestone--e.g. _Briza
media_, _Kœleria cristata_, and the exotic species _Stipa pennata_
and _Melica ciliata_.
The following are said to indicate a sufficiency of potassium salts,
In moister soils.
_Digraphis arundinacea._
_Phleum pratense._
_Avena pubescens._
_Arundo Phragmites._
_Molinia cærulea._
_Glyceria fluitans._
In drier soils.
_Anthoxanthum odoratum._
_Alopecurus pratensis._
_Agrostis alba._
__Holcus lanatus.__
_Arrhenatherum._
_Kœleria cristata._
_Briza media._
_Dactylis glomerata._
_Cynosurus cristatus._
_Poa pratensis._
_P. trivialis._
_P. compressa._
_Festuca elatior._
_Lolium perenne._
Grasses like _Bromus arvensis_ indicate the existence of clay in the
soil.
While the following are indicative of sand,
_Aira caryophyllea._
_A. præcox._
_A. canescens._
_Festuca ovina._
_Bromus sterilis._
And only if the sandy soil is moist and of better quality, owing to a
certain proportion of humus, the following,
_Anthoxanthum odoratum._
_Agrostis alba._
_Dactylis glomerata._
_Arrhenatherum avenaceum._
_Avena pubescens._
_Poa pratensis._
That the soil contains considerable quantities of common salt--sodium
chloride--may be inferred if the following grasses occur,
_Psamma arenaria._
_Elymus arenarius._
_Hordeum maritimum._
_Agropyrum junceum_, &c.
The existence of much humus is indicated by such shade grasses as
_Melica uniflora._
_M. nutans._
_Milium effusum._
_Bromus giganteus._
_B. asper._
_Brachypodium sylvaticum._
Whereas soils known as “sour,” though containing much vegetable
remains, may be suspected if the following grasses abound on them,
_Aira cæspitosa._
_Nardus stricta._
_Alopecurus geniculatus._
_Molinia cærulea_;
especially if sedges and rushes coexist with them.
When cuttings are made in forests, such grasses as the following are
very apt to appear, and may do harm to young plants,
_Festuca ovina_ and varieties.
_Agrostis alba._
_Holcus mollis._
_Aira flexuosa_, &c.
The grasses more especially indicative of particular classes of
forest-soils are chiefly the wood-species (see p. 28), and need not be
further specified. In gaps, borders, and copses--half-shade--we find
several common grasses--e.g.
_Anthoxanthum odoratum._
_Agrostis alba._
_Aira flexuosa._
_Holcus lanatus._
_Arrhenatherum avenaceum._
_Triodia decumbens._
_Dactylis glomerata._
_Festuca rubra._
_Brachypodium pinnatum._
_Hordeum sylvaticum._
Whereas
_Poa nemoralis_,
_Festuca sylvatica_,
_Agropyrum caninum_,
_Melica_,
_Milium_,
_Bromus asper_,
_B. giganteus_,
_Brachypodium sylvaticum_,
are more likely to be met with in the deep shade inside the forest.
On the other hand there are vagabond grasses which seem to show no
signs of preference for one soil over another--e.g. _Poa annua_--though
in some cases these _ruderal_ plants indicate the presence of rotting
substances, on ash-heaps and rubbish of various kinds.
With reference to the above, however, the student must not forget
that very complex relations are concerned in changes of soil, shade,
moisture, elevation, &c. and that although experienced observers can
draw conclusions of some value from the presence of _numerous species
and individuals_ on a given soil, no one must conclude too readily that
a soil is so and so, from observing solely that a particular kind of
grass will grow there.
An excellent example of what may be done by applying such knowledge
as exists of the habits of grasses, is afforded by the historic case
of the planting up of shifting sand-dunes with species like _Psamma
arenaria_, _Elymus arenarius_, _Agropyrum junceum_, &c. (together
with sand-binding species of sedges) and so not only fixing the sand,
but preparing it for gradual afforestation with bushes and eventually
trees, and so saving enormous tracts of land and sums of money, as has
been done on the West coasts of France.
Moreover, the action of ruderal plants--including grasses--is to
completely alter the nature of the poor soil and gradually fit it for
other plants. Coverings of grass greatly affect the actions of heat and
sunshine on the surface soil, and modify the effects of radiation and
evaporation, to say nothing of the penetrating and other effects of the
roots.
Rhizomes and stolons break up stiff soils; and every engineer
and forester knows how useful certain grasses are in keeping the
surface-soil from being washed down by heavy rains on steep hill-sides
or embankments.
On the other hand, luxuriant growths of tall grasses may do harm to
young plants, by their action as weeds and especially as shade-plants;
though foresters can employ them in the latter capacity, under
restrictions, to shelter young trees from the sun. Again, too much dry
grass near a forest offers dangers from fire; and it is a well known
fact that certain injurious animals, e.g. mice and other vermin, are
favoured by a covering of grass.
Graminaceæ are for the most part chalk-fleeing plants, in spite of the
fact that certain species can grow in very thin layers of soil on chalk
downs. They must be regarded as requiring moderate supplies of humus as
a rule, and even sand-loving grasses are not real exceptions.
The physiognomy of the grasses has always been regarded as a striking
one, and Humboldt classed it as one of his 19 types of vegetation.
As is well known they are sociable plants, often covering enormous
areas--prairies, alps, steppes, &c.--with a few species, alone or
densely scattered throughout a mixed herbage. They also represent
characteristically the sun-plants, the erect leaves exposing their
surfaces obliquely to the solar rays, and being often folded and nearly
always narrow.
The dead remains of these sociable grasses are an important factor in
protecting the soil against drought and in facilitating humification,
as well as in covering up plants during long winters or dry seasons,
keeping the ground warmer and moister, and generally lessening the
effect of extremes.
Many Graminaceæ are pronounced xerophytes, the epidermis often being
developed as a water-storing tissue, while the erect leaves roll
themselves in intense light, the stomata being situated accordingly.
The halophytic strand-plants _Psamma arenaria_, _Elymus arenarius_,
_Agropyrum junceum_, and other Dune-species, as well as species of
_Aira_, _Festuca_, _Anthoxanthum_, _Stipa_, _Lygeum_, _Aristida_,
&c. are examples. The heath-grasses--e.g. _Festuca ovina_, _Nardus
stricta_, _Molinia cærulea_--also come under this category.
Many of the strand-plants (halophytes) _Agropyrum_, _Psamma_, _Elymus_,
are covered with waxy bloom, and have long rhizomes which bind the sand
and form new soil, a property largely taken advantage of in certain
forest operations.
Other grasses, particularly annual species, show their adaptation to
xerophytic habits by forming bulbous store-houses at the base of the
culms--e.g. _Phleum arenarium_.
Some Graminaceæ are hydrophytes, such as _Arundo_, _Glyceria_,
&c., with large intercellular spaces in their tissues; while
many species--e.g. _Aira cæspitosa_, _Agrostis canina_, _Molinia
cærulea_--grow on wet moor-lands, forming perennial tufts, with or
without creeping rhizomes.
The mesophyte grasses are especially characteristic of what may be
termed carpets--a lawn is a good example on a small scale, though of
course we must remember that here the struggle for existence has been
artificially interfered with more or less. Such carpets consist of the
densely interwoven rootlets and rhizomes forming sod, and contain much
humus from the accumulated débris of former years. These grass-carpets
may be composed of nearly pure growths of a few species, or of very
many different grasses and other herbage. They are common in Arctic
regions, on Alps, and in temperate climates generally, where we know
them as meadows, hay-fields, pasture and lawns.
The Bamboos in the wider sense have a physiognomy of their own, e.g. in
India, and may drive out most other plants and form dense undergrowths
or jungle of interlaced stems and leaves and thorny shoots. Similar
growths occur on the Andes and elsewhere in South America. In some
parts of India and tropical Asia the taller bamboos form aggregates
comparable to dense forests, and such forests are common on the
banks of several large tropical rivers. Most of these Bamboos are
xerophytes. Bamboos are neither confined to the tropics, nor to
warmer regions, however, for species are known from distinctly cool
regions--e.g. South America--or even from near the snow line--e.g.
Chili, the Himalayas, Japan, &c., and the number of species known as
hardy is increasing annually, as is evident on examining our larger
English gardens.
The permanence and character of extensive grasslands, especially
prairies, savannahs, and steppes, are much affected by the periodical
firing they are exposed to in the dry season, and large tracts of
country in various parts of the world would doubtless bear forests or
other vegetation if not thus fired, while in other cases the herbage
would be differently constituted were firing discontinued.
The following chapter embodies an attempt to classify our British
grasses solely for purposes of identification when not in flower. It
is not claimed that the arrangement is the best possible, nor that it
is complete, and I need hardly say that corrections will be gratefully
received.