Any number of spikelets may compose the inflorescence, and their
arrangement is very various. In the spicate forms, with sessile
spikelets on the main axis, the latter is often dilated and flattened
(_Paspalum_), or is more or less thickened and hollowed out
(_Stenotaphrum_, _Rottboellia_, _Tripsacum_), when the spikelets are
sunk and buried within the cavities. Every variety of racemose and
paniculate inflorescence obtains, and the number of spikelets composing
those of the large kinds is often immense. Rarely the inflorescence
consists of very few flowers; thus _Lygeum Spartum_, the most anomalous
of European grasses, has but two or three large uniflorous spikelets,
which are fused together at the base, and have no basal glumes, but are
enveloped in a large, hooded, spathe-like bract.
[Illustration: FIG. 12.--Spikelet of _Cenchrus echinatus_ enclosed in a
bristly involucre.]
[Illustration: FIG. 13.--Flowers of Grasses (enlarged). 1,
_Piptatherum_, with the palea p; 2, _Poa_; 3, _Oryza_; l, Lodicule.]
_Flower._--This is characterized by remarkable uniformity. The perianth
is represented by very rudimentary, small, fleshy scales arising below
the ovary, called _lodicules_; they are elongated or truncate, sometimes
fringed with hairs, and are in contact with the ovary. Their usual
number is two, and they are placed collaterally at the anterior side of
the flower (fig. 13,) that is, within the flowering glume. They are
generally considered to represent the inner whorl of the ordinary
monocotyledonous (liliaceous) perianth, the outer whorl of these being
suppressed as well as the posterior member of the inner whorl. This
latter is present almost constantly in _Stipeae_ and _Bambuseae_, which
have three lodicules, and in the latter group they are occasionally more
numerous. In _Anomochloa_ they are represented by hairs. In
_Streptochaeta_ there are six lodicules, alternately arranged in two
whorls. Sometimes, as in _Anthoxanthum_, they are absent. In _Melica_
there is one large anterior lodicule resulting presumably from the union
of the two which are present in allied genera. Professor E. Hackel,
however, regards this as an undivided second pale, which in the majority
of the grasses is split in halves, and the posterior lodicule, when
present, as a third pale. On this view the grass-flower has no perianth.
The function of the lodicules is the separation of the pale and glume to
allow the protrusion of stamens and stigmas; they effect this by
swelling and thus exerting pressure on the base of these two structures.
Where, as in _Anthoxanthum_, there are no lodicules, pale and glume do
not become laterally separated, and the stamens and stigmas protrude
only at the apex of the floret (fig. 7). Grass-flowers are usually
hermaphrodite, but there are very many exceptions. Thus it is common to
find one or more imperfect (usually male) flowers in the same spikelet
with bisexual ones, and their relative position is important in
classification. _Holcus_ and _Arrhenatherum_ are examples in English
grasses; and as a rule in species of temperate regions separation of the
sexes is not carried further. In warmer countries monoecious and
dioecious grasses are more frequent. In such cases the male and female
spikelets and inflorescence may be very dissimilar, as in maize, Job's
tears, _Euchlaena_, _Spinifex_, &c.; and in some dioecious species this
dissimilarity has led to the two sexes being referred to different
genera (e.g. _Anthephora axilliflora_ is the female of _Buchloe
dactyloides_, and _Neurachne paradoxa_ of a species of _Spinifex_). In
other grasses, however, with the sexes in different plants (e.g.
_Brizopyrum_, _Distichlis_, _Eragrostis capitala_, _Gynerium_), no such
dimorphism obtains. _Amphicarpum_ is remarkable in having cleistogamic
flowers borne on long radical subterranean peduncles which are fertile,
whilst the conspicuous upper paniculate ones, though apparently perfect,
never produce fruit. Something similar occurs in _Leersia oryzoides_,
where the fertile spikelets are concealed within the leaf-sheaths.
_Androecium._--In the vast majority there are three stamens alternating
with the lodicules, and therefore one anterior, i.e. opposite the
flowering glume, the other two being posterior and in contact with the
palea (fig. 13, 1 and 2). They are hypogynous, and have long and very
delicate filaments, and large, linear or oblong two-celled anthers,
dorsifixed and ultimately very versatile, deeply indented at each end,
and commonly exserted and pendulous. Suppression of the anterior stamen
sometimes occurs (e.g. _Anthoxanthum_, fig. 7), or the two posterior
ones may be absent (_Uniola_, _Cinna_, _Phippsia_, _Festuca bromoides_).
There is in some genera (_Oryza_, most _Bambuseae_) another row of three
stamens, making six in all (fig. 13, 3); and _Anomochloa_ and
_Tetrarrhena_ possess four. The stamens become numerous (ten to forty)
in the male flowers of a few monoecious genera (_Pariana_, _Luziola_).
In _Ochlandra_ they vary from seven to thirty, and in _Gigantochloa_
they are monadelphous.
_Gynoecium._--The pistil consists of a single carpel, opposite the pale
in the median plane of the spikelet. The ovary is small, rounded to
elliptical, and one-celled, and contains a single slightly bent ovule
sessile on the ventral suture (that is, springing from the back of the
ovary); the micropyle points downwards. It bears usually two lateral
styles which are quite distinct or connate at the base, sometimes for a
greater length (fig. 14, 1), each ends in a densely hairy or feathery
stigma (fig. 14). Occasionally there is but a single style, as in
_Nardus_ (fig. 14, 7), which corresponds to the midrib of the carpel.
The very long and apparently simple stigma of maize arises from the
union of two. Many of the bamboos have a third, anterior, style.
Comparing the flower of Gramineae with the general monocotyledonous plan
as represented by Liliaceae and other families (fig. 15), it will be
seen to differ in the absence of the outer row and the posterior member
of the inner row of the perianth-leaves, of the whole inner row of
stamens, and of the two lateral carpels, whilst the remaining members of
the perianth are in a rudimentary condition. But each or any of the
usually missing organs are to be found normally in different genera, or
as occasional developments.
[Illustration: FIG. 14.--Pistils of grasses (much enlarged). 1,
_Alopecurus_; 2, _Bromus_; 3, _Arrhenatherum_; 4, _Glyceria_; 5,
_Melica_; 6, _Mibora_; 7, _Nardus_.]
[Illustration: FIG. 15.--Diagrams of the ordinary Grass-flower.
1, Actual condition;
2, Theoretical, with the suppressed organs supplied.
a, Axis.
b, Flowering glume.
c, Palea.
d, Outer row of perianth leaves.
e, Inner row.
f, Outer row of stamens.
g, Inner row.
h, Pistil.]
_Pollination._--Grasses are generally wind-pollinated, though
self-fertilization sometimes occurs. A few species, as we have seen, are
monoecious or dioecious, while many are polygamous (having unisexual as
well as bisexual flowers as in many members of the tribes
_Andropogoneae_, fig. 18, and _Paniceae_), and in these the male flower
of a spikelet always blooms later than the hermaphrodite, so that its
pollen can only effect cross-fertilization upon other spikelets in the
same or another plant. Of those with only bisexual flowers, many are
strongly protogynous (the stigmas protruding before the anthers are
ripe), such as _Alopecurus_ and _Anthoxanthum_ (fig. 7), but generally
the anthers protrude first and discharge the greater part of their
pollen before the stigmas appear. The filaments elongate rapidly at
flowering-time, and the lightly versatile anthers empty an abundance of
finely granular smooth pollen through a longitudinal slit. Some flowers,
such as rye, have lost the power of effective self-fertilization, but in
most cases both forms, self- and cross-fertilization, seem to be
possible. Thus the species of wheat are usually self-fertilized, but
cross-fertilization is possible since the glumes are open above, the
stigmas project laterally, and the anthers empty only about one-third of
their pollen in their own flower and the rest into the air. In some
cultivated races of barley, cross-fertilization is precluded, as the
flowers never open. Reference has already been made to cleistogamic
species which occur in several genera.
_Fruit and Seed._--The ovary ripens into a usually small ovoid or
rounded fruit, which is entirely occupied by the single large seed, from
which it is not to be distinguished, the thin pericarp being completely
united to its surface. To this peculiar fruit the term _caryopsis_ has
been applied (more familiarly "grain"); it is commonly furrowed
longitudinally down one side (usually the inner, but in _Coix_ and its
allies, the outer), and an additional covering is not unfrequently
provided by the adherence of the persistent palea, or even also of the
flowering glume ("chaff" of cereals). From this type are a few
deviations; thus in _Sporobolus_, &c. (fig. 16), the pericarp is not
united with the seed but is quite distinct, dehisces, and allows the
loose seed to escape. Sometimes the pericarp is membranous, sometimes
hard, forming a nut, as in some genera of _Bambuseae_, while in other
_Bambuseae_ it becomes thick and fleshy, forming a berry often as large
as an apple. In _Melocanna_ the berry forms an edible fruit 3 or 4 in.
long, with a pointed beak of 2 in. more; it is indehiscent, and the
small seed germinates whilst the fruit is still attached to the tree,
putting out a tuft of roots and a shoot, and not falling till the latter
is 6 in. long. The position of the embryo is plainly visible on the
front side at the base of the grain. On the other, posterior, side of
the grain is a more or less evident, sometimes punctiform, sometimes
elongated or linear mark, the hilum, the place where the ovule was
fastened to the wall of the ovary. The form of the hilum is constant
throughout a genus, and sometimes also in whole tribes.
[Illustration: FIG. 16.--Fruit of _Sporobolus_, showing the dehiscent
pericarp and seed.]
The testa is thin and membranous but occasionally coloured, and the
embryo small, the great bulk of the seed being occupied by the hard
farinaceous endosperm (albumen) on which the nutritive value of the
grain depends. The outermost layer of endosperm, the aleuron-layer,
consists of regular cells filled with small proteid granules; the rest
is made up of large polygonal cells containing numerous starch-grains in
a matrix of proteid which may be continuous (horny endosperm) or
granular (mealy endosperm). The embryo presents many points of interest.
Its position is remarkable, closely applied to the surface of the
endosperm at the base of its outer side. This character is absolute for
the whole order, and effectually separates Gramineae from Cyperaceae.
The part in contact with the endosperm is plate-like, and is known as
the _scutellum_; the surface in contact with the endosperm forms an
absorptive epithelium. In some grasses there is a small scale-like
appendage opposite the scutellum, the _epiblast_. There is some
difference of opinion as to which structure or structures represent the
cotyledon. Three must be considered: (1) the scutellum, connected by
vascular tissue with the vascular cylinder of the main axis of the
embryo which it more or less envelops; it never leaves the seed, serving
merely to prepare and absorb the food-stuff in the endosperm; (2) the
cellular outgrowth of the axis, the epiblast, small and inconspicuous as
in wheat, or larger as in _Stipa_; (3) the pileole or germ-sheath,
arising on the same side of the axis and above the scutellum, enveloping
the plumule in the seed and appearing above ground as a generally
colourless sheath from the apex of which the plumule ultimately breaks
(fig. 17, 4, b). The development of these structures (which was
investigated by van Tieghem), especially in relation to the origin of
the vascular bundles which supply them, favours the view that the
scutellum and pileole are highly differentiated parts of a single
cotyledon, and this view is in accord with a comparative study of the
seedling of grasses and of other monocotyledons. The epiblast has been
regarded as representing a second cotyledon, but this is a very doubtful
interpretation.
[Illustration: FIG. 17.--A Grain of Wheat. 1, back, and 2, front view;
3, vertical section, showing (b) the endosperm, and (a) embryo; 4,
beginning of germination, showing (b) the pileole and (c) the radicle
and secondary rootlets surrounded by their coleorrhizae.]
_Germination._--In germination the coleorhiza lengthens, ruptures the
pericarp, and fixes the grain to the ground by developing numerous
hairs. The radicle then breaks through the coleorhiza, as do also the
secondary rootlets where, as in the case of many cereals, these have
been formed in the embryo (fig. 17, 4). The germ-sheath grows vertically
upwards, its stiff apex pushing through the soil, while the plumule is
hidden in its hollow interior. Finally the plumule escapes, its leaves
successively breaking through at the tip of the germ-sheath. The
scutellum meanwhile feeds the developing embryo from the endosperm. The
growth of the primary root is limited; sooner or later adventitious
roots develop from the axis above the radicle which they ultimately
exceed in growth.
_Means of Distribution._--Various methods of scattering the grain have
been adopted, in which parts of the spikelet or inflorescence are
concerned. Short spikes may fall from the culm as a whole; or the axis
of a spike or raceme is jointed so that one spikelet falls with each
joint as in many _Andropogoneae_ and _Hordeae_. In many-flowered
spikelets the rachilla is often jointed and breaks into as many pieces
as there are fruits, each piece bearing a glume and pale. One-flowered
spikelets may fall as a whole (as in the tribes _Paniceae_ and
_Andropogoneae_), or the axis is jointed above the barren glumes so that
only the flowering glume and pale fall with the fruit. These
arrangements are, with few exceptions, lacking in cultivated cereals
though present in their wild forms, so far as these are known. Such
arrangements are disadvantageous for the complete gathering of the
fruit, and therefore varieties in which they are not present would be
preferred for cultivation. The persistent bracts (glume and pale) afford
an additional protection to the fruit; they protect the embryo, which is
near the surface, from too rapid wetting and, when once soaked, from
drying up again. They also decrease the specific gravity, so that the
grain is more readily carried by the wind, especially when, as in
_Briza_, the glume has a large surface compared with the size of the
grain, or when, as in _Holcus_, empty glumes also take part; in Canary
grass (_Phalaris_) the large empty glumes bear a membranous wing on the
keel. In the sugar-cane (_Saccharum_) and several allied genera the
separating joints of the axis bear long hairs below the spikelets; in
others, as in _Arundo_ (a reed-grass), the flowering glumes are
enveloped in long hairs. The awn which is frequently borne on the
flowering glume is also a very efficient means of distribution, catching
into fur of animals or plumage of birds, or as often in _Stipa_ (fig. 8)
forming a long feather for wind-carriage. In _Tragus_ the glumes bear
numerous short hooked bristles. The fleshy berries of some _Bambuseae_
favour distribution by animals.
The awn is also of use in burying the fruit in the soil. Thus in
_Stipa_, species of _Avena_, _Heteropogon_ and others the base of the
glume forms a sharp point which will easily penetrate the ground; above
the point are short stiff upwardly pointing hairs which oppose its
withdrawal. The long awn, which is bent and closely twisted below the
bend, acts as a driving organ; it is very hygroscopic, the coils
untwisting when damp and twisting up when dry. The repeated twisting and
untwisting, especially when the upper part of the awn has become fixed
in the earth or caught in surrounding vegetation, drives the point
deeper and deeper into the ground. Such grasses often cause harm to
sheep by catching in the wool and boring through the skin.
A peculiar method of distribution occurs in some alpine and arctic
grasses, which grow under conditions where ripening of the fruit is
often uncertain. The entire spikelet, or single flowers, are transformed
into small-leaved shoots which fall from the axes and readily root in
the ground. Some species, such as _Poa stricta_, are known only in this
viviparous condition; others, like our British species _Festuca ovina_
and _Poa alpina_, become viviparous under the special climatic
conditions.
II. CLASSIFICATION.--Gramineae are sharply defined from all other
plants, and there are no genera as to which it is possible to feel a
doubt whether they should be referred to it or not. The only family
closely allied is Cyperaceae, and the points of difference between the
two may be here brought together. The best distinctions are found in
the position of the embryo in relation to the endosperm--lateral in
grasses, basal in Cyperaceae--and in the possession by Gramineae of the
2-nerved palea below each flower. Less absolute characters, but
generally trustworthy and more easily observed, are the feathery
stigmas, the always distichous arrangement of the glumes, the usual
absence of more general bracts in the inflorescence, the split
leaf-sheaths, and the hollow, cylindrical, jointed culms--some or all of
which are wanting in all Cyperaceae. The same characters will
distinguish grasses from the other glumiferous orders, Restiaceae, and
Eriocaulonaceae, which are besides further removed by their capsular
fruit and pendulous ovules. To other monocotyledonous families the
resemblances are merely of adaptive or vegetative characters. Some
Commelinaceae and Marantaceae approach grasses in foliage; the leaves of
_Allium_, &c., possess a ligule; the habit of some palms reminds one of
the bamboos; and Juncaceae and a few Liliaceae possess an inconspicuous
scarious perianth. There are about 300 genera containing about 3500
well-defined species.
The great uniformity among the very numerous species of this vast family
renders its _classification_ very difficult. The difficulty has been
increased by the confusion resulting from the multiplication of genera
founded on slight characters, and from the description (in consequence
of their wide distribution) of identical plants under several different
genera.
No characters for main divisions can be obtained from the flower proper
or fruit (with the exception of the character of the hilum), and it has
therefore been found necessary to trust to characters derived from the
usually less important inflorescence and bracts.
Robert Brown suggested two primary divisions--Paniceae and Poaceae,
according to the position of the most perfect flower in the spikelet;
this is the upper (apparently) terminal one in the first, whilst in the
second it occupies the lower position, the more imperfect ones (if any)
being above it. Munro supplemented this by another character easier of
verification, and of even greater constancy, in the articulation of the
pedicel in the Paniceae immediately below the glumes; whilst in Poaceae
this does not occur, but the axis of the spikelet frequently articulates
_above_ the pair of empty basal glumes. Neither of these great divisions
will well accommodate certain genera allied to _Phalaris_, for which
Brown proposed tentatively a third group (since named _Phalarideae_);
this, or at least the greater part of it, is placed by Bentham under the
Poaceae.
The following arrangement has been proposed by Professor Eduard Hackel
in his recent monograph on the order.
A. Spikelets one-flowered, rarely two-flowered as in Zea, falling from
the pedicel entire or with certain joints of the rachis at maturity.
Rachilla not produced beyond the flowers.
a. Hilum a point; spikelets not laterally compressed.
[alpha] Fertile glume and pale hyaline; empty glumes thick,
membranous to coriaceous or cartilaginous, the lowest the largest.
Rachis generally jointed and breaking up when mature.
1. Spikelets unisexual, male and female in separate inflorescences
or on different parts of the same inflorescence.
1. _Maydeae_.
2. Spikelets bisexual, or male and bisexual, each male standing
close to a bisexual.
2. _Andropogoneae_.
[beta] Fertile glume and pale cartilaginous, coriaceous or papery;
empty glumes more delicate, usually herbaceous, the lowest usually
smallest. Spikelets falling singly from the unjointed rachis of the
spike or the ultimate branches of the panicle.
3. _Paniceae_.
b. Hilum a line; spikelets laterally compressed.
4. _Oryzeae_.
B. Spikelets one- to indefinite-flowered; in the one-flowered the
rachilla frequently produced beyond the flower; rachilla generally
jointed above the empty glumes, which remain after the fruiting glumes
have fallen. When more than one-flowered, distinct internodes are
developed between the flowers.
a. Culm herbaceous, annual; leaf-blade sessile, and not jointed to the
sheath.
[alpha] Spikelets upon distinct pedicels and arranged in panicles or
racemes.