GRASSES IN FLOWER.
When the flowering shoot of a grass pushes up into the light and air
from the enveloping leaves, it forms a more or less branched collection
of flowers known as the _Inflorescence_, and in all our grasses this
inflorescence consists of a principal stalk, _haulm_ or _culm_, on
which shorter stalks--branched or not--are arranged. The mode of
branching is usually such that the youngest branches are nearest the
top, and the oldest nearest the bottom. It is evident at once, on
comparing the Moor Mat-grass (_Nardus_), Vernal-grass (_Anthoxanthum_),
Cock’s-foot (_Dactylis_), Meadow-grass (_Poa_) that considerable
differences exist as to the extent of this primary branching of the
inflorescence.
In _Nardus_ (Fig. 2) we find a number of long cylindrical-tapering
bud-like structures each seated on one side of the principal stem, and
one over the other: in the Vernal-grass and Cock’s-foot we find tufts
of such bud-like structures closely crowded round the upper end of the
principal stalk, the whole forming an elongated tuft of tufts: in the
_Poa_ we find a number of radiating, slender, long branches springing
from the principal stalk, and each of these ramifies again, and yet
again, until each of the ultimate hair-like branches bears one of the
bud-like structures. See also _Catabrosa_ (Fig. 4).
[Illustration: Fig. 29. A spikelet of _Festuca elatior_, var.
_pratensis_, from which the glumes and one palea (the outer) have been
removed to show the flower in situ (× 12). The two lodicules are in
front: the inner palea behind. Strasburger.]
The first thing for the student to apprehend is the nature of the
bud-like structures referred to.
Each of these is in itself a small tuft or bud of leaflike organs or
scales arranged on a short twig (_Rachis_, _Rachilla_), as it were, and
is called a _Spikelet_, and the true flowers of the grass are contained
in the angles between the scales--the scales being popularly known as
“_chaff_”: technically as _Glumes_ and _Paleæ_.
In order to understand the structure of a spikelet the student
should carefully dissect a large one, such as that of an Oat (Fig.
1). Proceeding from outside, he will find two large scales, like two
boats, fixed below to the stalk (_rachis_) one just below the other,
and shutting together as if hinged. These are called the _glumes_--the
inner and outer glume respectively--and they enclose the rest of the
spikelet.
[Illustration: Fig. 30. Diagram of a spikelet of a grass as it would
appear if the internodes between each set of organs were elongated.
_g^1_ lower and _g^2_ upper glume. _P_ lower and _p_ upper palea of the
second oldest flower _F^2_. _f_ a barren flower represented only by the
axis and paleæ. Above it a single palea and the termination of the axis
(_a_) of the spikelet.]
Inside them the axis or stalk (_rachis_) is continued for a short
distance only and on its sides are hinged two other pairs of more or
less boat-shaped scales, smaller and more delicate than the glumes, and
known as the _pales_ (_paleæ_), while a third pair of still smaller
pales is fixed to the end of the axis. In each case one smaller and
more delicate _inner_ palea is hinged just inside its more obvious
_outer_ palea. In the closed condition of the spikelet each of the
three pairs of pales is shut together, and pressed close to the axis,
and the pair of glumes shut in the whole.
On opening each of the lower pairs of pales we find a _flower_ inside;
but the terminal pair usually contain only the barren end of the axis.
Hence the latter is barren and the former are fertile.
Each fertile flower is found on careful dissection to consist of a
small swollen _Ovary_, or young grain, covered with silky hairs and
with a couple of delicate plumes (the _Stigma_) at its apex, and three
long and slender _Stamens_; while the magnifying glass will show two
tiny scales at the base--the _Lodicules_. All our ordinary grasses have
their flowers thus constructed--a pair of _lodicules_, three dangling
_stamens_ and an _ovary_ with a feathery two-plumed _stigma_: each such
flower is also enclosed in its pair of _pales_, and the several pairs
of pales of each spikelet, with their contents, are enclosed in the
pair of _glumes_ (Figs. 29-32).
Returning now to the inflorescence. It is clear that we have to
distinguish between the entire branched total Inflorescence, and the
Spikelets or partial inflorescences of which it is composed. In Botany
it is agreed to call any inflorescence consisting of a stalk or axis on
which the flowers are arranged without stalks--i.e. sitting directly
on it--and so that the youngest are above and the older below, a
_Spike_, and each spikelet is _a little spike_.
[Illustration: Fig. 31. Diagram of a spikelet of Wheat dissected (×
about 5) showing--from below upwards--the two glumes, two paleæ, two
lodicules, three stamens, and the ovary of the typical grass. Oliver.]
[Illustration: Fig. 32. Diagram of a spikelet of _Anthoxanthum_
dissected (× about 8), and showing--from below upwards--two outer and
two (awned) inner glumes, two paleæ, two stamens, and the ovary. There
are no lodicules. Oliver.]
On looking at the total inflorescence of the _Nardus_ we see that we
have a number of spikelets seated on the sides of an axis: this is then
a spike of spikelets, or, shortly, a _Spike_[9] (Fig. 5). _Mibora_ and
_Lepturus_ afford other examples. In _Panicum_, _Cynodon_ (Fig. 2) and
_Spartina_ we have groups of such spikes.
The _Poa_ inflorescence is, however, different. It consists of a loose
branched system of spikelets. Botanists term such a loosely branching
system, where each branch ends in a flower, a _panicle_: here then
we have a panicle of spikelets, or, shortly, a _Panicle_. _Aira_,
_Agrostis_, _Calamagrostis_, _Avena_, _Catabrosa_ (Fig. 4) and many
others afford further examples.
In _Dactylis_ we have a condition of affairs between the two extremes
given: the inflorescence is not so close a spike as _Nardus_, and
not so open a panicle as _Poa_--it is rather a spike-like panicle,
partaking of the nature of both. A special type of this (Foxtail)
occurring in certain grasses--e.g. _Phleum_, _Alopecurus_, _Phalaris_
and _Lagurus_,--is so characteristic as to be worth noting (Fig. 3).
There is also another aspect of these inflorescences which is not
without interest as showing how diagnostic characters may be obtained
from purely external features, easily observed in the field. We have
seen that in _Nardus_ the spikelets are arranged on one side only
of the rachis, or main axis, so that about three quarters of the
circumference of the latter is bare; whereas in _Lolium_--with which
_Agropyrum_ and _Brachypodium_ agree in this respect--the spikelets are
on opposite sides, leaving the intervening two quarters, i.e. half its
surface, of the circumference of the axis naked.
In _Cynosurus_ and the simpler forms of _Dactylis_, we find the
spikelets crowded round about three quarters of the surface of the
rachis, leaving the fourth quarter naked; and, finally, in _Phleum_,
_Alopecurus_, _Hordeum_, and _Anthoxanthum_ the spikelets cover the
entire surface.
In the first (_Nardus_) and third examples (_Cynosurus_, _Dactylis_)
where the spikelets are turned to one side, the inflorescence is said
to be _secund_.
The next point to notice is that not every grass has so many as two
fertile flowers and one barren one in its spikelet, as the Oat has.
A spikelet may have one (_Phleum_), two (_Aira_) or three (_Avena_)
or more (_Poa_) fertile flowers, and no barren ones or several, the
number of flowers being counted by the number of pairs of pales found
inside the pair of glumes. Moreover every flower is not necessarily
fertile (e.g. _Arrhenatherum_, _Holcus_) and several grasses have
one or more flowers in each spikelet containing _stamens_ only--male
flowers--while others may have _ovaries_ only--female flowers. In some
exotics the male and female flowers are on different parts of the same
plant (Maize) or even on different plants (_Gynerium_), an arrangement
not met with in our grasses. Accordingly, it is of importance in
determining a grass to discover how many flowers the spikelet contains,
and whether any are male only, or barren, &c., as well as to make out
the character of its inflorescence.
In the following lists I have brought together some of the chief points
with illustrative examples.
SPIKELETS with only one perfect flower (without rudimentary ones).
_Psamma._
_Milium._
_Nardus._
_Alopecurus._
_Phleum._
_Hordeum._
_Agrostis._
And species of the rare grasses _Calamagrostis_, _Mibora_, _Lepturus_,
_Spartina_, _Cynodon_, _Gastridium_, _Lagurus_, _Polypogon_, _Leersia_.
SPIKELETS with one perfect flower and one or more male or rudimentary
ones.
_Digraphis._
_Anthoxanthum._
_Holcus._
_Arrhenatherum._
And the rare genera _Hierochloe_ and _Panicum_.
SPIKELETS with at least two perfect flowers.
_Molinia._
_Arundo._
_Avena._
_Aira_ (some species).
_Bromus._
_Cynosurus._
_Dactylis._
_Triodia._
_Sesleria._
_Kœleria._
_Melica_ (one species).
_Briza._
_Poa trivialis._
_Catabrosa._
SPIKELETS with at least three perfect flowers and usually more.
_Elymus._
_Agropyrum._
_Brachypodium._
_Lolium._
_Festuca._
_Poa._
_Glyceria._
INFLORESCENCE, a spike of single spikelets.
_Agropyrum._
_Brachypodium._
_Lolium._
_Nardus._
And varieties of _Festuca Myurus_ and _F. loliacea_, &c.
INFLORESCENCE, a spike of pairs or tufts of three or more spikelets.
_Elymus_ (pairs).
_Hordeum_ (threes).
_Cynosurus_ (clusters).
INFLORESCENCE, a cylindrical closely tufted spike-like panicle.
_Phleum._
_Alopecurus._
_Psamma._
_Anthoxanthum._
INFLORESCENCE, a compact more or less tufted panicle.
_Molinia._
_Kœleria._
_Triodia._
_Dactylis._
_Aira præcox._
_A. canescens._
_Sesleria._
And rare grasses like _Polypogon_, _Gastridium_, &c.
INFLORESCENCE, a loose plume-like or branched panicle.
_Avena._
_Bromus._
_Arrhenatherum._
_Catabrosa._
_Aira_ (except _A. præcox_ and
_A. canescens_).
_Arundo._
_Digraphis._
_Milium._
_Agrostis._
_Melica._
_Holcus._
_Poa_ (most of the species).
_Glyceria._
_Briza._
_Festuca_ (except _F. Myurus_).
And the rare _Hierochloe_.
The _Glumes_ are always present in our grasses, and rudimentary only
in the rare grass _Leersia_; but _Lolium_ and _Nardus_ have only _one_
glume to the spikelet, and _Hierochloe_, _Digraphis_ and _Anthoxanthum_
(Fig. 32) have four or six. Our other grasses have _two_, but often
unequal in size.
In shape they are usually boat-like, pointed or obtuse (_Briza_) and
frequently with a distinct keel (_Anthoxanthum_, _Digraphis_, _Phleum_,
&c.) or with ridges, green lines (veins) and other characteristic
markings (e.g. _Digraphis_). The tip may be extended into a stiff long
point or awn (_Agropyrum_, _Phleum_, _Nardus_) and the keel, ribs,
and awn may have hairs or serrulæ on them. The rule is, however, that
the glumes are not awned. In texture the glumes may be herbaceous and
green-brown or purple (e.g. _Melica_) or membranous or stiff, or
scarious (i.e. browned, as if scorched) at the edges. In _Hordeum_ some
of the glumes are so narrow and pointed as to resemble stiff awns. In
_Catabrosa_ the glumes are truncate, as if bitten off at the top.
The _Paleæ_ are also often more or less boat-shaped, or flat ovate or
oblong scales, usually more delicate than the glumes and frequently
pointed, or (especially the outer pale) awned at the tip: in some
cases, however, the awn springs from the middle or base of the back of
the pale, and the latter may be bifid at its apex. The pale has usually
a distinct middle nerve. The inner pale is commonly the smaller and
more delicate of the two, and is sometimes difficult to see.
CONSPICUOUSLY AWNED GRASSES.
_Avena._
_Arrhenatherum_ (Fig. 33).
_Hordeum._
_Bromus._
_Agropyrum caninum._
_Lolium temulentum._
_Brachypodium sylvaticum._
_Festuca Myurus._
And a few rare grasses like _Panicum_, _Polypogon_, _Lagurus_.
GRASSES WITH NO TRUE AWNS.
_Kœleria. Milium. Digraphis. Elymus. Festuca_ (except _F.
Myurus_ and _F. uniglumis_). _Briza. Poa. Glyceria. Catabrosa._
_Molinia. Melica. Psamma. Agrostis alba._
And a few rare forms like _Leersia_, _Hierochloe_, &c.
GRASSES WITH INCONSPICUOUS AWNS, OR MERE POINTS, TO SOME OF THE GLUMES
OR PALES.
_Arundo._
_Phleum._
_Alopecurus._
_Holcus._
_Nardus._
_Agropyrum repens._
_Lolium perenne._
_Brachypodium pinnatum._
_Dactylis._
_Cynosurus._
_Sesleria._
[Illustration: Fig. 33. _Arrhenatherum._ 1 unopened and 2 open anther
(× 12). 3, spikelets open and exposing the stamens and stigmas; 4,
the pollen escaping and being dusted on to the stigmas (× about 5).
Kerner.]
As regards the flower proper, all our British grasses except
_Anthoxanthum_ (which has two only) have three stamens; but many exotic
grasses have six stamens, and a few have a large number--even 40. The
stamens have slender filaments and large versatile anthers, which
dangle from between the paleæ when the flowers are mature, scattering
their clouds of fine pollen in the wind (Fig. 33).
All our ordinary grasses except _Nardus_--where there is a simple
straight hairy style--have two spreading feathery stigmatic plumes,
which stand out right and left from between the paleæ when the pollen
is flying about on the wind. (Fig. 33.)
Much interesting speculation has been expended in attempting to explain
the morphological or theoretical significance of the parts of the
spikelet of a grass. If we project the various organs on a flat surface
in the form of a plan, keeping their relative positions intact, we
obtain a diagram such as that shown in Fig. 34.
[Illustration: Fig. 34. Diagram of a spikelet of a grass. The two
glumes--_g^1_ outer, _g^2_ inner--embrace four flowers, of which 1 is
the lowermost and 4 the uppermost.]
The comparison of numerous cases, and the study of the development of
the parts on the microscopic growing point dissected out from young
buds, have suggested that the inner and outer glumes are bracts, or
covering leaves, at the base of the true spikelet. In like manner
the two paleæ are bracteoles which subtend the flower proper. On this
assumption they can be compared with the corresponding structures in
other plants; whereas any attempt to compare the paleæ or glumes with
the sepals and petals of ordinary flowers breaks down.
A curious interest attaches to the awns so often found on the backs of
paleæ, and especially to those where the (sub-terminal) awn springs
from just below the bifid apex (e.g. _Avena_, &c.).
Hackel showed by comparison with a rolled leaf attached to its sheath
and ligule (e.g. _Psamma_) that such an awn as that of _Bromus
Alopecurus_ attached to its palea stands in the relation of a leaf to
its sheath, the part of the palea above the insertion corresponding
to the ligule, the awn itself to the lamina, and the palea below its
insertion to the sheath. This view is rendered the more probable by the
anatomy of the awn and by the observations of Schmid, who has shown
that the awns of cereals contain chlorophyll-tissue and a vascular
bundle, and have stomata, and his experiments led him to conclude that
in the young condition they transpire and assimilate, and probably even
contribute to the nutrition of the ripening grain. When dry and mature
the awns subserve biological functions of quite another kind, and as
we shall see are of importance in the distribution and sowing of the
grains. (Fig. 42.)
Returning to the floral diagram, we see that the two lodicules, the
three stamens and the ovary still remain to be explained. Much
discussion has been held regarding the lodicules. Functionally
they are said to aid in the divarication of the paleæ when the
period of anthesis arrives, and the stamens and stigmatic lobes
are to be exposed, by swelling and driving the valve-like paleæ
apart. Morphologically they have been explained as representing the
rudimentary perianth, here reduced to two minute scales, but in some
exotic grasses (_Bambusa_, _Stipa_, &c.) three lodicules, or even
more, are present. (Figs. 35, 36.) On the other hand they may be, and
probably are, scales of the nature of minute bracteoles and of no
significance to the flower itself.
[Illustration: Fig. 35. Floral diagram of ordinary grass. Each pair
of paleæ--_i.p._ inner and _o.p._ outer palea--encloses three stamens
(_s_), two lodicules (_l_) and the ovary. _st_, stigmatic plumes. _a_,
axis.]
[Illustration: Fig. 36. Floral diagram of a Bamboo, showing six
stamens, three inner (_i.s._) and three outer (_a.s._), and three
lodicules (_l_) in addition to the ovary. _i.p._ inner and _o.p._ outer
palea. _a_, axis.]
If this is so the flower of the grass is perfectly naked, and consists
in the typical case of three stamens and one carpel. The development
of the ovary lends no support to the view that there are two carpels:
the stigmatic plumes are not separate styles. Nor does the fact that
some grasses have six stamens lend support to the idea that the flower
is derived from the trimerous type so common in Monocotyledons: other
numbers may occur--e.g. as few as two (_Anthoxanthum_, Fig. 32) or even
one only (_Uniola_, _Cinna_, &c.) or as many as 20 or 40 in certain
other exotic grasses.
Even when three stigmatic plumes are developed, as in some Bamboos,
close investigation does not confirm the view that the ovary consists
of more than one carpel.
We must therefore regard the flower of the grass as typically composed
of one carpel and three stamens, with no perianth whatever. It is
subtended by one or more bracteoles (the lodicules), and enclosed in a
pair of bracteoles one higher than the other (the paleæ). The glumes
are bracts of the partial inflorescence--the spikelet.
That there are some departures from this type in detail does not
invalidate the importance of the fact that most grasses conform to it.
I now pass to the consideration of a grouping of our ordinary grasses
according to their floral arrangements.
The student should distinctly understand that the following notes are
intended to serve as an introduction to the floral characters of our
grasses, and not to replace the study of the Flora. I have dealt
with this section of the subject less in detail, because our best
Floras give so much information that it seemed undesirable to do more
than attempt to guide the reader in the recognition of the genera and
principal species by means of external features easily observed by
anyone with a little care. The detailed and critical examination of
species, varieties and rare forms should always be done with reference
to a complete Flora.