THE INFLORESCENCE AND FLOWER.
The flowers of grasses are reduced to their essential organs, the
stamens and the pistil. The flowers are aggregated together on distinct
shoots constituting the inflorescence of grasses. Sooner or later all
the branches of a grass-plant terminate in inflorescences which usually
stand far above the foliage leaves. As in other flowering plants, in
grasses also different forms of inflorescence are met with. But in
grasses the unit of the inflorescence is the =spikelet= and not the
flower.
The forms of inflorescence usually met with are the spike, raceme and
panicle. When the spikelets are sessile or borne directly along an
elongated axis as in _Enteropogon melicoides_ the inflorescence is a
=spike=. If the spikelets borne by the axis are all stalked, however
short the pedicels may be, it is a =raceme=. It must, however, be
remembered that true spikes are very rare. An inflorescence may appear
to be a spike, but on a close examination it will be seen to consist of
spikelets more or less pedicelled. Such an inflorescence, strictly
speaking, is a =spiciform raceme=. The branches of the inflorescence in
_Paspalum scrobiculatum_ or _Panicum javanicum_ are racemes and the
whole inflorescence is a compound raceme. The inflorescence is a
=panicle= when the spikelets are borne on secondary, tertiary or further
subdivided branches. Panicles differ very much in appearance according
to the relative length and stoutness of the branches. In _Eragrostis
tremula_ the panicle is very diffuse, in _Eragrostis Willdenoviana_ less
so. The panicle in _Sporobolus coromandelianus_ is pyramidal and the
branches are all verticillate, the lower being longer than the upper.
The branches of a panicle are usually loose, spreading or drooping in
most grasses. But in some species of grasses such as _Pennisetum
Alopecuros_ and _Setaria glauca_, the paniculate inflorescences become
so contracted that the pedicels and the short branches are hidden and
the inflorescence appears to be a spike. Such inflorescences as these
are called =spiciform panicles=. The inflorescences in several species
of Andropogon consist of racemes so much modified as to appear exactly
like a spike. What looks like a spike in these cases consists of a
jointed axis and each joint bears a pair of spikelets, one sessile and
the other pedicelled.
The name =rachis= is given to the axis of the spike, raceme and panicle,
whether the axis is the main one or of the branch. The rachis of the
inflorescence is usually cylindrical. In some grasses it is zigzag as in
_Pennisetum cenchroides_. It is very much flattened in _Paspalum
scrobiculatum_, but somewhat trigonous in _Digitaria sanguinalis_. In
very many grasses the rachis is continuous, but in a few cases it
consists of internodes or joints which disarticulate at maturity. Many
species of Andropogon have such jointed rachises. Sometimes the joints
become greatly thickened and the surface hollowed out, the spikelets
fitting in the cavities as in Rottboellia and Manisuris.
In panicles, especially when they are diffuse, the primary branches may
be disposed irregularly or in verticils on the main axis. For example in
the panicle of _Eragrostis Willdenoviana_, the branches are irregularly
disposed, whereas in _Sporobolus coromandelianus_ the branches are
verticillate. In both these grasses fleshy cushions are developed in the
axils of the branches. These swellings help to spread out the branches
especially at the time of anthesis. The branches at the top spread out
earlier than those below.
Sometimes at the base of the rachises, main or secondary, glandular
streaks are seen as in the rachises of _Sporobolus coromandelianus_.
These glands secrete a viscid juice at the time of anthesis.
[Illustration: Fig. 17.--The Spikelet of Dinebra arabica.
1 and 2. Empty glumes; 3, 4, 5, and 6. flowering glumes with flowers.]
The =spikelet= may be considered as a specialised branch consisting of a
short axis, the =rachilla= bearing a series of modified bracts, the
=glumes=, the lower pair being empty but the others bearing flowers in
their axils. The glumes are two-ranked and imbricating. As a type for
the spikelet that of an Eragrostis or Dinebra may be chosen. (See fig.
17.) In this spikelet the rachilla bears a number of glumes alternating
and imbricating. The first two glumes at the base of the spikelet do not
bear any flowers and so these two glumes are usually called empty
glumes. This is the case in almost all the species of grasses. The third
and the subsequent glumes are regularly arranged on the slender rachilla
alternately in two rows. In the axils of each of these glumes there is a
flower, except perhaps in the topmost glume. The flower is usually
enclosed by the glume and another structure found opposite the glume and
differing very much from the glume. This is the =palea=. It is attached
to the axis of the flower and its back is towards the rachilla.
Generally there are two nerves in a palea and its margins are enclosed
within those of the glume. The palea is homologous with the prophyllum
which it very much resembles. The prophyllum is usually found in the
branches of grasses, but it is not confined to grasses alone. It occurs
in the branches of some species of Commelina.
The spikelets vary very much in their structure. The spikelets in
grasses of several genera consist of only four glumes. As usual the
first two glumes are empty and the remaining two are flower-bearing
glumes. Both these glumes may have perfect flowers as in Isachne or the
terminal one may contain a perfect flower, the lower having either a
staminate flower or only a palea. Very often the spikelets are unisexual
and the male and female spikelets may be on the same plant as in _Coix
Lachryma-Jobi_ and _Polytoca barbata_, or they may be on different
plants as in _Spinifex squarrosus_.
The glumes of a spikelet are really modified bracts and some
differentiate the flowering glumes from the empty ones, by giving them
different names. The first two empty glumes are called glumes by all
agrostologists. Some in Europe call the flowering glume lower palea to
distinguish it from the real palea which they call the upper palea. Some
American Authors have recently adopted for the flowering glume the term
=lemma= introduced by Piper.
Considerable variation is met with in the case of the empty glumes.
Generally they are unequal, the first being smaller. Very often the
first glume becomes very small and it may be altogether absent. In some
species of Panicum the first glume is very small, in Digitaria it is
very minute and in Paspalum and Eriochloa it is entirely suppressed. The
flowering glumes are generally uniform when there are many. In the
spikelet having only four glumes the fourth glume differs from the
others mainly in texture. Instead of being thin and herbaceous it
becomes rigid and hard, smooth or rugose externally as in Panicum.
Flowering glumes instead of being like empty glumes, become very thin,
shorter and hyaline in Andropogon. Sometimes the flowering glumes are
awned. All of them may be awned as in Chloris or only the fourth glume
as in Andropogon.
The palea is fairly uniform in its structure in many grasses, but it is
also subject to variation. It becomes shorter in some and is absent in
others. Instead of having two nerves, it may have one and rarely more
than two. The palea can easily be distinguished from the glume, because
its insertion in the spikelet is different from that of the glume.
[Illustration: Fig. 18.--Flower of Chloris.
1. lodicules; 2. stamens; 3. ovary.]
The =lodicules= are small organs and they are the vestiges of the
perianth. In most grasses there are only two, but in Ochlandra and other
bamboos we meet with three lodicules. There are also some species with
many lodicules. In shape they are mostly of some form referable to the
cuneate form. They are of somewhat elongated form in Aristida and
Chloris. The function of the lodicules seems to be to separate the glume
and its palea so as to enable the stamens to come out and hang freely at
the time of anthesis. So it is only at the time of the opening of the
flowers that the lodicules are at their best. Then they are fairly
large, fleshy and thick and conspicuous. In the bud stage they are
usually small and after the opening of the flower they shrivel up and
are inconspicuous. There are also species of grasses in which the
lodicules are not found.
The =stamens= are three in number in the majority of grasses and six are
met with in Leersia, Hygrorhiza and Bamboos. Each stamen consists of a
very delicate long filament and an anther basifixed to the filament. But
as the anthers are long and the connective is reduced to its minimum,
they appear as if versatile when the stamens are out. When there are
three stamens one stands in front of the flowering glumes and the other
two in front of the palea, one opposite each edge of the palea. The
relative positions of the parts of the floret are shown in the floral
diagrams. (See figs. 18 and 19.)
[Illustration: Fig. 19.--Floral diagrams.
The first is that of Chloris, second of Panicum and the third of Oryza.]
The =pistil= consists of an ovary and two styles ending in plumose
stigmas. The ovary is 1-celled and 1-ovuled. It is one carpelled
according to the views of Hackel and his followers and there are also
some who consider it as 3-carpelled because of the occurrence of three
styles in the pistil of some bamboos.
The =rachilla= is usually well developed and elongated in many-flowered
spikelets, while in 1-flowered spikelets it remains very small so that
the flower appears to be terminal. It often extends beyond the insertion
of the terminal flower and its glume, and then lies hidden appressed to
the palea. This may be seen in the spikelets of the species of Cynodon.
This prolonged rachilla sometimes bears a minute glume, which is of
course rudimentary. Usually the glumes are rather close together on the
rachilla so that the internodes are very short; but in some grasses, as
in _Dinebra arabica_, the glumes are rather distant and so the
internodes are somewhat longer and conspicuous. In some species of
Panicum the rachilla is jointed to the pedicel below the empty glumes,
whereas it is articulated just above these glumes in _Chloris barbata_.
Sometimes the rachilla is articulated between the flowers. This is the
case in the spikelet of _Dinebra arabica_.
Pollination in most grasses is brought about by wind, though in a few
cases self-pollination occurs. The terminal position of the
inflorescence, its protrusion far above the level of the foliage leaves,
the swinging and dangling anthers, the abundance of non-sticking pollen
and the plumose stigmas are all intended to facilitate pollination by
wind. Furthermore the stamens and the stigmas do not mature at the same
time. In some grasses the stamens mature earlier, (=protandry=) while in
others the stigmas protrude long before the stamens (=protogyny=). As
the result of the pollination the ovary developes into a dry 1-seeded
indehiscent fruit. The seed fills the cavity fully and the pericarp
fuses with the seed-coat and so they are inseparable. Such a fruit is
termed a =caryopsis= or =grain=. Though in the vast majority of grasses
the pericarp is inseparable, in a few cases it is free from the
seed-coat as in _Sporobolus indicus_ and _Eleusine indica_.
[Illustration: Fig. 20.--Longitudinal section of a portion of the grain
of Andropogon Sorghum. x 280 P. Pericarp; Sc. seed-coat; A. aleurone
layer; E. endosperm; S. scutellum; Rs. root-sheath; Rc. root-cap; R.
radicle; Pl. plumule; G. growing point.]
[Illustration: Fig. 21.--A portion of the section of the grain of
Andropogon Sorghum. x 500 P. pericarp; I. seed-coat; A. aleurone layer;
S. starch.]
The caryopsis consists of an embryo on one side at the base and the
endosperm occupies the remaining portion. The embryo can be made out on
the side of the grain facing the glume, as it is outlined as an oval
area. On the other face of the grain which is towards the palea, the
hilum is seen at the base. The grain varies in shape considerably. It
may be rounded, oval, ellipsoidal, narrow and cylindrical, oblong terete
or furrowed. There is considerable variation as regards the colour also.
The =embryo= consists of an =axis= and a =scutellum=. The axis, which is
differentiated into the plumule directed upward and the radicle
downward, is small and straight and it is covered more or less by the
edges of the scutellum. The scutellum is attached to the axis at about
its middle and its outer surface is in contact with the endosperm. This
is an important organ as its function is to absorb nourishment from the
endosperm during germination. The scutellum is considered to represent
the first leaf or cotyledon. The endosperm consists mostly of starch.
Just outside the endosperm and within the epidermis lies a layer of
cells containing much proteid substance. This layer is called the
=aleurone layer=. (See fig. 21.) As an illustration of the caryopsis,
the grain of Andropogon Sorghum may be studied. All the structural
details are shown in fig. 20 which is a longitudinal section of the
grain.
The primary axis of the embryo is enclosed by a closed sheath both above
and below. The sheath which envelopes the radicle is called =coleorhiza=
and that of the plumule, =pileole= or =germ-sheath=.