THE FRUIT AND SEED.
The stigma of an ordinary grass consists of two divaricating plume-like
structures composed of thin-walled cells. When the paleæ open these
stigmatic plumes protrude, one on either side, and readily catch pollen
shed from the dangling stamens and carried by the wind, and since the
pollen of the same flower is usually shed at a time when the stigmas
of many neighbouring plants are mature, there is every opportunity for
cross-fertilisation. (Fig. 33.)
In some cases, however, e.g. _Anthoxanthum_, _Alopecurus_, the flowers
are proterogynous, the stigmatic plumes being ready for pollination
some time before the pollen is shed from the anthers of the same
flower; whereas in most of our grasses the pollen begins to scatter
before the stigmas are ready (protandrous). Among exotic grasses, many
are diœcious or monœcious--i.e. the flowers contain stamens
only or ovary only, on each plant, or on different inflorescences
of the same plant respectively--and even in our own _Holcus_ and
_Arrhenatherum_ this state of affairs is partially represented, since
one flower of the spikelet is male only.
In some grasses, e.g. Rye, however, it appears improbable that
cross-fertilisation ever occurs, since the paleæ do not open, and the
pollen falls on to the stigma direct; and in _Leersia_ and the foreign
_Amphicarpum_ the spikelets are completely cleistogamous, those of the
latter being on stalks close to the ground which push the flowers into
the soil, where pollination and fructification are accomplished.
Hybrid grasses are by no means uncommon. To say nothing of the numerous
cross-bred Wheats and Barleys, artificial hybrids have been raised
between Wheat and Rye. In the Maize an astonishing number of selected
cross-breeds have been obtained, and, among others, certain forms in
which the seeds have a violet outer layer and a sugary endosperm, are
found to transmit these characters to the resulting seed of a variety
which would normally have produced seed with white outer layers and
starchy endosperm, if the pollen of the former is used on the stigma of
the latter. Such direct influences of the pollen are termed _Xenia_.
[Illustration: Fig. 37. Pollen-grains adherent to the papillæ of the
stigma, on which they germinate, sending the pollen-tubes down between
the cells. Kerner. × about 100.]
When the pollen-grain, having adhered to the hairy stigma, has begun
to germinate, the resulting pollen-tube creeps down between the cells
of the stigma, and hands over its enclosed nuclei to the embryo-sac,
where fertilisation of the egg-cell is accomplished, by the fusion
of one of the pollen nuclei with the nucleus of the egg-cell. As the
resulting embryo developes, the sac becomes filled with endosperm-cells
charged with starch-grains or sugar, and in the ripe seed the embryo is
always found affixed laterally and below to this endosperm--a point of
distinction from Sedges, where the embryo is buried in the endosperm.
The ripe seed fills the ovary, and its outer walls usually fuse with
those of the carpel, forming the well-known _Caryopsis_ or “grain.”
[Illustration: Fig. 38. Longitudinal median section of the caryopsis
of a grass--e.g. _Lolium_ (× about 35). _p_, pericarp; _t_, attachment
to axis; _m_, position of micropyle; _E_, endosperm; _A_, its aleurone
layer; _l_, folded leaves of plumule; _r_, radicle; _r´´_, secondary
roots; _RC_, root-cap; _s_, scutellum. The dark line _e_ represents the
surface where the face of the scutellum is applied to the endosperm and
where absorption of the latter takes place.]
If such a “grain” is carefully examined, three chief parts are visible
in addition to the embryo. (Fig. 38.) Firstly, we find on the outside
the fused seed- and fruitcoats, differing in the number of layers and
in the microscopic characters of the cells, some of which characters
can be employed in diagnoses. (Fig. 39.)
Secondly, the great mass of the “grain” internally is composed
of delicate cells filled in most cases with starch-grains, the
sizes, shapes and arrangement of which can also be employed for
diagnoses--e.g. the compound grains of the Aveneæ and Festuceæ
are different from the simple polyhedral or rounded grains of the
Andropogoneæ and Maydeæ, and some races of Maize have sugar and soluble
starch instead of grains of the latter.
[Illustration: Fig. 39. Transverse section (highly magnified) through a
grain of _Brachypodium pinnatum_ taken about half-way up. 1, epidermis;
2, pericarp; 3, remains of the true seed-coat; 4, vascular bundle; 5,
remains of nucellus; _P_, epidermis of nucellus; _G_, aleurone layer:
remarkable in being several cells thick; _E_, endosperm. Harz.]
Thirdly, the outermost layer or layers of cells of the endosperm are
filled with proteids, and are known as the Aleurone layer. (Fig. 38,
_A_.)
The embryo consists of the folded embryonic leaves in bud (plumule)
above, which will grow up on germination as the shoot or “spear,” the
short primary root (radicle) below, with in many cases two or more
secondary rootlets already showing in its tissues, and from the common
“collar” uniting these a more or less prominent shield-shaped organ
(scutellum) standing out laterally in contact with the endosperm, the
dissolved contents of which it absorbs on germination. (Fig. 38, _s_.)
Although typical grasses form a caryopsis as described, exceptions
occur. In the exotic _Sporobolus_, _Eleusine_, _Crypsis_ and
_Heleochloa_ the fruit becomes truly dehiscent, the seed being loose in
the fruit, and the latter opens and allows it to fall out; and in many
Bamboos the seed is loose in the _achene_, while in a few cases--e.g.
_Melocanna_--the fruit is fleshy and may be as big as a walnut.
Returning to the typical grasses. When the fruit ripens in the
spikelet, several events may happen.
[Illustration: Fig. 40. _Triodia decumbens. a_, “seed,” nat. size;
_b_, ditto, × 6. Note the basal hair-tuft and rachilla, and the
ciliate, toothed outer palea with a short awn-point, enclosing the
inner more delicate palea. Between the two lies the caryopsis. Nobbe.]
In most of our grasses the caryopsis comes away trapped between its
two paleæ, and the latter bring away with them the small piece of
the axis of the spikelet on which they stand: this bit of axis--the
_rachilla_--often affords valuable characters in diagnosis. (Fig. 41.)
It is the pair of paleæ enclosing their caryopsis which goes by the
name of “seed” in most of our grasses. (Fig. 40.)
In a few cases, however, e.g. _Panicum_, the spikelet comes away as a
whole, so that here the “seed” consists of the glumes, enclosing one,
two or more pairs of paleæ with their contained caryopses.
Even among our native grasses, however, cases occur where the
separation takes place below some of the glumes, and so the “seed,” as
met with in samples, consists of glumes as well as enclosed paleæ and
caryopsis--e.g. _Anthoxanthum_, _Alopecurus_, &c.--and some care is
necessary in examining grass “seed” in these circumstances (see p. 134).
[Illustration: Fig. 41. Diagram of a spikelet of a grass--e.g.
_Festuca_--comprising six matured flowers and their paleæ, embraced
by the two glumes (_g^1g^2_) at the moment of disarticulation as the
fruits ripen. The small piece of axis (_a_) left attached to each
segment is the rachilla _R_. At _p^2_ the tip of the inner palea is
visible protruding from the outer one _p^1_: in the rest it is still
enclosed in the latter.]
Interesting biological adaptations are met with in the distribution
of grass “seeds.” The very small and light fruits of _Agrostis_
easily fall and are scattered by the wind, but in many cases the
glumes (_Holcus_) or paleæ (_Briza_) are expanded and serve as “wings”
offering extensive surfaces to the wind. In _Arundo_, _Calamagrostis_,
_Aira_, &c., fine silky hairs attached to the rachilla serve a similar
function, reminding us of the _coma_ of true seeds and the _pappus_ of
Composites. In _Hordeum jubatum_ of the prairies, the axis breaks up
and the disarticulated portions with their attached tufts of fruits
are blown away by the wind, and something similar occurs in our own
_H. murinum_ to a less extent. In the exotic _Spinifex_ whole heads of
fruits are thus detached and blown over the sands as “tumble weeds.”
In _Stipa pennata_ we have an example of perhaps the most complex of
all such adaptations: the exceedingly long awn terminating the palea
is plumose at the upper end and twisted below, and the hard sharp
rachilla at the base of the fruit is furnished with short, stiff hairs
directed upwards. The plumed awn serves as a wind surface, the whole
fruit flying like an arrow through the air. The stiff hairs below serve
to fix the lower end between particles of soil, and by their alternate
drying and wetting, the warping of these and of the twisting and
untwisting awn drives the sharp base into the soil. (Fig. 42.) Similar
mechanisms exist in _Avena_ and others.
These bristles and awns also subserve dissemination in other ways,
especially by clinging to the wool and fur of sheep and other animals,
and cases occur where the twisting awns and reflexed hairs on the hard
pointed fruit-base drive the latter into the bodies of sheep with fatal
effects--e.g. _Stipa capillata_ in Russia, _S. spartea_ in America,
_Aristida hygrometrica_ in New Zealand, _Heteropogon contortus_ in New
Caledonia.
[Illustration: Fig. 42. Awned fruit of _Stipa_. The reflexed stiff
hairs and hard point favour penetration into the soil. The long twisted
awn performs hygroscopic movements, and its terminal plume offers
surface to the wind. Lubbock.]
The driving action of even small reflexed asperities on awns is well
illustrated by the fruits of _Hordeum_, which are often made by
children to creep up the sleeve.
When we come to examine the external features of the “seeds” of
grasses--usually the caryopsis enclosed in one or more paleæ, but
sometimes in glumes as well--the following diagnostic characters are of
importance.
The size varies from lengths of 2 mm. or less (_Poa_, _Aira_)
(Figs. 43, 44) to 15-20 mm. or more (_Arrhenatherum_) (Fig. 47)
and distinction must be made between the various characters of the
caryopsis devoid of its coverings (paleæ, glumes) and such as really
belong to the latter.
The caryopsis proper may be short and stout and devoid of a
groove--“Millet-seed” type (Fig. 45)--but is oftener elongated, like a
grain of Wheat, and then has the characteristic longitudinal groove on
the face opposite that where the embryo is situated (Fig. 46). Details
of shape--e.g. cylindroid, fusiform, flattened, &c.--are also of
diagnostic value.
[Illustration: Fig. 43. _Poa annua. d_, the “seed,” nat. size; _a_
and _b_, ditto, × about 7; _c_, caryopsis, natural size (above) and
× about 7. Note the ribs and silky keel. No “web.” Nobbe. Cf. Figs.
54-56.]
In many cases the “seed,” consisting of the caryopsis closely
compressed between the paleæ, is boat-shaped, e.g. _Lolium perenne_,
_Festuca elatior_. If this “boat” is long and narrow it may be termed
barge-shaped, e.g. _Brachypodium sylvaticum_, &c. (Figs. 71, 72): if
short, broad, and open or shallow, the term coracle-shaped seems to
apply, e.g. _Bromus arvensis_ (Fig. 73).
[Illustration: Fig. 44. _Aira cæspitosa._ The minute “seed” (nat. size)
is seen to the right, and the caryopsis (nat. size) to the left of _c_,
the caryopsis, × 8. _a_ and _b_, the “seed,” × 10. The basal awn is
about as long as the palea, and the rachilla is very hairy. Nobbe.]
[Illustration: Fig. 45. _Phleum pratense._ Millet-seed type. _a_,
“seed,” nat. size; _b_, ditto, × about 7; _d_, caryopsis, nat. size;
_c_, ditto, × 7. The rounded caryopsis in _b_ is only loosely covered
by one palea. Nobbe.]
[Illustration: Fig. 46. Boat-shaped “seed” of _Glyceria fluitans_. _a_,
nat. size; _b_ and _c_, × about 6. _d_, caryopsis, × about 10. Note the
5-7 strong nerves, and the slender cylindrical rachilla. Cf. Figs. 57
and 58. Nobbe.]
[Illustration: Fig. 47. _Arrhenatherum avenaceum._ The “double seed”
(_a_ and _b_) consists of two pairs of paleæ with their enclosures. The
lower pair (γ) are devoid of fruit, and the outer palea has a long
twisted and kneed basal awn: the upper pair (β) enclose the fruit,
and the palea has a short straight awn. Note the stiff basal hairs.
_a_, nat. size; _b_, × about 6; _c_, caryopsis, nat. size; _d_, ditto,
× 6. Nobbe.]
[Illustration: Fig. 48. _Lolium temulentum._ Type of boat-shaped “seed”
with sub-terminal awn arising from between two teeth. _c_, “seed,” nat.
size; _a_ and _b_, ditto, × 6. Note the large smooth rachilla. Nobbe.
Cf. Fig. 57.]
Further important distinctive characters are obtained from the absence
or presence of awns, and the peculiarities--length, stiffness, &c.--of
the latter. The awns may be straight, hooked, or kneed (i.e. sharply
bent); twisted or not; smooth or serrulate. They may be inserted at the
base or near the middle of the back of the investing palea, or glume,
or near its apex, as indicated by the terms basal, dorsal, sub-terminal
(see Figs. 47-50).
Some difficulty arises in connection with terminal awns. In some
cases there is a true awn--i.e. a distinct bristle or hair--at the
apex of the palea, and apparently continuing its substance without
interruption; but in most instances close examination shows that this
awn arises from between two minute teeth, and is really inserted at the
back of the slight depression between them--e.g. _Lolium temulentum_
(Fig. 48), _Brachypodium pinnatum_ (Fig. 77), &c.
In another class of cases the awn appears to be really the prolongation
of the palea--e.g. _Nardus_ (Fig. 81), _Festuca Myurus_ (Fig. 80),
&c.--and when it runs out into a distinct bristle we may speak of a
terminal awn without staying to discuss whether or no it is really
terminal in development.
[Illustration: Fig. 49. _Agrostis Spicaventi. c_, “seed,” nat.
size; _a_ and _b_, ditto, × about 9. The long slender awn is inserted
below the bifid tip of the palea. Nobbe.]
In _Arundo_, _Cynosurus_ and some Fescues, where the palea tapers off
into a stiff long point, I have not spoken of it as an awn, but have
described the palea as tapering into a sharp point (awn-point). It
must be admitted that the distinction is somewhat artificial, but it
has its advantages in practice.
[Illustration: Fig. 50. _Bromus sterilis. a_, “seed,” nat. size; _b_
and _c_, ditto, × about 2. Nobbe. Cf. Figs. 80 and 81.]
[Illustration: Fig. 51. _Festuca sylvatica. a_, “seed,” nat. size;
_b_ and _c_, ditto, × 8. Nobbe. Cf. Fig. 78.]
The rachilla (Fig. 41), when present, often affords good characters,
and in a few cases is relied on for the distinction of “seeds”
otherwise much alike--e.g. _Lolium perenne_ and _Festuca elatior_; and
similarly with the presence or absence of hairs (_Digraphis_, _Arundo_,
&c.) or “web” (_Poa_) at the base of the “seed.” (Figs. 55, 69.)
On germination the primary root of the embryo usually emerges and
at once plunges into the soil, but soon ceases to grow, and the
secondary roots (and subsequently adventitious rootlets from the lower
internodes) soon give the peculiar fibrous character so well known in
grass-roots.
The plumule either pushes out from the same end of the caryopsis as
the root (e.g. Wheat, Rye) or drives its way between the coats to the
opposite end (e.g. Barley) and appears as a pointed cylinder of rolled
leaves (the “spear”) the outermost of which is sheath only, no lamina
being developed. As Darwin showed, the pointed apex of such a plumule
is hardened, facilitating the piercing of the soil above, and when the
blade attains the open air it performs spiral movements during growth,
indicating that similar attempts to rock from side to side have aided
the plumule in forcing its way through the soil. It has also been shown
that the apices of roots and plumules are sensitive to differences of
temperature, of light-intensity, moisture and contact, all of which aid
the seedling in establishing its position in the soil and in liberating
the “spear.” For particulars the student should read Darwin’s _Power of
Movement in Plants_.
With regard to the established seedlings of grasses, many interesting
details of structure are to be found in them. I have not sufficient
material to draw up a diagnostic arrangement of grass-seedlings, but it
is evident that such could be done. It may be useful to illustrate this
by the following tabular view of the characters of the larger seedlings
of our common cereals, in part adapted from Vesque and Percival; though
I find that some variations may occur, especially in the development of
the auricles.
SEEDLINGS.
I. First expanded aerial leaves broad, bright green, with 18-24 ribs.
_Barley._
The blade tends to twist to the left: auricles, when well developed,
long, pointed, and claw-like, embracing the glabrous sheath in
front. Ligule long, membranous, pointed and irregularly toothed. The
plumule emerges at the upper end of the grain, and the embryo has 5-6
secondary roots.
II. First aerial leaves narrower, with not more than 11-13 ribs.
A. _Ligule long, rounded and toothed._
_Wheat._
The blade tends to twist to the left: auricles filiform and embrace
the densely pubescent sheath. Leaves green. Embryo with three roots.
B. _Ligule short and toothed. Leaves with a reddish cast._
(_a_) Blade and sheath softly hairy, the latter with scattered
long hairs, the former tending to twist to the left.
_Rye._
The first leaf-sheath purple. Auricle-claws more triangular and
smaller than those of wheat, and the accompanying bristles are fewer
and shorter. Embryonic roots four.
(_b_) Blade and sheath glabrous or merely ciliate or silky, the
former tending to twist to the right.
_Oat._
Auricles filiform. The plumule emerges above. The embryo has three
roots.
A curious phenomenon is observed in some grasses growing in high
latitudes, or mountainous regions, or in moist situations. The
flowers, or even entire spikelets, grow out into minute leafy buds,
with rudimentary roots at the base, and fall off like the bulbils
of other monocotyledons, taking root directly in the damp soil.
The phenomenon must be looked upon as a case of apogamy, since the
development of sexual organs is entirely passed over; the parts which
would normally have become ovary and stamens being transformed into
leaves. In some species or varieties--e.g. _Poa alpina_, _Festuca
ovina_--this viviparous condition may coexist with normal flowers
and spikelets; in others--e.g. _Poa laxa_, var. _stricta_--only the
viviparous state occurs.
In the following arrangement the student should note that the terms
“Seed” and “Fruit” are used in the ordinary sense of the farmer and
seedsman: by the former is meant the “seed” as it comes in samples
into the market, when the true fruit or grain (_Caryopsis_) is almost
invariably invested by adherent “chaff”--i.e. paleæ or glumes or
both. When the word Caryopsis is employed, I mean it strictly in the
botanical sense explained above. In _Hordeum_, for example, we never
see the true fruit, the grain consisting of the caryopsis with paleæ
so closely adherent to it, that we are apt to take them as part of
the grain itself. The true seed, in the strict botanical sense, is
never seen as a naturally separate organ in our native grasses; and,
as already explained, only very few exotic grasses ever shed it--e.g.
_Sporobolus_.