CHAPTER VIII.

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_.

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.”

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.

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.

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).

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.

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.

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).

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.

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.

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_.