CHAPTER VI.. Grasses in Flower.

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).

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.

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.

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 (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.

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.

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.

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.