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| Fig. 3.—One-flowered spikelet of Agrostis. | Fig. 4.—Two-flowered spikelet of Aira. |
| b, Barren glumes; f, flowering glumes.(Both Enlarged.) | |
Inflorescence.—This possesses an exceptional importance in grasses, since, their floral envelopes being much reduced and the sexual organs of very great uniformity, the characters employed for classification are mainly derived from the arrangement of the flowers and their investing bracts. Various interpretations have been given to these glumaceous organs and different terms employed for them by various writers. It may, however, be considered as settled that the whole of the bodies known as glumes and paleae, and distichously arranged externally to the flower, form no part of the floral envelopes, but are of the nature of bracts. These are arranged so as to form spikelets (locustae), and each spikelet may contain one, as in Agrostis (fig. 3) two, as in Aira (fig. 4) three, or a great number of flowers, as in Briza (fig. 5) Triticum (fig. 6); in some species of Eragrostis there are nearly 60. The flowers are, as a rule, placed laterally on the axis (rachilla) of the spikelet, but in one-flowered spikelets they appear to be terminal, and are probably really so in Anthoxanthum (fig. 7) and in two anomalous genera, Anomochloa and Streptochaeta.
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| Fig. 5.—Spikelet of Briza. | Fig. 6.—Spikelet of Triticum. |
| (Both enlarged.) | |
| Fig. 7.—Spikelet of Anthoxanthum (enlarged) without the two lower barren glumes, showing the two upper awned barren glumes (g) and the flower. |
In immediate relation with the flower itself, and often entirely concealing it, is the palea or pale (“upper pale” of most systematic agrostologists). This organ (fig. 13, 1) is peculiar to grasses among Glumiflorae (the series to which belong the two families Gramineae and Cyperaceae), and is almost always present, certain Oryzeae and Phalarideae being the only exceptions. It is of thin membranous consistence, usually obtuse, often bifid, and possesses no central rib or nerve, but has two lateral ones, one on either side; the margins are frequently folded in at the ribs, which thus become placed at the sharp angles. This structure was formerly regarded as pointing to the fusion of two organs, and the pale was considered by Robert Brown to represent two portions soldered together of a trimerous perianth-whorl, the third portion being the “lower pale.” The pale is now generally considered to represent the single bracteole, characteristic of Monocotyledons, the binerved structure being the result of the pressure of the axis of the spikelet during the development of the pale, as in Iris and others.
The flower with its pale is sessile, and is placed in the axis of another bract in such a way that the pale is exactly opposed to it, though at a slightly higher level. It is this second bract or flowering glume which has been generally called by systematists the “lower pale,” and with the “upper pale” was formerly considered to form an outer floral envelope (“calyx,” Jussieu; “perianthium,” Brown). The two bracts are, however, on different axes, one secondary to the other, and cannot therefore be parts of one whorl of organs. They are usually quite unlike one another, but in some genera (e.g. most Festuceae) are very similar in shape and appearance.
| Fig. 8.—Spikelet of Stipa pennata. The pair of barren glumes (b) are separated from the flowering glume, which bears a long awn, twisted below the knee and feathery above. About ¾ nat. size. |
The flowering glume has generally a more or less boat-shaped form, is of firm consistence, and possesses a well-marked central midrib and frequently several lateral ones. The midrib in a large proportion of genera extends into an appendage termed the awn (fig. 4), and the lateral veins more rarely extend beyond the glume as sharp points (e.g. Pappophorum). The form of the flowering glume is very various, this organ being plastic and extensively modified in different genera. It frequently extends downwards a little on the rachilla, forming with the latter a swollen callus, which is separated from the free portion by a furrow. In Leptaspis it is formed into a closed cavity by the union of its edges, and encloses the flower, the styles projecting through the pervious summit. Valuable characters for distinguishing genera are obtained from the awn. This presents itself variously developed from a mere subulate point to an organ several inches in length, and when complete (as in Andropogoneae, Aveneae and Stipeae) consists of two well-marked portions, a lower twisted part and a terminal straight portion, usually set in at an angle with the former, sometimes trifid and occasionally beautifully feathery (fig. 8). The lower part is most often suppressed, and in the large group of the Paniceae awns of any sort are very rarely seen. The awn may be either terminal or may come off from the back of the flowering glume, and Duval Jouve’s observations have shown that it represents the blade of the leaf of which the portion of the flowering glume below its origin is the sheath; the twisted part (so often suppressed) corresponds with the petiole, and the portion of the glume extending beyond the origin of the awn (very long in some species, e.g. of Danthonia) with the ligule of the developed foliage-leaf. When terminal the awn has three fibro-vascular bundles, when dorsal only one; it is covered with stomate-bearing epidermis.
| Fig. 9 (left).—Spikelet of Leersia. f, Flowering glume; p, pale. |
| Fig. 10 (right).—Spikelet of Setaria, with an abortive branch (h) beneath it. b, Barren glumes; f, flowering glume; p, pale. |
The flower with its palea is thus sessile in the axil of a floriferous glume, and in a few grasses (Leersia (fig. 9), Coleanthus, Nardus) the spikelet consists of nothing more, but usually (even in uniflorous spikelets) other glumes are present. Of these the two placed distichously opposite each other at the base of the spikelet never bear any flower in their axils, and are called the empty or barren glumes (figs. 3, 8). They are the “glumes” of most writers, and together form what was called the “gluma” by R. Brown. They rarely differ much from one another, but one may be smaller or quite absent (Panicum, Setaria (fig. 10), Paspalum, Lolium), or both be altogether suppressed, as above noticed. They are commonly firm and strong, often enclose the spikelet, and are rarely provided with long points or imperfect awns. Generally speaking they do not share in the special modifications of the flowering glumes, and rarely themselves undergo modification, chiefly in hardening of portions (Sclerachne, Manisuris, Anthephora, Peltophorum), so as to afford greater protection to the flowers or fruit. But it is usual to find, besides the basal glumes, a few other empty ones, and these are in two- or more-flowered spikelets (see Triticum, fig. 6) at the top of the rhachilla (numerous in Lophatherum), or in uniflorous ones (fig. 10) below and interposed between the floral glume and the basal pair.