Fig. 293.—Wheat-grain germinating: g the plumule; b the first leaf succeeding the cotyledon; r1 the primary root; r2 lateral root.
Fig. 294.—Older seedling of the Wheat: s the second sheathing-leaf; l first foliage-leaf.
The ripe Grass-fruit, in some species of Bamboo, is a berry; in some other Grasses a nut with loosely lying seed, in some even a capsule, but otherwise a “caryopsis.” In some instances it is loosely enveloped by the pales (Oat), in others firmly attached to these (Barley), and finally, in others, “naked,” i.e. it is entirely free from the pales (Wheat and Rye). On the ventral side there is a groove (Fig. [288] E); on the anterior side (dorsal suture), which is turned towards the inner pale, it is convex, and at the base on this side, inside the testa, lies the embryo (Fig. [288] D). The apex of the fruit is often hairy (Fig. [293]). The skin (Fig. [291], a) is formed by the pericarp and testa, and in some cases (Barley) the pales also form the outer portion. The endosperm (b) is large, and formed of parenchymatous, starch-containing cells: aleurone (proteid) grains may also be found among them. When the starch-grains and the aleurone-grains adhere together the endosperm becomes “horny,” but is “floury” when the starch-grains lie loosely with air between them. In the most external region, just beneath the skin, 1–several layers of nearly cubical cells (filled principally with aleurone-grains and fat) are found, the aleurone-layer (Fig. [292]). The embryo (Fig. [291] c-d) contains large quantities of fatty oil; the large shield-like structure, attached to the embryo and turned inwards towards the endosperm (c), is the cotyledon (scutellum); it remains enclosed in the seed during germination, and dissolves the endosperm by means of the peculiar epithelial cells developed on the dorsal surface. The radicle, on germination, is obliged to perforate a mass of cells derived from the suspensor and which form the “root-sheath” (coleorhiza, Fig. [293]) round its base. In addition to the tap-root, lateral roots are frequently developed before germination; these quickly break through, and later on are followed by others which appear at the base of the leaf (Figs. [293], [294]).
The DISTRIBUTION OF THE FRUIT is most frequently effected by the wind. The spirally-twisted and hygroscopic awn which persists on the fruits of some species (Avena, Stipa, etc.) assists in their dissemination, and even helps to bury them in the ground.
The two preceding orders are more closely related to each other than they are to the Gramineæ.
The generic differences are chiefly founded on the form of the inflorescence, the number and sex of the flowers in the spikelets, the shape and relative length of the pales, awns, etc. In addition to these the structure of the fruit and seed presents a great many differences; some have compound starch-grains, while in others they are single; some have 1 layer of aleurone-cells, others have several (Fig. [292]), etc.
1. Bambuseæ. Tall Grasses with woody, very siliceous stems which bear many branches in the axils of the leaves. 6 stamens. Bambusa (Bamboo).
2. Oryzeæ. Oryza sativa (Rice) is a herbaceous marsh-plant, with panicle and small, 1-flowered spikelets, with two small glumes and two large, boat-shaped, strongly siliceous pales. 6 stamens.—Leersia. Lygeum. Pharus. Zizania aquatica.
3. Maydeæ. Zea mais (Indian-corn, Maize); the spikelets are unisexual; the ♂-spikelets in a terminal panicle; the ♀-spikelets closely crowded and arranged in many rows in a thick, axillary spike, enclosed by large sheathing-leaves. The ♀-spikelets are 1-(2-) flowered; the ovary bears one, long, filamentous style, with bifid stigma.—Euchlæna; Coix.