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
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.