Section Xv. the Seed.

380. Seeds are the final product of the flower, to which all its parts and offices are subservient. Like the ovule from which it originates, a seed consists of coats and kernel.

381. =The Seed-coats= are commonly two (320), the outer and the inner. Fig. 414 shows the two, in a seed cut through lengthwise. The outer coat is often hard or crustaceous, whence it is called the Testa, or shell of the seed; the inner is almost always thin and delicate.

382. The shape and the markings, so various in different seeds, depend mostly on the outer coat. Sometimes this fits the kernel closely; sometimes it is expanded into a wing, as in the Trumpet-Creeper (Fig. 415), and occasionally this wing is cut up into shreds or tufts, as in the Catalpa (Fig. 416); or instead of a wing it may bear a Coma, or tuft of long and soft hairs, as in the Milkweed or Silkweed (Fig. 417). The use of wings, or downy tufts is to render the seeds buoyant for dispersion by the winds. This is clear, not only from their evident adaptation to this purpose, but also from the fact that winged and tufted seeds are found only in fruits that split open at maturity, never in those that remain closed. The coat of some seeds is beset with long hairs or wool. Cotton, one of the most important vegetable products, since it forms the principal clothing of the larger part of the human race, consists of the long and woolly hairs which thickly cover the whole surface of the seed. There are also crests or other appendages of various sorts on certain seeds. A few seeds have an additional, but more or less incomplete covering, outside of the real seed-coats called an

383. =Aril, or Arillus.= The loose and transparent bag which encloses the seed of the White Water-Lily (Fig. 418) is of this kind. So is the mace of the nutmeg; and also the scarlet pulp around the seeds of the Waxwork (Celastrus) and Strawberry-bush (Euonymus). The aril is a growth from the extremity of the seed-stalk, or from the placenta when there is no seed-stalk.

384. A short and thickish appendage at or close to the hilum in certain seeds is called a CARUNCLE or STROPHIOLE (Fig. 419).

385. The various terms which define the position or direction of the ovule (erect, ascending, etc.) apply equally to the seed: so also the terms anatropous, orthotropous, campylotropous, etc., as already defined (320, 321), and such terms as

HILUM, or Scar left where the seed-stalk or funiculus falls away, or where the seed was attached directly to the placenta when there is no seed-stalk.

RHAPHE, the line or ridge which runs from the hilum to the chalaza in anatropous and amphitropous seeds.

CHALAZA, the place where the seed-coats and the kernel or nucleus are organically connected,--at the hilum in orthotropous and campylotropous seeds, at the extremity of the rhaphe or tip of the seed in other kinds.

MICROPYLE, answering to the Foramen or orifice of the ovule. Compare the accompanying figures and those of the ovules, Fig. 341-355.

386. =The Kernel, or Nucleus=, is the whole body of the seed within the coats. In many seeds the kernel is all Embryo; in others a large part of it is the Albumen. For example, in Fig. 423, it is wholly embryo; in Fig. 422, all but the small speck (g) is albumen.

387. =The Albumen or Endosperm= of the seed is sufficiently characterized and its office explained in Sect. III., 31-35.

388. =The Embryo= or Germ, which is the rudimentary plantlet and the final result of blossoming, and its development in germination have been extensively illustrated in Sections II. and III. Its essential parts are the Radicle and the Cotyledons.

389. =Its Radicle or Caulicle= (the former is the term long and generally used in botanical descriptions, but the latter is the more correct one, for it is the initial stem, which merely gives origin to the root), as to its position in the seed, always points to and lies near the micropyle. In relation to the pericarp it is

Superior, when it points to the apex of the fruit or cell, and

Inferior, when it points to its base, or downward.

390. =The Cotyledons= have already been illustrated as respects their number,--giving the important distinction of Dicotyledonous, Polycotyledonous and Monocotyledonous embryos (36-43),--also as regards their thickness, whether foliaceous or fleshy; and some of the very various shapes and adaptations to the seed have been figured. They may be straight, or folded, or rolled up. In the latter case the cotyledons may be rolled up as it were from one margin, as in Calycanthus (Fig. 424), or from apex to base in a flat spiral, or they may be both folded (plicate) and rolled up (convolute), as in Sugar Maple (Fig. 11.) In one very natural family, the Cruciferae, two different modes prevail in the way the two cotyledons are brought round against the radicle. In one series they are

Accumbent, that is, the edges of the flat cotyledons lie against the radicle, as in Fig. 425, 426. In another they are

Incumbent, or with the plane of the cotyledons brought up in the opposite direction, so that the back of one of them lies against the radicle, as shown in Fig. 427, 428.

391. As to the situation of the embryo with respect to the albumen of the seed, when this is present in any quantity, the embryo may be Axile, that is occupying the axis or centre, either for most of its length, as in Violet (Fig. 429), Barberry (Fig. 48), and Pine (Fig. 56); and in these it is straight. But it may be variously curved or coiled in the albumen, as in Helianthemum (Fig. 430), in a Potato-seed (Fig. 50), or Onion-seed (Fig. 60), and Linden (Fig. 414); or it may be coiled around the outside of the albumen, partly or into a circle, as in Chickweed (Fig. 431, 432) and in Mirabilis (Fig. 52). The latter mode prevails in Campylotropous seeds. In the cereal grains, such as Indian Corn (Fig. 67) and Rice (Fig. 430a), and in all other Grasses, the embryo is straight and applied to the outside of the abundant albumen.

392. The matured seed, with embryo ready to germinate and reproduce the kind, completes the cycle of the vegetable life in a phanerogamous plant, the account of which began with the seed and seedling.