Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/164

Rh lo-i BOTANY [REPRODUCTIVE ORGANS. character, it has received the name of sarcosperm or sarcoderm. The nature of the testa depends upon that of the pericarp. When the pericarp is dehiscent then the seed-covering is of a strong and tough character; but when Fig. 309. Fio. 308. The seed of the Pea (Pisuni), deprived of one-half of its integument or spermodenn. The outer covering, called either cpispenn, exospenn, or testa, is marked te, the Inner, called endopleura. e. Within these integuments is the nucleus, consisting of cotyledons or seed-lobes c containing nourishing matter, the gemmule or young leaf-bud g, the radicle or young root r, the tigelle or stalk between root and bud t. The seed is attached to the placenta^)/ by a cord or funiculus /. The nourishing vessels rap enter the nucleus at the chalaza ch, and the root of the embryo points to the micropyle or foramen m. FIG. 309. Seed of Asclepias, with a cluster of hairs arising from the edges of the micropyle, and by some considered as a hairy aril. These hairs are for the purpose of scattering the seed. the pericarp is indehiscent and encloses the seed for a long period, its outer covering has a proportional softness. The cells of the testa are often coloured, and have projections and appendages of various kinds. Thus in Abrus precatorius and Adenanthera pavonina it is of a bright red colour ; in French beans it is beautifully mottled ; in the Almond it is veined ; in the Tulip and Primrose it is rough ; in the Snapdragon it is marked with depressions ; in Cotton and Asclepias (fig. 309) it has hairs attached to it; and in Mahogany, Bignonia, and Firs it is expanded in the form of wing-like appendages (fig. 310). In Collomia, Acantho- diura, and other seeds, it contains spiral cells, from which, when moistened with water, the fibres uncoil in a beautiful manner ; so also in Cobasa scandens and Galempelis scaber. In the testa of the seed of Ulmus campestris the cells are compressed, and their sinuous boundaries are traced out by minute rectangular crystals adhering to their walls ; and in such plants as the Flax (Linum) the cells are converted into mucilage. These structural peculiarities of the testa in different plants have relation to the scattering of the seed and its germination upon a suitable nidus. But in some plants the pericarps assume structures which subserve the same purpose; this especially occurs in small pericarps enclosing Jngle seeds, as achenes, caryopsides, &c. Thus Plate VII I. in Compositae (fig. 197) and Valerian, the pappose limb of the calyx forms a parachute to the pericarp ; in Labiate and some Compositoe spiral cells are formed in the epicarp; and the epicarp is prolonged as a wing in Fraxinus (fig. 281) and Acer (fig. 296). The endopleura (fig. 308,e) is usually a thin and transparent layer of cells underneath the testa, frequently incorporated with it. It represents the outer layer of the cells of the nucleus, constituting the tercine of the ovule, though frequently the embryo -sac is incorporated with it. Some times there is an additional covering to the seed, formed after fertilization, to which the name arilhts has been given (fig. 311). This is seen in the Passion-flower, where the covering arises from the placenta or extremity of the funiculus at the base of the ovule and passes upwards towards the apex, leaving the micropyle uncovered. In the Nutmeg and Spindle-tree this additional coat com mences at the side of the exostome, and is formed from above downwards, constituting in the former case a Plate XII. lacmiated scarlet covering called mace. In such instances it has been called by some an arillode (fig. 312). This anllode, after growing downwards, may be reflected upwards so as to cover the foramen. The fleshy scarlet covering formed around the naked seed in the Yew is by some considered of the nature of an aril. On the testa, at various points, there are produced at times cellular bodies, which are not dependent on fertilization, to which the name of strophioles, or caruncules, has been given, the Plate II seeds being strophiolate or carunculate. These tumours % 5 - may occur near the base of the seed, as in Polygala, or at tho apex, as in Bicinus, where they are swellings of the exostome ; or they may occur in the course of the raphe, as in Blood-root and Asarabacca. The funiculi of the ovules frequently attain a great length in the seed, and in some Magnolias, when the fsuit dehisces, they appear as long scarlet cords suspending the seeds outside. The liilum or umbilicus of the seed is usually well marked, as a scar of varying size, in the Calabar bean and in some species of Macuna and Dolichos, extending along a large portion of the edge of the seed ; and it frequently exhibits marked colours, being black in the Bean, white in many species of Phaseolus, &c. The micropyle (fig. 308, m) of the seed, with its exostome and endostome, may be recog nizable by the naked eye, as in the Pea and Bean tribe, Iris, &c., or it may be very minute or microscopic. It indicates the true apex of the seed, and is important as marking the point to which the root of the embryo is directed. At the micropyle in the Bean is observed a small process of integument, which, when the young plant sprouts, is pushed up like a lid, and is called cmbryotega. The chalaza (fig. 311, ch) is often of a different colour from the rest of the seed. In the Orange (fig. 313) it is of a reddish-brown colour, and is easily recognized at one end of the seed when the integuments are carefully removed. In anatropal seeds the raphe forms a distinct ridge along one side of the seed (fig. 314). Fit*. 310. Fig. 313. FIG. 310. Seed of Fir (Finns), with a membranous r.ppendage to to the testa, called a wing. The seed is said to be winged. FIG. 311. Young anatropal seed of the White Water-lily (NympJia a alba), cut vertically. It is attached to the placenta by the funiculus or umbilical cord f, cellular prolongations from which form an aril a a. The vessels of the cord are prolonged to the base of the nucleus n by means of the raphe r, composed of cells and spiral vessels. The base of tho nucleus is indicated by the chain/a eh, while the apex is at the micropyle m. The covering of the seed is marked t. n is the nucleus or perisperm, surrounded by its covering, and enclosing the embryo-sac, in which the endosperm is formed. The embryo e, with its suspensor. is contained in the sac, the radicle pointing to the micropyle m. FIG. 312. Arillode a, or false aril, of the Spindle-tree (Vuonymus), arising from the micropyle/. FIG. 313. Anatropal seed of the Orange (Citrus Aurantium) opened to show the chalaza c, which forms a brown spot at one end. The position of the seed as regards the pericarp resembles that of the ovule in the ovary, and the same terms are applied erect, ascending, pendulous, suspended, curved, &c. These terms have no reference to the mode in which the fruit is attached to the axis. Thus the seed may be erect while the fruit itself is pendent, in the ordinary meaning of that term. The part of the seed next the axis or the ventral suture is its face, the opposite side being the back. Seeds exhibit great varieties of form. They may be flattened laterally (compressed), or from above