Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/166

Rh 156 BOTANY [rtEPIlODUCTlVE ORGANS. tlio embryo, especially in Monocotyledons, and these arise as cellular processes from the outer or pericambial layer of the plerorne in the hypocotyledonary portion of the axis. The primary root of the embryo is, by reason of its forma tion, always directed to the micropyle In Monocotyledons and some Dicotyledons the young root rises deep in the tissues of the embryo, which, when it sprouts, form a sheath or coleorhiza around it. The lateral roots have also sheaths. The embryo then consists of cotyledons (figs. 52, 53, 58, c), or the first leaves of the plant produced upon a cellular axial mass. To that part of this axis immediately beneath the cotyledons the terms hypocotyledonary portion, caulicle, or tigellus (t) have been applied, and continuous backwards with it is the young root or radicle (r), the descending axis, their point of union being the collum, collar, or neck. The terminal growing bud of the axis is called the plumule or gemmule (g), and represents the ascending axis. That extremity of the embryo which produces the first leaves or cotyledons is called the cotyledonary extremity of the embryo, while the opposite is the radicular extremity. The radicular extremity being continuous with the suspensor points towards the micropyle (fig. 318), or the summit of the nucleus, an important fact in practical botany; while the cotyledonary extremity is pointed towards the base of the nucleus or the chalaza. Hence, by ascertaining the position of the micropyle and chalaza, the two extremities of the embryo can in general be discovered. In some rare instances, in consequence of a thickening in the coats of the seed, as in Ricinus and some other Euphorbiacese, there is an alteration in the micropyle, so that the radicle does not point directly to it. It is in many cases difficult to recognize the parts in an embryo ; thus in Cuscuta, the embryo appears as an elongated axis without divisions ; and in Caryocar butyro- sum, the mass of the embryo is made up by the radicular extremity and tigelle, in a groove of which the cotyledonary extremity lies embedded. In some Moriocotyledonous embryos, as in Orchidaceae, the parts of the embryo of the ripe seed are not differentiated, and only become so after germination. In parasitic plants which form no chloro phyll, as Orobanche, Monotropa, &c., the embryo remains without differentiation, consisting merely of a mass of cells until the ripening of the seed. When the embryo is surrounded by the endosperm on all sides except its radicular extremity it becomes internal or intrarius ; when lying outside the endosperm, and only coming into contact with it at certain points, it is external or extramus. When the embryo follows the direction of the axis of the seed, it is axile or axial (fig. 318), and it may be either external, so as to come into contact with the endosperm only by its cotyledonary apex, or internal. In the latter case the radicular extremity may, as in some Conifers, become incorporated with the endosperm apparently by means of a thickened suspensor. When the embryo is not in the direction of the axis, it becomes abaxile or abaxial ; and in this case it may be either straight or curved, internal or external. In campylotropal seeds the embryo is curved, and in place of being embedded in endosperm, is frequently external to it, folio wing the concavity of the seed (fig. 319), and becoming peripherical, with the chalaza situated in the curvature of the embryo, as in Caryophyllacese It has been already stated that the radicle of the embryo is directed to the micropyle, and the cotyledons to the chalaza. In some cases, by the growth of the integuments, the former is turned round so as not to correspond with the apex of the nucleus, and then the embryo has the radicle directed to one side, and is called excentric, as is seen in Primulaceae, Plantaginaceae, and many Palms, especially the Date. The position of the embryo in iifferent kinds of seeds varies. In an orthotropal seed the embryo is inverted or antitropal, the radicle pointing to the apex of the seed, or to the part opposite the hiluiu. Again, in an anatropal seed the embryo is erect or homo- tropal (fig. 318), the radicle being directed to the base of the seed. In some anatropal seeds, as in Castor oil, tho exostome is thickened or carunculate, and the endostomo does not correspond exactly to it, so that the radicle of the embryo is directed to a point a little removed from the exostome. In curved or campylotropal seeds the embryo is folded so that its radicular and cotyledonary extremities are approximated, and it becomes ampkitropal. In this instance the seed may be exalbuminous, and the embryo may be folded on itself ; or albuminous, the embryo surrounding more or less completely the endosperm, and being peripherical. According to the mode in which the seed is attached to the pericarp, the radicle may be directed upwards or downwards, or laterally, as regards the ovary. In an orthotropal seed attached to the base of the pericarp it is superior, as also in a suspended anatropal seed. In other anatropal seeds the radical is inferior. When the seed is horizontal as regards the pericarp, the radicle is either centrifugal, when it points to the outer wall of the ovary ; or centripetal, when it points to the axis or inner wall of the ovary. Fig. 319. Fie. 3:20. Fig. 322. FIG. 318. The seed of tho Pansy (Viola tricolor) cut vertically. The embryo pi is axial, in the midst of fleshy albumen al. The seed is anatropul, and the embryo is homotropal ; tho cotyledons co point to the base of the nucleus or eh iilax.a c/r, while the radicle, or the other extremity of the embryo, points to the foramen, close to the hilum 7i. The hilum or base of the seed, and thu chalaza or base of the nucleus are united by means of the raphe r. FIG. 319. Seed of the Red Campion (Lychnis), cut vertically, showing the peripherical embiyo, with its two cotyledons and its radicle. The embryo is curved round the albumen, so that its cotyledons and radicle both come near the hilum. The embryo is sometimes called ampliitropal. FIG. 320. Mature Dicotyledonous embryo of the common Almond, with one of the cotyledons removed; r, radicle; (, tiRelle or cuulicule; c, one of the cotyledons left; ie, cicatrix left at the place where the other cotyledon was attached; y, gemmulc composed of several small leaves. FIG. 321. Transverse section of the seed of the Wallflower (Cheiranthus,). Tho radicle r is folded on the edges of the cotyledons c, which are said to be accumbent. FIG. 322. Transverse section of the seed of the Dame s Violet (Hesperis) The radicle r is folded on the back of the cotyledons c, which arc said to be incum bent Plants in which there are two cotyledons produced in the embryo are Dicotyledonous. The form of this embryo varies much ; and although sometimes resembling in its general aspect that of Monocotyledons, yet it is always distinguished by a division taking place at the cotyledonary extremity (fig. 317), by which two, more or less evident, lobes are formed. The two cotyledons thus formed are opposite to each other (figs. 58 and 320). The cotyledons are not always, however, of the same size. Thus, in a species of Hiraja, one of them is smaller than the other ; and in Carapa guianensis there appears to be only one, in consequence of the intimate union which takes place between the two. The union between the cotyledonary leaves may continue after the young plant begins to germinate. Such embryos have been called pseudo- monocotyledonous. The texture of the cotyledons varies. They may be thick, as in the Pea (fig. 58), exhibiting only slight traces of venation, with their flat internal surfaces in contact, and their backs more or less convex ; or they may be in the form of thin and delicate laminse,