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called the nucleus ; and consists either of albumen and embryo, or of the latter only.
The albumen (perispermium, Juss. ; endospermium, Rich.; medulla seminis, Jungius; secundina interna, Malpighi) (Plate VI. fig. 5. a, 1. a, 9. a, &c.), when present, is a body enclosing the embryo, and interposed between it and the integuments of the seed when there are any : it is of different degrees of hardness, varying from fleshy to bony, or even stony, as in some palms. It is in all cases destitute of vascularity, and has been usually considered as the amnios in an indurated state : but Brown is of opinion that it is formed by a deposition or secretion of granular matter in the cellules of he amnios, or in those of the nucleus itself.
The former origin is certainly that of Santalum, Viscum, and Loranthus, as traced in the progress of its formation by Mr. Griffith; but it is deserving of enquiry whether bodies of very different natures and origins are not confounded under the common name of albumen. Has, for example, the albumen of Ranunculus, and other “albuminous" exogens, the same origin as that of Solanaceæ, Scrophulariaceæ, &c. ?
The albumen is often absent, is frequently much smaller than the embryo, but is also occasionally of much greater size. This is particularly the case in monocotyledons, in some of which the embryo scarcely weighs a few grains, while the albumen weighs many ounces, as in the cocoa-nut. It is almost always solid, but in Anonacex and Myristicaceæ it is perforated in every direction by dry cellular tissue, which appears to originate in the remains of the nucleus in which the albumen has been deposited: in this state it is said to be ruminated.
The embryo (or corculum) (Plate VI. fig. 1. b, &c.) is a fleshy body occupying the interior of the seed, and constituting the rudiment of a future plant. In most plants one embryo only is found in each seed. It nevertheless occurs, not unfrequently, that more than one is developed within a single testa, as occasionally in the Orange and the Hazel nut, and very commonly in Coniferæ, Cycas, the Onion, and the Mistletoe. Now and then a union takes place of these embryos.
It is distinguished into three parts; viz., the radicle (Plate VI. fig. 2.b, &c.) (rhizoma or rostellum); cotyledons (fig. 2. a, &c.); and plumule (or gemmule) (fig. 2. c.); from which is also by some distinguished the cauliculus or neck (scapus, scapellus, or tigelle). Mirbel admits but two principal parts; viz., the cotyledons, and what he calls the blastema, which comprises radicle, plumule, and cauliculus.
Upon certain differences in the structure of the embryo, modern botanists have divided the whole vegetable kingdom into three great portions, which form the basis of what is called the natural system. These are, 1. Dicotyledons ; 2. Monocotyledons; and, 3. Acotyledons. In order to understand exactly the true nature of the embryo in each of these, it will be requisite first to describe it fully as it exists in dicotyledons, and then to explain its organisation in the two others.
If a common DicotyLEDONOus embryo (Plate VI. fig. 2.), that of the Apple for example, be examined, it will be found to be an obovate, white, fleshy body, tapering and solid at the lower end, and compressed and deeply divided into two equal opposite portions at the upper end; the lower tapering end is the radicle, and the upper divided end consists of two cotyledons. Within the base of the cotyledons is just visible a minute point, which is the plumule. The imaginary line of division between the radicle and the cotyledons is the caulicule. If the embryo be placed in circumstances favourable for germination, the following phenomena occur: the caulicule will extend so as to separate the cotyledons from the radicle by an interval, the extent of which varies in different plants ; the radicle will become elongated downwards, forming a little root; the cotyledons will either elevate themselves above the earth and unfold, or, remaining under ground, will part with their amylaceous matter and shrivel up; and the plumule will lengthen upwards, giving birth to a stem and leaves. Such is the normal or proper appearance of a dicotyledonous embryo.
The exceptions to it chiefly consist, 1. in the cohesion of the cotyledons in a single mass, instead of their unfolding; 2. in an increase of their number; 3. in their occasional absence ; and, 4., in their inequality. A cohesion of the cotyledons takes place in those embryos which Gærtner called pseudomonocotyledonous, and Richard macrocephalous. In the Horsechestnut, the embryo consists of a homogeneous undivided mass, with a curved horn-like prolongation, of one side directed towards the hilum. If a section be made in the direction of the axis of the horn-like prolongation, through the whole mass of the embryo, a slit will be observable above the middle of the horn, at the base of which lies a little conical body. In this embryo the slit indicates the division between the two bases of a pair of opposite confluent cotyledons; the conical body is the plumule, and the horn-like prolongation is the radicle. In Castanea nearly the same structure exists, except that the radicle, instead of being curved and exserted, is straight, and enclosed within the projecting base of the two cotyledons; and in Tropæolum, which is very similar to Castanea in structure, the bases of the cotyledons, are slit into four little teeth enclosing the radicle. The germination of these seeds indicates more clearly that the cotyledonary body consists of two and not of one cotyledon ; at that time the bases of the cotyledons, which had been previously scarcely visible, separate and lengthen, so as to extricate the radicle and plumule from the testa, within which they had been confined.
In number the cotyledons vary from two to a much more considerable number, four occur in Boraginaceæ, Brassicaceæ and elsewhere; in Coniferæ they vary from two to more than twelve.
Instances of the absence of cotyledons occur, 1. in Cuscuta (Plate VI. fig. 19.), to which they may be supposed to be deficient in consequence of the absence of leaves in that genus; 2. in Lentibulaceæ; and, 3., in Cyclamen, in which the radicle enlarges exceedingly. To these a fourth instance has by some been added in Lecythis, of which Richard gives the following account: - The kernel is a fleshy almond-like body, so solid and homogeneous that it is extremely difficult to discover its two extremities until germination takes place: at that period one of the ends forms a little protuberance, which subsequently bursts through the integuments of the seed, and extends itself as a root; the other end produces a scaly plumule, which in time forms the stem. The great mass of the kernel is supposed by Richard to be an enlarged radicle. I, however, see no reason for calling the two-lobed part of the embryo (Plate VI. fig. 17. c) a plumule, instead of cotyledons. An inequality of cotyledons is the most unusual circumstance with dicotyledons, and forms a distinct approach to the structure of monocotyledons: it occurs in Trapa and Sorocea, in which they are extremely disproportionate. In Cycas they are also rather unequal; but in a much less degree.
The embryo of MONOCOTYLEDONS (Plate VI. fig. 1, B, &c.) is usually a solid, cylindrical, undivided, homogeneous body, slightly conical at each extremity, with no obvious distinction of radicle, plumule, or cotyledons. In germination the upper end swells and remains within the testa (fig. 10. cb, &c.); the lower lengthens, opens at the point, and emits one or more radicles : and a thread-like green body is protruded from the upper part of the portion which is lengthened beyond the testa. Here the portion remaining within the testa is a single cotyledon; the body which lengthens, producing radicles from within its point, is the cauliculus; and the threadlike protruded green part is the plumule. If this is compared with the germination of dicotyledons, an obvious difference will be at once perceived in the manner in which the radicles are produced: in monocotyledons they are emitted from within the substance of the radicular extremity, and are actually sheathed at the base by the lips of the passage through which they protrude ; while in dicotyledons they appear at once from the very surface of the radicular extremity, and consequently have no sheath at their base. Upon this difference Richard proposed to substitute the term Endorhiza for monocotyledons, and Exorhize for dicotyledons. Some consider the former less perfect than the latter : endorhiza being involute, or imperfectly developed : exorhizæ evolute, or fully developed. Dumortier adds to these names endophyllous and exophyllous ; because the young leaves of monocotyledons are evolved from within a sheath (coleophyllum or coleoptilum), while those of dicotyledons are always naked. The sheath at the base of the radicle of monocotyledons is called the coleorhiza by Mirbel. Another form of monocotyledonous
embryo is that of Araceæ and their allies, in which the plumule is not so intimately combined with the embryo as to be undistinguishable, but is indicated externally by a little slit above the base (Plate VI. fig. 6. B e), within which it lies until called into developement by germination.
The exceptions to what has been now described ought, like those of dicotyledons, rather to be called remarkable modifications. Much stress has been laid upon them by several writers, who have thought it requisite to give particular names to their parts. To me, however, it appears
far advisable to explain their analogies without the unnecessary creation of new and bad names. In Graminaceæ (Plate VI. fig. 4.) the embryo consists of a lenticular body lying on the outside of the base of the albumen on one side, and covered on its inner face by that body, and on its outer face by the testa : if viewed on the face next the testa, a slit will be observed of the same nature as that in the side of the embryo of Araceæ; within this cleft a small conical projection is discovered, pointing towards the apex of the seed. If the embryo be then divided vertically through the conical projection, it will be seen that the latter (c) is a sheath including other little scales resembling the rudiments of leaves; that that part of the embryo which lies next the albumen (d), and above the conical body, is solid; and that the lower extremity of the embryo (e) contains within it the indication of an internal radicle, as in other monocotyledons. In this embryo it is to be understood that the conical projection is the plumule ; that part of the embryo lying between it and the albumen, a single scutelliform cotyledon ; and the lower point of the embryo, the radicle. In Wheat there is a second small cotyledon on the outside of the embryo, inserted a little lower down than the scutelliform cotyledon. This last is called scutellum by Gærtner, who thought it of the nature of vitellus. Richard considered the scutelliform cotyledon a particular modification of the radicle, and called it hypoblastus ; the plumule a form of cotyledon, or blastus ; the anterior occasional cotyledon a peculiar appendage, or epiblastus ; and the radicle a protuberance of the caulicule, or radiculoda. He, further, in reference to this opinion, termed embryos of this descrip