Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/167

Rh COTYLEDONS.] BOTANY 157 flattened on both sides, and having distinct venation, as in Ricinus, Jatropha, Euouymus, ite. In the former case they are called fleshy, or seminal lobes ; in the latter, foliaccous, or seminal leaves. The cotyledons usually form the greater part of the mature embryo, and this is remarkably well seen in such exalbuminous seeds as the Bean and Pea. Cotyledons are usually entire and sessile. But they occasionally become lobed, as in the Walnut and the Lime, where there are five lobes ; or petiolate, as in Geranium mollc ; or auriculate, as in the Ash. Like leaves in the bud, cotyledons may be either applied directly to each other, or may be folded in various ways. In Geranium the cotyledons a.s twisted and doubled; in Convolvulus they are corrugated ; and in the Potato and in B unias, they are spiral, the same terms being applied as to the foliage leaves. The radicle and cotyledons may be either straight or curved in various ways. Thus, in some Cruciferous plants, as the Wallflower, the cotyledons are applied by their faces, and the radicle (fig. 321) is folded 011 their edges, so as to be lateral; the cotyledons are here accumbent. In others, as the Hesperis, the cotyledons (fig. 322) are applied to each other by their faces, and the radicle, r, is folded on their back, so as to be dorsal, and the cotyledons are incumbent. Again, the cotyledons are conduplicate when the radicle is dorsal, and enclosed between their folds. In other divisions the radicle is folded in a spiral manner, and the cotyledons follow the same course. present, and they are arranged in a whorl. This occurs in Conifene, especially in the Fir (fig. 323), Spruce, and Larch, in which six, nine, twelve, and even fifteen have been observed. They are linear, and resemble in their form and mode of development the clustered or fasciculated leaves of the Larch. Plants having numerous cotyledons are occasion ally denominated polycotyledonoiis. In the gymnospermous genus Welwitschia, there are two cotyledons which last throughout its life (more than a century), and in the course of time they grow to an enormous size, being sometimes six feet long and two or three feet in breadth. They con stitute the only leaves of the plant. In species of Strepto- carpus the cotyledons are also permanent, and act the part of leaves. One of them is frequently largely developed, while the other is small or abortive. In those plants in which there is only a single cotyledon (fig. 53) in the embryo, hence called Monocotylcdonous, the embryo usually has a cylindrical form more or less rounded at the extremities, or elongated and fusiform, often oblique. The axis is usually very short compared with the cotyledon, which in general encloses the plumule by its lower portion, and exhibits on one side a small slit which indicates the union of the edges of the vaginal or sheathing portion of the leaf (fig. 324). In Grasses, by the enlargement of the embryo in a particular direction, the endosperm is pushed on one side, and thus the embryo comes to lie outside at the base of the endosperm. The lamina of the cotyledon is not developed. Upon the side of the embryo next the endosperm and enveloping it is a large shield- shaped body, termed the scutellum. This is by some authors considered to be an outgrowth from the hypocotyle- donary portion of the axis or expansion of the radicular extremity, enveloping more or less the cotyledon and plumule, in some cases, as in Maize, completely investing it ; in other cases, as in Rice, merely sending small prolon gations over its anterior face at the apex. By others this scutellum is considered as the true cotyledon, and the sheathing structure covering the plumule is regarded as a ligule or axillary stipule. In several other Monocotyle- douoiis plants, as Ruppia and Zostera, this scutellar struc ture is well seen, and in these casca its homulogy, as an expansion of the radicular extremity of the embryo, is clearly discernible ; and this is further borne out by such cases as that of Caryocar butyrosum among Dicotyledons, where the radicular swelling occupies most of the embryo (fig. 32G). In some Grasses, as Oats and Rice, a projection I m,. -e t Fig. 323. Fig. 324. Fig. o25. Fig. 32(5. FIG. 323. Folycotyledonous embryo of the Pine (Pinus} beginning to sprout- The axis ( shows its radicular portion r and cotyledonary portion c. The cotyledons c are numerous. Within the cotyledons the primordial leaves are seen, constituting the plumule or first bud of the plant. FIG. 324. Embryo of a species of Arrow-grass (Triylochin liarrtlierf), showing a uniform conical mass, with a slit s near the lower part. The cotyledon c envelops the young bud, which protrudes at the slit during germination. The radicle is developed from the lower part of the axis r. FIG. 325. Grain of Wheat (n-iticum) germinating. The embryo lies at one Bide of the grain g. The radicular port ion of the embryo gives off rootlets r r r rr, covered with cellular hairs. The pi incipal root is the central one ; the others being developed subsequently in succession. The roots pass through sheaths c c c. The ascending axis t, sheathed in the cotyledon, rises upwards. FIG. 326. Embryo of Caryocar butyrusum. &amp;lt;, thick tigelle or caulicule, forming nearly the whole mass, becoming narrowed and curved at its extremity, and applied to the groove *. In the figure this narrowed portioi is slightly separated from the groove ; c, two rudimentary cotyledons. of cellular tissue is seen upon the side of the embryo oppo site to the scutellum, that is, on the anterior side. This has been termed the epiblast. It is very large in Rice. This by some was considered the rudimentary second cotyledon ; but this is impossible, as it arises outside, and after the first cotyledon. It is merely an outgrowth of the radicular extremity like the scutellum. The radicular portion of the axis is usually shorter than the cotyledon, and more dense in structure ; but in some instances it becomes much larger, giving rise to what has been called a macropodow embryo. 5. Female Organs and Reproduction in Cryptogams. The female organs of reproduction, like the male organs, Female have not been demonstrated in all Cryptogams. In all yg aus in vascular Cryptogams, and in Mosses and Hepaticrc, certain Q!&quot;^ usually flask-shaped bodies, which have been termed gams. archegonia or pistillidia (fig. 327), represent the female organs producing cells or spores, which germinate and form new plants (fig. 54). These archegonia have the general structure of a large cell, the central cell or oosphere (c) surrounded by a layer of smaller cells. From the apex of this oosphere leads a canal, which is bounded by four rows of small cells and constitutes the neck, and in it is a large cell full of soft mucilaginous matter, which lias been formed from the central cell, and is the canal cell. Upon the wall of the oosphere, turned towards the neck, a small portion different from the surrounding part is the receptive spot. Fertilization is effected by the antherozoids freed from the antheridium, penetrating the neck of the archegonium, and eventually reaching the receptive spot of the central cell or oosphere ; they then enter the oosphere and coalesce with it. The fertilized oosphere is termed an oospore, and it then may escape from the archegonium, and sooner or later germinates ; or it germinates within the archegonial cells. These archegonia are, in vascular Crypto gams, produced upon a cellular expansion formed asexually from a spore, and termed the prothallits (fig. 245), which is of varying size and form. Both archegonia and antheridia may be formed on one prothallus, or only one form of organ may be produced, thus indicating a tendency to diclinism. In Characerc the female organ has a peculiar structure,
 * In many gymnosperms more than two cotyledons are