Page:EB1911 - Volume 11.djvu/273

Plants in which there are two cotyledons produced in the embryo are dicotyledonous. The two cotyledons thus formed are opposite to each other (figs. 42 and 45), but are not always of the same size. Thus, in Abronia and other members of the order Nyctaginaceae, one of them is smaller than the other (often very small), 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 pseudomonocotyledonous. The texture of the cotyledons varies. They may be thick, as in the pea (fig. 42), exhibiting no 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 laminae, flattened on both sides, and having distinct venation, as in Ricinus, Jatropha, Euonymus, &c. 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.

Fig. 43.		Fig. 46. Fig. 47.
Fig. 44.	Fig. 45.	Fig. 48.
Fig. 43.—Seed of Pansy (Viola tricolor) cut vertically. The embryo pl is axial, in the midst of fleshy endosperm al. The seed is anatropal, and the embryo is homotropal; the cotyledons co point to the base of the nucellus or chalaza ch, while the radicle, or the other extremity of the embryo, points to the micropyle, close to the hilum h. The hilum or base of the seed, and the chalaza or base of the nucellus are united by means of the raphe r.
Fig. 44.—Seed of the Red Campion (Lychnis), cut vertically, showing the peripheral embryo, 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 (amphitropal).
Fig. 45.—Mature dicotyledonous embryo of the Almond, with one of the cotyledons removed. r, Radicle; t, young stem or caulicle; c, one of the cotyledons left; i, line of insertion of the cotyledon which has been removed; g, plumule.
Fig. 46.—Exalbuminous seed of Wallflower (Cheiranthus) cut vertically. The radicle r is folded on the edges of the cotyledons c which are accumbent.
Fig. 47.—Transverse section of the seed of the Wallflower (Cheiranthus), showing the radicle r folded on the edges of the accumbent cotyledons c.
Fig. 48.—Transverse section of the seed of the Dame’s Violet (Hesperis). The radicle r is folded on the back of the cotyledons c, which are said to be incumbent.

Cotyledons are usually entire and sessile. But they occasionally become lobed, as in the walnut and the lime; or petiolate, as in Geranium molle; 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 are twisted and doubled; in convolvulus they are corrugated; and in the potato and in Bunias, they are spiral,—the same terms being applied as to the foliage leaves. The radicle and cotyledons are either straight or variously curved. Thus, in some cruciferous plants, as the wallflower, the cotyledons are applied by their faces, and the radicle (figs. 46, 47) is folded on their edges, so as to be lateral; the cotyledons are here accumbent. In others, as Hesperis, the cotyledons (fig. 48) 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.

In many gymnosperms more than two cotyledons are present, and they are arranged in a whorl. This occurs in Coniferae, especially in the pine, fir (fig. 49), 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 termed polycotyledonous. In species of Streptocarpus the cotyledons are permanent, and act the part of leaves. One of them is frequently largely developed, while the other is small or abortive.

Fig. 49.	Fig. 50.	Fig. 51.	Fig. 52.
Fig. 49.—Polycotylodonous embryo of the Pine (Pinus) beginning to sprout. t, Hypocotyl; r, radicle. The cotyledons c are numerous. Within the cotyledons the primordial leaves are seen, constituting the plumule or first bud of the plant.
Fig. 50.—Embryo of a species of Arrow-grass (Triglochin), 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. 51.—Grain of wheat (Triticum) germinating, showing (b) the cotyledon and (c) the rootlets surrounded by their sheaths (coleorrhizae).
Fig. 52.—Embryo of Caryocar. t, Thick hypocotyl, forming nearly the whole mass, becoming narrowed and curved at its extremity, and applied to the groove s. In the figure this narrowed portion is slightly separated from the groove; c, two rudimentary cotyledons.

In those plants in which there is only a single cotyledon in the embryo, hence called monocotyledonous, 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. 50). 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 (figs. 22, 51). 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 an outgrowth from the base of the cotyledon, 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 prolongations 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 (see Grasses). In many aquatic monocotyledons (e.g. Potamogeton, Ruppia and others) there is a much-developed hypocotyl, which forms the greater part of the embryo and acts as a store of nutriment in germination; these are known as macropodous embryos. A similar case is that of Caryocar among Dicotyledons, where the swollen hypocotyl occupies most of the embryo (fig. 52). In some grasses, as oats and rice, a projection of cellular tissue is seen upon the side of the embryo opposite 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 is now generally regarded as an outgrowth of the sheath of the true cotyledon.  (A. B. R.) 

FRUIT AND FLOWER FARMING. The different sorts of fruits and flowers are dealt with in articles under their own headings, to which reference may be made; and these give the substantial facts as to their cultivation. See also the article Horticulture.

Great Britain

The extent of the fruit industry may be gathered from the figures for the acreage of land under cultivation in orchards and small fruit plantations. The Board of Agriculture returns concerning the orchard areas of Great Britain showed a continuous expansion year by year from 199,178 acres in 1888 to 234,660 acres in 1901, as will be learnt from Table I. There was, it is true, an exception in 1892, but the decline in that year is explained by the circumstance that since 1891 the agricultural returns have been collected only from holdings of more than one acre, whereas they were previously obtained from all holdings of a quarter of an acre or more. As there are many holdings of less than an acre in extent upon which fruit is grown, and as fruit is largely raised also in suburban and other gardens which