6. Semina, the seeds, fig 47, the most essential of all the organs of fructifica tion, being those to which all the others are subservient. The seeds are compos ed of several parts, the most important of which is the embryo, or germ. Linnmus calls it corcnlum, a little heart, in allusion to its shape in the walnut, in which, as well as in the bean, and other leguminous plants, it is readily observed. Its position is either upright, horizontal, or reversed, It is generally lodged within the sub stance, of the seed, except in grasses. Cotyledons, the cotyledons, or seed-lobes, are intimately connected with the em bryo ; they are almost universally two in number, though in the fir tribe they are more numerous. When the seed has sent its root into the ground, these organs ge nerally rise above the surface, and per form the functions of leaves till the proper foliage is produced. Plants, therefore, for the most part, are properly denominated dicotyledons. Such as are called monoco tyledons have really no proper cotyledon, and the first part that appears above the ground from their seed is a real leaf. Al bumen, the white, makes up the chief bulk of some seeds ; but never rises out of the ground, nor assumes the office of leaves, being destined solely to nourish the em bryo till its roots can perform their office.. It may be observed in grasses, corn, and palm-trees : in some it is farinaceous; in others as hard as a stone ; witness the date. The nutricious matter, which in these plants constitutes the albumen, is in others lodged in the substance of the cotyledons. Vitellus, the yolk, was first named by Gmrtner, and is supposed by him to furnish nourishment to the em bryo. Dr. Smith, however, has first sug gested, that the Vitellus is rather a subter raneous cotyledon, see his "Introduction to Botany," 292. Testa, the skin, a sin gle or double membrane, envelops the parts hitherto described, bursting irregu larly when its contents swell in germina tion. Mum, the scar, is the point of at tachment, through which nourishment is conveyed to the seed while growing. This point is always considered as the base of the seed in description.
Seeds are often accompanied by appen dages or accessory parts, as pellicula, the pellicle. which adheres to their outside in the form of a fine skin, sometimes downy, sometimes of a mucilaginous substance. An instance of the latter occurs in Salvia verhenaca, whose seeds are celebrated for extracting particles of dust from the eye, by enveloping them in its mucilage, which swells on the application of mois ture. Arillus, the tunic, is a complete or
partial covering of a seed, fixed tons base only, and more or less closely enfolding its other parts. In the euonymus it is pulpy and orange-coloured, the seed itself being crimson. The mace, which enfolds the nutmeg, is of this nature. Many of the orchis tribe are enveloped in a mem branous tunic, extending beyond the out line of the seeds, and giving them a light chaffy appearance. The elegant wood. sorrel has an elastic arillus, like a little bag, serving to project the seeds to a dis tance. In the carex the same part is in some degree inflated and membranous. The covering of the seeds in the cyno glossum is considered by Dr. Smith as a testa, rather than an arillus. Pappus, the seed-down, in its most strict sense, is the chaffy, feathery, or bristly crown of seve ral seeds that have no seed-vessel as in the dandelion, thistle, scabious, and others. In a more general sense, pappus is ap plied to any feathery or downy appen dage to seeds, even though lodged in a pericarpium. Cauda, a tail, is an elon gated appendage, originating from the permanent style. It is generally feathery, as in the virgin's bower, clematis. Ros trum, a beak, has a similar origin, but usually belongs to a seed-vessel. Ala, a wing, is a dilated membranous appen dage, serving to waft seeds along in the air. To all the above may be added va rious spines, hooks, scales, and crests, generally serving to attach such seeds as are furnished with them to the rough coats of animals, and so to promote their dispersion. This appears to be the final purpose of the awns of grasses in gene ral.
6. Receptaculum, fig. 48, the receptacle, is the common base or point of connection of the parts of fructification. It is essen tial, inasmuch as it must exist in some form or other. This part, however, comes chiefly into notice when it assumes any peculiar form, as in compound flowers ; the dandelion, daisy, and thistle, for in stance. In some of this class it is naked, scaly, hairy, or cellular, and such circum stances afford excellent generic charac ters. Such of the natural order of Pro teacere as have aggregate flowers are also furnished with as conspicuous a recepta cle as the compound flowers. The re ceptacle of the seeds is a term used for the part to which they are attached in a seed-vessel.