Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/165

SEED AND EMBRYO.] downwards (depressed). They may be round, oval, triangu lar, polygonal, rolled up like a snail, as in Physostemou, or coiled up like a snake, as in Ophiocaryon paradoxum. The endosperm formed in the embryo-sac of angiosperrns after fertilization, and found previous to it in gymnosperms, consists of cells containing protoplasmic substance and starchy or fatty matter, or both, destined for the nutriment of the embryo. It arises by free-cell division from the protoplasm of the embryo-sac, and may occupy the whole cavity of the embryo-sac, or may be formed only at certain portions of it, at the apex, as in llhinanthus, at the base, as in Vaccinium, or in the middle, as in Veronica. As the endosperm increases in size along with the embryo-sac and the embryo, the substance of the original nucleus of the ovule is gradually absorbed, and usually at last disappears except the layer constituting the endopleura, and in some cases this goes also. Sometimes, however, as in Mus- acea). Cannaceae, Zingiberacece, no endosperm is formed ; the cells of the original nucleus, becoming filled with food-materials for the embryo, arc not absorbed, but remain surrounding the embryo-sac with the embryo, and constitute the perisperm. Again, in other plants, as Nympheeaceae (fig. 311) and Piperacete, both endosperm and perisperm are present. It was from observations on cases such as these that old authors, imagining a resemblance betwixt the plant-ovule and the animal ovum, applied the name albumen to the outer nutrient mass or perisperm, and designated the endosperm as vitellus. The term albumen is very generally used as including all the nutrient matter stored up as a separate mass in the ovule, but it would be advisable to discard the name as implying a definite chemical substance. There is a large class of plants in which although at first after fertilization a mass of endosperm is formed, yet, as the embryo increases in size, the nutrient matter from the endospermic cells passes out from them, and is absorbed by the cells of the embryo plant. In the mature seed, in such cases, there is no separate mass of tissue containing nutrient food-material apart from the embryo itself. Such a seed is said to be cxalbuminous, as in Composite, Cruciferse, and most Legu- minosoe. When either endosperm or perisperm or both are present the seed is said to be albuminous. The albumen varies much in its nature and consistence, and furnishes important characters. It may be farinaceous or mealy, consisting chiefly of cells filled with starch, as in cereal grains, where it is abundant ; fleshy or cartilaginous, consisting of thicker cells which are still soft, as in the Coco-nut, and which sometimes contain oil, as in the oily albumen of Croton, Ricinus, and Poppy ; horny, when the cell-walls are slightly thickened and capable of distension, as in Date and Coffee; and they sometimes become greatly thickened, filling up the testa as a hard mass, as in Vegetable Ivory. The albumen may be uniform throughout, or it late XII, may present a mottled appearance, as in the Nutmeg, the 1 - . seeds of Anonacese, and some Palms, where it is called ruminated. This mottled appearance is due to a protrusion of a dark lamella of the endopleura between folded pro tuberances of albumen. The endosperm within the embryo- sac is developed from the protoplasm of that sac, but in many cases as it grows inward the whole protoplasm is not converted into a solid mass, but a cavity is left in the centre which is usually filled with fluid, as in the Coco-nut. In the thickened albumen of this Palm, as well as in that of the Attalea funifera, the Date, and the Doum Palm, the small cavity in the centre arid radiating spaces are well seen under the microscope. The albumen is a store of matter laid up for the nourishment of the embryo. The relative size of the embryo and of the endosperm varies much. In Mono cotyledons the embryo is usually small, and the endosperm large, and the same is true in the case of Coffee and many y 155 other plants amongst Dicotyledons. The opposite is the case in other plants, as in the Labiate, Plumbaginacete, <fec. In angiosperms after fertilization the embryonal vesicle The em- undergoes changes by which the embryo plant is eventually bryo. formed. The portion of the vesicle nearest the apex of the embryo-sac coalesces with it. The lower portion enlarges and lengthens greatly, and divides by transverse partition until a large terminal cell is formed at the extremity of a suspensor or proembryo (figs. 316, 317), formed of smaller cells. It is this terminal cell which immediately forms the Fig. 315. Fig. 316. Fid. 314. Entire anatropal seed of the Orange (Citrus Aurantium), with its rugose or wrinkled testa, and the raphe r ramifying in the thickness of the testa on one side. FIG. 315. Seed of Wallflower (Cheimntlws) cut vertically. The seed is exalbu- minous or aperispennic all the nourishing matter being incorporated with the cotyledons e. On removing the integument which is marked by the dark lines, the embryo alone Is found in the interior. The radicle r is folded on the edges of the cotyledons, which are accumbent. The plant belongs to the division of Cruciferse, called Pleurorhizew. FIG. 316. Section of the ovule of (Enothera, showing the pollen-tube t, with its enlarged extremity applied to the end of the embryo-sac, and introverting it slightly ; the germinal vesicle in the sac has been impregnated, and has divided into two parts, the upper part forming a confcrvoid septate suspensor s, and 1 the lower dividing into four parts, which form a globular mass the rudimentary embryo, surrounded by endospermal cells e. FIG. 317. A Dicotyledonous embryo, in different stages of development, within, the seed. At 1 it appears as a globular cellular mass at the extremity of a cellular cord or suspensor ; at 2 it becomes more ovoid ; it enlarges still more at 3 ; and at 4 it presents two distinct portions, a, the radicle attached to the suspensor, and 6, the two cotyledons. embryo. By longitudinal growth the terminal cell becomes pushed into the endosperm (when present) of the sac, and becomes completely surrounded by it. In the lower part of the central cell of gymnosperms a cell is produced which divides until a proembryo is formed, which elongates and bursts through the central cell. The divisions in this proembryo take place so that several suspensors are formed, penetrating the endosperm. Usually, each of these elongates and bears a large cell at its extremity, which becomes an embryo ; thus polyembryony occurs. The terminal cell of the proembryo in both angiosperms and gymnosperms in creases in size, and divides repeatedly, until a cellular axis is formed. This is the rudiment of the embryo. Upon this axial mass, one, two, or more rounded cellular protuberances appear, the indications of the first leaves or cotyledons ; and at the point where the embryo joins the proembryo is intercalated a mass of cellular tissue forming the hypophysis, from which subsequently the young root or radicle is formed. The axial mass continues to grow, and soon a differentiation is observable into an outer peripheral layer, the dermatogen or primordial epidermis, covering over the central mass, which soon is distinguishable into a plerome or axial mass, from which the fibro-vascular bundles develop, and a portion between it and the dermatogen, the periblem or primordial cortex. After this has been formed a differ entiation, takes place in the hypophysis, by which an upper and a lower layer of cells is formed. The upper divides into two layers, the lower of which forms a con tinuation of the dermatogen of the axial mass, the upper being continuous with the central portion of the axis. The lowermost cells of the hypophysis form the first layer

of the root-cap. Lateral roots arc frequently given off from