well marked, as a scar of varying size; in the calabar bean and in
some species of Mucuna and Dolichos it extends along a large
portion of the edge of the seed; it frequently exhibits marked
colours, being black in the bean, white in many species of Phaseolus,
&c. The micropyle (fig. 35, m) of the seed may be recognizable by
the naked eye, as in the pea and bean tribe, Iris, &c., or it may be
very minute or microscopic. It indicates the true apex of the seed,
and is important as marking the point to which the root of the embryo
is directed. At the micropyle in the bean is observed a small
process of integument, which, when the young plant sprouts, is
pushed up like a lid; it is called the embryotega. The chalaza (fig.
38, ch) is often of a different colour from the rest of the seed. In the
orange (fig. 40) it is of a reddish-brown colour, and is easily recognized
at one end of the seed when the integuments are carefully removed.
In anatropal seeds the raphe forms a distinct ridge along one side
of the seed (fig. 41).
The position of the seed as regards the pericarp resembles that of the ovule in the ovary, and the same terms are applied—erect, ascending, pendulous, suspended, curved, &c. These terms have no reference to the mode in which the fruit is attached to the axis. Thus the seed may be erect while the fruit itself is pendent, in the ordinary meaning of that term. The part of the seed next the axis or the ventral suture is its face, the opposite side being the back. Seeds exhibit great varieties of form. They may be flattened laterally (compressed), or from above downwards (depressed). They may be round, oval, triangular, polygonal, rolled up like a snail, as in Physostemon, or coiled up like a snake, as in Ophiocaryon paradoxum.
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| Fig. 37. | Fig. 38. | Fig. 39. | Fig. 40. | Fig. 41. |
| Fig. 37.—Seed of Pine (Pinus), with a membranous appendage w to the testa, called a wing. | ||||
| Fig. 38.—Young anatropal seed of the white Water-lily (Nymphaea alba), cut vertically. It is attached to the placenta by the funicle f, cellular prolongations from which form an aril a a. The vessels of the cord are prolonged to the base of the nucellus n by means of the raphe r. The base of the nucellus is indicated by the chalaza ch, while the apex is at the micropyle m. The covering of the seed is marked i. n is the nucellus or perisperm, enclosing the embryo-sac es, i | ||||
| Fig. 39.—Arillode a, or false aril, of the Spindle-tree (Euonymus), arising from the micropyle f. | ||||
| Fig. 40.—Anatropal seed of the Orange (Citrus Aurantium) opened to show the chalaza c, which forms a brown spot at one end. | ||||
| Fig. 41.—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. | ||||
The endosperm formed in the embryo-sac of angiosperms after
fertilization, and found previous to it in gymnosperms, consists of
cells containing nitrogenous and starchy or fatty matter, destined
for the nutriment of the embryo. It occupieds the whole cavity of
the embryo-sac, or is formed only at certain portions of it, at the
apex, as in Rhinanthus, 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 nucellus
of the ovule is gradually absorbed. Sometimes, however, as in
Musaceae, Cannaceae, Zingiberaceae, no endosperm is formed;
the cells of the original nucellus, becoming filled with food-materials
for the embryo, are not absorbed, but remain surrounding the
embryo-sac with the embryo, and constitute the perisperm. Again,
in other plants, as Nymphaeaceae (fig. 38) and Piperaceae, 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 in the seed, 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 exalbuminous,
as in Compositae, Cruciferae and most Leguminosae (e.g. pea, fig. 35).
When either endosperm or perisperm or both are present the seed
is said to be albuminous.
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| Fig. 42.—The dicotyledonous embryo of the Pea laid open. c, c, The two fleshy cotyledons, or seed-lobes, which remain under ground when the plant sprouts; r, the radicular extremity of the axis whence the root arises; t, the axis (hypocotyl) bearing the young stalk and leaves g (plumule), which lie in a depression of the cotyledons f. |
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; the cell-walls sometimes become greatly thickened, filling up the testa as a hard mass, as in vegetable ivory (Phytelephas). The albumen may be uniform throughout, or it may present a mottled appearance, as in the nutmeg, the seeds of Anonaceae and some Palms, where it is called ruminated. This mottled appearance is due to a protrusion of a dark lamella of the integument between folded protuberances of albumen. A cavity is sometimes left in the centre which is usually filled with fluid, as in the coco-nut. The relative size of the embryo and of the endosperm varies much. In Monocotyledons the embryo is usually small, and the endosperm large, and the same is true in the case of coffee and many other plants amongst Dicotyledons. The opposite is the case in other plants, as in the Labiatae, Plumbaginaceae, &c.
The embryo consists of an axis bearing the cotyledons (fig. 42, c), or the first leaves of the plant. To that part of this axis immediately beneath the cotyledons the terms hypocotyl, caulicle or tigellum (t) have been applied, and continuous backwards with it is the young root or radicle (r), the descending axis, their point of union being the collar or neck. The terminal growing bud of the axis is called the plumule or gemmule (g), and represents the ascending axis. The radicular extremity points towards the micropyle, while the cotyledonary extremity is pointed towards the base of the ovule or the chalaza. Hence, by ascertaining the position of the micropyle and chalaza, the two extremities of the embryo can in general be discovered. It is in many cases difficult to recognize the parts in an embryo; thus in Cuscuta, the embryo appears as an elongated axis without divisions; and in Caryocar the mass of the embryo is made up by the radicular extremity and hypocotyl, in a groove of which the cotyledonary extremity lies embedded (fig. 52). In some monocotyledonous embryos, as in Orchidaceae, the embryo is a cellular mass showing no parts. In parasitic plants also which form no chlorophyll, as Orobanche, Monotropa, &c., the embryo remains without differentiation, consisting merely of a mass of cells until the ripening of the seed. When the embryo is surrounded by the endosperm on all sides except its radicular extremity it is internal (see figs. 19, 20); when lying outside the endosperm, and only coming into contact with it at certain points, it is external, as in grasses (e.g. wheat, fig. 22). When the embryo follows the direction of the axis of the seed, it is axile or axial (fig. 43); when it is not in the direction of the axis, it becomes abaxile or abaxial. In campylotropal seeds the embryo is curved, and in place of being embedded in endosperm, is frequently external to it, following the concavity of the seed (fig. 44), and becoming peripherical, with the chalaza situated in the curvature of the embryo, as in Caryophyllaceae.
It has been already stated that the radicle of the embryo is directed to the micropyle, and the cotyledons to the chalaza. In some cases, by the growth of the integuments, the former is turned round so as not to correspond with the apex of the nucellus, and then the embryo has the radicle directed to one side, and is called excentric, as is seen in Primulaceae, Plantaginaceae and many palms, especially the date. The position of the embryo in different kinds of seeds varies. In an orthotropal seed the embryo is inverted or antitropal, the radicle pointing to the apex of the seed, or to the part opposite the hilum. Again, in an anatropal seed the embryo is erect or homotropal (fig. 43), the radicle being directed to the base of the seed. In curved or campylotropal seeds the embryo is folded so that its radicular and cotyledonary extremities are approximated, and it becomes amphitropal (fig. 44). In this instance the seed may be exalbuminous, and the embryo may be folded on itself; or albuminous, the embryo surrounding more or less completely the endosperm and being peripherical. According to the mode in which the seed is attached to the pericarp, the radicle may be directed upwards or downwards, or laterally, as regards the ovary. In an orthotropal seed attached to the base of the pericarp it is superior, as also in a suspended anatropal seed. In other anatropal seeds the radicle is inferior. When the seed is horizontal as regards the pericarp, the radicle is either centrifugal, when it points to the outer wall of the ovary; or centripetal, when it points to the axis or inner wall of the ovary. These characters are of value for purposes of classification, as they are often constant in large groups of genera.
