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The integuments of a seed are called the testa; the rudiment of a future plant, the embryo (Plate VI. fig. 1. b, &c.); and a substance interposed between the embryo and the testa, the albumen (fig. 1. a, 5. a, &c.)
The testa, called also lorica by Mirbel, perisperm and episperm by Richard, and spermodermis by De Candolle, according to some consists, like the pericarp, of three portions; viz., 1. the external integument, tunica externa of Willdenow, testa of De Candolle; 2. the internal integument, tunica interna of Willdenow, endopleura of De Candolle, hilofère and tegmen of Mirbel; and, 3., of an intervening substance answering to the sarcocarp, and called sarcodermis by De Candolle this last is chiefly present in seeds with a succulent testa, and by many is considered a portion of the outer integument, which is the most accurate mode of understanding it.
According to Schleiden, the integuments of the seed experience many changes during the period of ripening, so that their original number can rarely be recognised. They are sometimes all consolidated so as to form but one; or they are broken up into many layers, having no relation to the original number of integuments. In Menyanthes, which has but one integument of the ovule, the seed appears to have two, because of the separation and lignification of the epidermis of that integument; and in Canna there are five layers of tissue resembling integuments, although the ovule has not even one complete integument.
The cellular tissue of the integuments of the seed is very often reticulated. In most Bignoniaceæ, and many other plants, the epidermis is in this state, and in Casuarina there is a layer of spiral vessels below the epidermis, very thin and delicate, and extremely minute. In Swietenia febrifuga there is, below the epidermis, a thick layer of large spiral cells, which have little cohesion with each other, and which form a multitude of rather large fusiform sacs lying confusedly (?); this is the most complete case of spiral cells in seeds. with which I am acquainted, and it is accompanied by the presence of a bundle of numerous slender spiral vessels in the raphe.
The outer integument is either membranous, coriaceous, crustaceous, bony, spongy, fleshy, or woody; its surface is either smooth, polished, rough, or winged, and sometimes is furnished with hairs, as in the cotton and other plants, which, when long, and collected about either extremity, form what is called the coma (sometimes also, but improperly, the pappus). It consists of cellular tissue disposed in rows, with or without bundles of vessels intermixed: in colour it is usually of a brown or similar hue: it is readily separated from the inner integument. In Maurandya Barclayana it is formed of reticulated cellular tissue; in Collomia linearis, some Salvias and others, it is caused by elastic spirally twisted fibres enveloped in mucus, and springing outwards when the mucus is dissolved. In the genus Crinum it is of a very fleshy succulent character, and has been mistaken for albumen, from which it is readily known by its vascularity. According to Brown, a peculiarly anomalous kind of partition, which is found lying loose within the fruit of Banksia and Dryandra, without any adhesion either to the pericarp or the seed, is a state of the outer integument; it is said, that in those genera the inner membrane (secundine) of the ovule is, before fertilisation, entirely exposed, the primine being reduced to half, and open its whole length; and that the outer membranes (primines) of the two collateral ovules, although originally distinct, finally contract an adhesion by their corresponding surfaces, and together constitute the anomalous dissepiment. But it may be reasonably doubted whether the integument here called secundine is not primine, and the supposed primine arillus.
The inner membrane (secundine) of the ovule, however, in general appears to be of greater importance as connected with fecundation, than as affording protection to the nucleus at a more advanced period. For in many cases, before impregnation, its perforated apex projects beyond the aperture of the testa, and in some plants puts on the appearance of an obtuse, or even dilated, stigma; while in the ripe seed it is often either entirely obliterated, or exists only as a thin film, which might readily be mistaken for the epidermis of a third membrane, then frequently observable.
"This third coat (tercine) is formed by the proper membrane or cuticle of the nucleus, from whose substance in the unimpregnated ovule it is never, I believe, separable, and at that period is very rarely visible. In the ripe seed it is distinguishable from the inner membrane only by its apex, which is never perforated, is generally acute and more deeply coloured, or even sphacelated."
Mirbel has, however, justly remarked that the primine and the secundine are, in the seed, very frequently confounded; and that, therefore, the word testa is better employed, as one which expresses the outer integument of the seed without reference to its exact origin, which is practically of little importance. The tercine is also, no doubt, often absent. He observes that these mixed integuments often give rise to new kinds of tissue; that in Phaseolus vulgaris the testa consists, indeed, of three distinct layers, but of those the innermost was the primine; and that the others, which represent nothing that pre-existed in the ovule, have a horny consistence, and are formed of cylindrical cellules, which elongate in the direction from the centre to the circumference. And this is probably the structure of the testa of many Leguminosæ.
It sometimes happens that the endopleura (or tercine?) thickens so much as to have the appearance of albumen, as in Cathartocarpus fistula. In such a case it is only to be distinguished from albumen by gradual observation from the ovule to the ripe seed.
One of the innermost integuments is occasionally present in the form of a fleshy sac, interposed between the albumen and the ovule, and enveloping the latter. It is what was called the vitellus by Gærtner, and what Richard, by a singular prejudice, considered a dilatation of the radicle of the embryo: to his macropodal form of which he referred the embryo of such plants. Instances of this are found in Nymphæa and its allies, and also in Zingiberaceæ, Peppers, and Saururus. Brown, who first ascertained the fact, considers this sac to be always of the same nature and origin, and as the vesicula colliquamenti or amnios of Malpighi.
The end by which the seed is attached to the placenta is called the hilum or umbilicus (Plate VI. fig. 5. c, 17. e, 11. c,
&c.); it is frequently of a different colour from the rest of the seed, not uncommonly being black. In plants with small seeds it is minute, and recognised with difficulty; but in some it is so large as to occupy fully a third part of the whole surface of the seed, as in the Horsechestnut, Sapotaceæ, and others. Seeds of this kind have been called nauca, by Gærtner. In grasses the hilum is indicated by a brownish spot situated on the face of the seed, and is called by Richard spilus. The centre of the hilum, through which the nourishing vessels pass, is called by Turpin the omphalodium. Sometimes the testa is enlarged in the form of irregular lumps or protuberances about the umbilicus; these are called strophiola or caruncule; and the umbilicus, round which they are situated, is said to be strophiolate or carunculate. Mirbel has ascertained that in Euphorbia Lathyris the strophiole is the fungous foramen of the primine; and it is probable that such is often the origin of this tubercle: but at present we know little upon the subject.
The foramen in the ripe seed constitutes what is called the micropyle: it is always opposite the radicle of the embryo; the position of which is, therefore, to be determined without dissection of the seed, by an inspection of the micropyle,—often a practical convenience.
In some seeds, as the Asparagus, Commelina, and others (fig. 188.), there is a small callosity at a short distance from the hilum this callosity gives way like a lid at the time of germination, emitting the radicle, and has been named by Gærtner the embryotega.
At the apex of the seed, in the Orange and many other plants, may be perceived upon the testa a small brown spot, formed by the union of certain vessels proceeding from the hilum this spot is the chalaza (Plate VI. fig. 11. b). In the orange it is beautifully composed of dense bundles of spiral vessels and spiral ducts, without woody fibre. The vessels which connect the chalaza with the hilum constitute a particular line of communication, called the raphe: in most plants this consists of a single line passing up the face of the seed; but in many Aurantiaceæ and Clusiaceæ it ramifies upon the surface of the testa.
The raphe is always a true indication of the face of the seed; and it is very remarkable that the apparent exceptions to this rule only serve to confirm it. Thus, in some species of Euonymus in which the raphe appears to pass along the back, an examination of other species shows that the ovules of such species are in fact resupinate; so that, with them, the line of vascularity representing the raphe is turned away from its true direction by peculiar circumstances. In reality, the chalaza is the place where the secundine and the primine are connected; so that in orthotropous seeds, or such as have the apex of the nucleus at the apex of the seed, and in which, consequently, the union of the primine and secundine takes place at the hilum, there can be no apparent chalaza, and consequently no raphe: the two latter can only exist as distinct parts in anatropous or amphitropous seeds, where the base of the nucleus corresponds to the geometrical apex of the seed. Hence, also, there can never be a chalaza without a raphe, nor a raphe without a chalaza.
Something has already been said about the aril (figs. 189. and 190.) when speaking of the ovule; but it more properly comes under consideration along with the ripe seed. As a general rule, it may be stated that every thing proceeding from the placenta, and not forming part of the seed, is referable to the aril. Even in plants like Hibbertia volubilis and Euonymus europæus, in which it is of unusual dimensions, it is scarcely visible in the unimpregnated ovary; and it is stated by Brown, that he is not acquainted with any case in which it covers the foramen of the testa before impregnation. The term aril has been misapplied in many cases to the testa, as in Orchidaceae; and even to a pair of opposite confluent bracts, as in Carex: of these errors, the former arose from imperfect observation, the latter from ignorance of the fundamental principles of Organography.
The mass enclosed within the testa or outer integument is