Some seeds are winged, as it is called, i. e. furnished with an extension of the testa beyond the margin of the seed. This not unfrequently consists of aggregated fibre-cells, the spiral fibre being very distinct, as in the seeds of Eccremocar´pus scáber ([Pl. I.] fig. 52). In the seeds of another curious plant in this respect, viz. Collómia grandiflóra, the fibre-cells are separate, so as to resemble hairs, and very mucilaginous, and in the dry seed are closely pressed to its surface. If a portion of the testa of these seeds, which can be procured at the seed-shops, be cut off, laid on a slide, a cover applied, and when the object is in focus, a drop of water be added, in a short time water softens the mucilaginous walls of the cells, the power of the spiral fibres comes into play, and the cells expand so as to form a very interesting object; the cells, in their expansion, apparently writhing like so many minute worms ([Pl. I.] fig. 35).
The seed itself, which is contained within the testa or seed-coat, consists essentially of the young plant or embryo. This is composed of three parts, viz. the plúmule (plumula, a little feather), or the young stem; the rad´icle (radicula, a little root), or the young root; and one or two, rarely more, imperfectly developed or rudimentary leaves, the cotyle´dons (κοτυληδὠν, a cup).
These structures are closely packed in the seed, and are not easily recognized at first. By keeping seeds moist for a day or two until they begin to grow, or germinate as the seed-growth is called, they are readily detected, and may then be more easily found in the dry seed.
When somewhat advanced in growth, they are familiar to every one, although they may not be recognized by their names. In table “mustard and cress,” the whole consists of these organs of the two plants; the white stalk directed downwards being the radicle, the two green leaf-like lobes the cotyledons, and between the latter directed upwards is the very minute plumule, which is more easily seen when the plants have been allowed to grow larger. This structure of the seed is important to be known, because the absence or presence and the number of cotyledons afford characters, corresponding with those already mentioned in respect to the veins of the leaves and the structure of the stem, for distinguishing the great divisions of the Vegetable Kingdom. Thus, the Exogens are Dicotyledons (δις, twice), their seeds having two cotyledons; while the Endogens are Monocotyledons (μὁνος, single), having one only; and the Cryptogam´ic plants are Acotyledons (α, without), their seeds (spores) having none of these organs.
Some seeds consist entirely of the embryo, surrounded by the testa. But in many others there is also present a usually whitish, firm cellular substance, called the albúmen (albumen, white of egg).
The albumen of seeds often affords good specimens of secondary deposit, the cells being almost entirely filled with it. An example may be found in a section of vegetable ivory, of which ornaments are sometimes made; its structure resembles essentially that of the plum-stone. In other instances the cells contain secreted matters, as starch, oil, &c.; and sometimes the cotyledons also contain starch and oil. An example of the former exists in the albumen of wheat; and of the latter, in the horse-chestnut, the filbert, and mustard-seed.
The albumen and cotyledons serve to supply the embryo with nutriment until the roots have grown sufficiently to enable them to absorb it from the soil; the cotyledons also serve as temporary leaves.
The form and relative position of the radicle and cotyledons serve to distinguish certain groups of plants. This may be illustrated by the natural order Cruciferæ, or that containing the mustard, wall-flower, &c.
Thus, in one group, which may be represented by the wall-flower, the cotyledons are flat or plane ([Pl. I.] figs. 43 & 44), the radicle being applied to their edges. This is best seen in a transverse section (fig. 43). They are then called accum´bent (accumbo, to lie against); and the botanical sign is O=. In the second group, the cotyledons are plane ([Pl. I.] fig. 38), with the radicle applied to the back of one of them, as in the seed of the common shepherd’s purse (Capsel´la bur´sa pastóris) ([Pl. VII.] fig. 19). They are then termed in´cumbent (incumbo, to lie upon), and the sign is O||. While in the third group the cotyledons are folded in the middle, like the leaves of a book (Pl. I. figs. 49 & 50), and the radicle is enclosed between them, as in the white mustard (Sina´pis alba). The cotyledons are then called condu´plicate (conduplico, to fold); and their sign is O> >.
The plants above-mentioned are evidently all Dicotyledonous, or their seeds have two cotyledons; and they contain no albumen.