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).