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tion macropodal. In these ideas, however, Richard was wrong, as is now well known.
From what has been stated, it is apparent that dicotyledons are not absolutely characterised by having two cotyledons, nor monocotyledons by having only one. The real distinction between them consists in their endorhizal or exorhizal germination, and in the cotyledons of dicotyledons being opposite or verticillate, while they are in monocotyledons solitary or alternate. Some botanists have, therefore, recommended the substitution of other terms in lieu of those in common use. Cassini suggests isodynamous or isobrious for dicotyledons, because their force of developement is equal on both sides; and anisodynamous or anisobrious for monocotyledons, because their force of developement is greater on one side than on the other. Another writer, Lestiboudois, would call dicotyledons exoptiles, because their plumula is naked; and monocotyledons endoptiles, because their plumule is enclosed within the cotyledon; but there seems little use in these proposed changes, which are, moreover, as open to objections as the terms in
In the Library of Useful Knowledge an apparently just explanation of the analogy between the embryo of monocotyledons and dicotyledons has been given ; and I take the liberty of reproducing it here:
“ 1. The embryo of an Arum is like that of a Palm, only there is a slit on one side of it through which the plumule easily escapes; 2. in Rice (Oryza) this slit is very much lengthened and widened; 3. in Barley the plumule projects beyond the slit, leaving a flat cotyledon on one side; and, 4., in Wheat the embryo has the structure of Barley, with this most important exception, that at the base of the plumule in front there is a rudimentary cotyledon, alternate with the large flat one, on the opposite side of the plumule. Hence we are to infer that the monocotyledonous embryo of a Palm is analogous to that of a dicotyledon, of which one of the cotyledons is abstracted, and the other rolled round the plumula and consolidated at its edges. And this is the view that must be taken of the monocotyledonous embryo in general, all the modifications of which seem reducible to this standard.
“ Thus in Sea-wrack (Zostera marina), of which the embryo is an oblong almond-shaped body with a cleft on one side, in the cavity of which a long flexuose process is placed, the latter is the plumule, and the former at one end the cotyledon, and the radicle at the other; in Ruppia maritima, whose embryo is an oblong body, cut suddenly off at one end, on which a sort of curved horn crouches, the latter is the plumule, and the former chiefly cotyledon; and so in Frog-bit (Hydrocharis morsus ranæ), the embryo of which is an oblong fleshy kernel with a hole on one side, in which there lies a short cylinder, the latter is the plumule, and the former the cotyledon.”
The ACOTYLEDONOUS embryo is not exactly, as its name seems to indicate, an embryo without cotyledons; for, in that case, Cuscuta would be acotyledonous. On the contrary, it is an embryo which does not germinate from two fixed invariable points, namely the plumule and the radicle, but indifferently from any point of the surface; as in some Araceæ, and in all flowerless plants. See Mohl, Bemerkungen über die Entwicklung und den Bau der Sporen der Cryptogamischen Gewächse : Regensb. 1833.
For further illustrations of the embryo, consult Plate VI. and the explanation of its figures.
The direction of the embryo is either absolute or relative. Its absolute direction is that which it has independently of the parts that surround it. In this respect it varies much in different genera; it is either straight (Plate VI. fig. 5.), arcuate (fig. 9.), falcate, uncinate, coiled up (fig. 8.) (cyclical), folded up, spiral (fig. 19.), bent at right angles (Plate V. fig. 28.) (gnomonical, Link), serpentine, or in figure like the letter S (sigmoid).
Its relative position is determined by the relation it bears to the chalaza and micropyle of the seed; or, in other words, upon the relation that the integuments, the raphe, chalaza, hilum, micropyle, and radicle bear to each other. If the sacs of the ovule are in no degree inverted, but have their common point of origin at the hilum, there being (necessarily) neither raphe nor chalaza visible, the radicle will in that case be at the extremity of the seed most remote from the hilum,
and the embryo inverted with respect to the seed, as in Cistus, Urtica, and others, where it is said to be antitropal. But if the ovule undergoes the remarkable extension of one side already described in speaking of that organ, when the sacs are so inverted that their orifice is next the hilum, and their base at the apex of the ovule, then there will be a raphe and chalaza distinctly present; and the radicle will, in the seed, be at the end next the hilum, and the embryo will be erect with respect to the seed, or orthotropal, as in the Apple, Plum, &c. On the other hand, supposing that the sacs of the embryo suffer only a partial degree of inversion, so that their foramen is neither at the one extremity nor the other, there will be a chalaza and a short raphe; and the radicle will point neither to the apex nor to the base of the seed, but the embryo will lie, as it were, across it, or be heterotropal, as is the case in the Primrose. When an embryo is so curved as to have both apex and radicle presented to the hilum, as in Reseda, it is amphitropal. It is, however, becoming customary to apply to the seed the same names as those used in expressing the modifications of the ovule; this will probably become the universal practice, and then all terms referring to the position of the embryo will become superfluous.
In the words of Gærtner an embryo is ascending when its apex is pointed to the apex of the fruit; descending, if to the base of the fruit; centripetal, if turned towards the axis of the fruit; and centrifugal, if towards the sides of the fruit: those embryos are called wandering, or vagi, which have no evident direction.
The cotyledons are generally straight, and placed face to face; but there are numberless exceptions to this. Some are separated by the intervention of albumen (Plate VI. fig. 11.); others are naturally distant from each other without any intervening substance. Some are straight, some waved, others arcuate or spiral. When they are folded with their back upon the radicle, they are called incumbent ; if their edges are presented to the same part, they are accumbent ; terms chiefly used in speaking of Brassicaceæ.
16. Of Naked Seeds.
By naked seeds has been understood, by the school of Linnæus, small seed-like fruit, like that of Labiatæ, Boraginaceæ, Grasses, and Cyperaceæ. But as these are distinctly covered by pericarps, as has been shown above, the expression in the sense of Linnæus is obviously incorrect, and is now abandoned. Hence it has been inferred that there is no such thing in existence as a naked seed; that is to say, a seed which bears on its own integuments the organ of impregnation. To this proposition botanists had assented till the year 1825,
, when Brown demonstrated the existence of seeds strictly naked; that is to say, from their youngest state destitute of pericarp, and receiving impregnation through their integuments without the intervention of style or stigma, or any stigmatic apparatus. That most learned botanist has demonstrated that seeds of this description are uniform in Coniferæ and Cycadaceæ, in which no pericarpial covering exists. But we have no knowledge at present of such an economy obtaining in other plants except Gnetaceæ, as a constant character. It does, however, happen, as the same observer has pointed out, that in particular species the ovary is ruptured at an early period by the ovules, which thus, when ripe, become truly naked seeds: remarkable instances of which occur in Ophiopogon spicatus, Leontice thalictroides, and Peliosanthes Teta.
THE COMPOUND ORGANS IN FLOWERLESS PLANTS, OR
We have now passed in review all the different organs which exist in the most perfectly formed plants; that is to say, in those whose reproduction is provided for by the complicated apparatus of stamens and pistils, and, according to Schultz, which have latex, with its peculiar tissue.
Let us next proceed to consider those lower tribes, some of which are scarcely distinguishable from animals, where there is less evident trace of sexes, in which nothing constructed like the embryo is to be detected, whose fluids have a simple motion of rotation, and which seem to have no other provision made for the perpetuation of their races than a dissolution of their cellular system. In what I may have to say about them, I shall not, however, do any thing more than give a mere enumeration and description of their organs, and an explanation of the numerous peculiar terms employed by writers in speaking of them. All speculative considerations are in this case left out of view : those who wish to be informed upon such points may consult the Introduction to the Natural System of Botany.
Filices, or Ferns, are plants consisting of a number of leaves, or fronds as they used to be called, attached to a stem which is either subterraneous or lengthened above the ground, sometimes rising like a trunk to a considerable height. They are the largest of known vegetables in which no organs fructification analogous to those of phænogamous plants have been discovered. Their stems are often arborescent, acquiring as much as the height of fifty or sixty feet, or even more,