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at the apex.

and in that case are usually unbranched, of the same thickness at the upper and lower ends, and grow exclusively The surface of the stem is often hairy or shaggy, sometimes spiny, and in all cases is more or less copiously furnished with callous points, which render it rough like shagreen leather, or covered with roots, sometimes entangled into a compact layer much thicker than the trunk itself, and appearing to be the extension of the callous points.

The anatomy of tree ferns has been skilfully elucidated by Mohl, to whose treatise upon the subject (Martius, Plant. Crypt. Bras. p. 40.) the reader is referred for the details of their curious organisation. I must content myself with a very general statement. The trunk is covered with a hard rind, occupying the place of bark, two or three lines thick, and consisting of hard brown parenchymatous and prosenchymatous tissue, the latter, if present, being on the inside. Within the rind is a mass of parenchymatous thinner-sided tissue, which is analogous to the horizontal cellular system of exogens and endogens. The wood is formed by concave or sinuous plates, whose section has a lunate or wavy form, and which are closely arranged in a circle next to the rind, enclosing a column of parenchyma, just as the wedges of wood in exogens enclose a similar column of pith; and in like manner there are openings between the plates, through which the subcortical and medullary parenchymas communicate. Each plate consists externally of several layers of hard brown prosenchyma, next within which is a pale stratum of thinsided parenchyma, and in the centre of all is a soft pale mass of trachenchyma, consisting of large scalariform and spiral vessels (sometimes line in diameter) mixed with soft parenchyma. Externally the stem is marked with long, or rhomboidal scars, the surface of which is broken into numerous hard ragged projections which represent the broken communication between the trunk and the leaves, by the fall of which the scars are produced. Next the apex of a trunk the scars are always arranged with great regularity, but towards the lower part of the stem they become much longer, irregular in form, and are separated by deep furrows; from

which it is to be inferred, that, although in these plants no new parts are added, except at the point of the trunk, yet that the parts after being formed do grow both in length and breadth.

Below the scars of the leaves are often (always?) found elliptical or roundish perforations, filled with a powdery matter. These have no obvious analogy in other plants, unless they are to be compared to the perforations in the rhizoma of Nymphæa.

It may be easily understood, that, taking such a structure as is now described for the type of Ferns, the name Acrogens (or point-growers) is well applied to them; and that all the modifications of structure which exist in the small species are mere reductions of developement, or adaptations of the same type to peculiar circumstances.

Their petioles, or stipes (rachis, W.; peridroma, Necker), consist of sinuous strata of indurated, very compact tissue, connected by cellular matter; and the wood of those which have arborescent trunks is formed by the cohesion of the bases of such petioles round a hollow or solid cellular axis. The organs of reproduction are produced from the back or under side of the leaves. In Polypodiaceæ, or what are more commonly called dorsiferous ferns, they originate, either upon the epidermis or from beneath it, in the form of spots, at the junctions, margins, or extremities of the veins. As they increase in growth they assume the appearance of small heaps of granules, which heaps are called sori. If examined beneath the microscope, these granules, commonly called sporangia, thecæ, capsules, or conceptacles, are found to be little, brittle, compressed bags formed of cellular membrane, partially surrounded by a thickened longitudinal ring (gyrus, annulus, gyroma), which sometimes at the vertex loses itself in the cellularity of the membrane, and at the base tapers into a little stalk. The sporangia burst with elasticity by aid of their ring, and emit minute particles named spores or sporules, from which new plants are produced: as from seeds, in vegetables of a higher order. Interspersed with the sporangia are often intermixed articulated hairs; and, in those genera in which the sporangia originate beneath the epidermis, the sori, when

mature, continue covered with the superincumbent portion of the epidermis, which is then called the indusium or involucrum (membranula, Necker; glandulæ squamosa, Guettard). In Trichomanes and Hymenophyllum, the sporangia are seated within the dilated cup-like extremities of the lobes of the frond, and are attached to the vein which passes through their axis, which is then called their receptacle. In Gleicheniaceæ, the sporangia have a transverse complete, instead of a vertical incomplete, ring, and they are nearly destitute of stalks; in others the sori occupy the whole of the under surface of the leaf, which becomes contracted, and wholly alters its appearance: the sporangia have no ring, and the cellular tissue of their membrane is not reticulated, but radiates regularly from the apex.

In these plants it has been in vain endeavoured to discover traces of organs of fecundation. Nevertheless, as it was difficult for sexualists to believe that plants of so large a size were destitute of such organs, it has been considered indispensable that they should be found; and, accordingly, while all seem to agree in considering the sporangia as female organs, a variety of other parts have been dignified by the title of male organs: thus, Micheli and Hedwig found the latter in certain stipitate glands of the leaf; Stahelin, Hill, and Schmidel, in the elastic ring; Kolreuter, in the indusium ; Gleichen, in the stomates; and Von Martius, in certain membranes enclosing the spiral vessels. None of these opinions are now adopted. M. Bory de St. Vincent contends that impregnation may take place in plants without the agency of pollen, and he affirms that hybrid ferns exist; which, if true, would render it impossible to deny the existence, in this large order, of sexual organs; but where are they? (Comptes Rendus, v. 125.)

In Ophioglossaceæ, a remarkable tribe of Ferns, the fertile leaf is rolled up in two lines parallel with its axis or midrib, and at maturity opens regularly by transverse valves along its whole length, emitting a fine powder, which, when magnified, is found to consist of particles of the same nature as the spores found in the sporangia of other ferns; here there are no sporangia, the metamorphosed leaf probably performing

their functions. Such is my view of the structure of Ophioglossaceae; but by other botanists it is described as a dense spike of two-valved capsules, dehiscing transversely.

2. Equisetaceæ.

In these plants, which may, I think, be as properly considered the lowest form of flowering plants, the stem is hollow, jointed, and bears a toothed sheath at each joint. The cylinder of the stem is pierced by longitudinal fistulæ, which alternate with furrows on the outside of the stem; there is also a bundle of ringed vessels connected with the fistulæ.

The organs of reproduction are arranged in a cone, consisting of scales bearing on their lower surface an assemblage of cases, called sporangia, thecæ, folliculi, or involucra, which dehisce longitudinally inwards. In these sporangia are contained two sorts of granules; the one very minute and lying irregularly among a larger kind, wrapped in two filaments, fixed by their middle, rolled spirally, having either extremity thickened, and uncoiling with elasticity. By Hedwig the apex of the larger granules was supposed to be a stigma, and the thickened ends of the filament anthers, the small granules being the pollen. It is certain that the larger granules, round which the elastic filaments are coiled, are the reproductive particles; and it seems to me that they may be compared to the naked seeds of Coniferæ, the only order to which Equisetaceæ appear to have much resemblance; but Mr. Griffith differs from me upon this point. This excellent observer states that the club-shaped bodies which Hedwig referred to stamens are elaters, and are developed in or on a loose membranous coat, and later than the central body, spore, or seed.

3. Lycopodiaceæ.

These are leafy plants with the habit of gigantic mosses. Their leaves and stem have the same structure as those plants, except that the former are sometimes provided with stomates, and the latter with a central bundle of vessels.

Their organs of reproduction are kidney-shaped two-valved cases, called thecæ, sporocarpia, conceptacles, or capsules, either 1. filled with minute powder-like granules, which, in consequence of lateral compression, from being spherical, acquire the figure of irregular polygons; or 2. containing three or four roundish fleshy bodies, marked at the apex by a threelegged line, and each of which is at least fifty times larger than the granules contained in the first kind of theca; the latter are said by Brotero to burst with elasticity, an observation which requires verification. The first kind of theca is found in all species of Lycopodiacea; the second is only found in a a few. The contents of both are believed to be sporules; but no satisfactory explanation has yet been offered of the cause of their difference in size, and probably also in structure. I would suggest that the powder-like grains are true sporules, and that the large ones are buds or viviparous organs, as has already been stated by Haller and Willdenow. A writer in the Transactions of the Linnean Society has figured and described the growth of the larger grains of Lycopodium denticulatum, and he considers that they exhibit the germination of a dicotyledonous plant; but, independently of any mistrust which may attach to the account, it is obvious enough that his own drawings and description represent a mode of germination analogous, not to that of dicotyledons, but rather to that of monocotyledons, and also reducible to the laws which govern the incipient vegetation of a bud.

The powder-like sporules are inflammable, and have been supposed by Haller, Linnæus, and others, to be pollen, while the larger have been considered seeds; and to a part of the surface of the theca the office of stigma has been attributed. The thecæ themselves have been fancied to be male apparatus by Koelreuter and Gærtner.

4. Marsileaceæ.

This very curious little order consists of plants differing from each other so much, that, although consisting of only four genera, it is necessary to subdivide it into two distinct tribes.

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