the "germ-cell;" and, by the softening of the membrane at its apex, they are even enabled to enter its cavity, within which a minute "embryonic vesicle" was previously distinguishable. This embryonic vesicle, when fertilized by the antherozoids which

Archegonium of Pteris serrulata :-A, as seen from above; a, a, a, cells surrounding the base of the cavity; b, c, d, successive layers of cells, the highest enclosing a quadrangular orifice :-B, side view, showing, A, A, cavity containing the germ-cell; B, B, walls of the archegonium, made up of the four layers of cells, b, c, d, e, and having an opening on the summit; c, c, antherozoids within the cavity; g, large extremity; h, thread-like portion; i, small extremity in contact with the germ-cell and dilated.

move actively round it, becomes the primordial cell of a new plant, the development of which speedily commences.' By the usual process of duplicative subdivision a globular homogeneous mass of cells is at first formed; but rudiments of special organs soon begin to make their appearance; the germ grows at the expense of the nutriment prepared for it by the prothallium; and it soon bursts forth from the cavity of the archegonium, which organ in the meantime is becoming atrophied. In the very beginning of its development, the tendency is seen in the cells of one extremity to grow upwards, so as to evolve the stem and leaves, and in those of the other extremity to grow downwards, 1 See Hofmeister, in "Ann. of Nat. Hist.," 2d Ser. vol. xiv, p. 272. The study of the development of the spores of Ferns, and of the act of fertilization and of its products, may be conveniently prosecuted as follows:-Let a frond of a Fern, whose fructification is mature, be laid upon a piece of fine paper, with its spore-bearing surface downwards; in the course of a day or two, this paper will be found to be covered with a very fine brownish dust, which consists of the discharged spores. This must be carefully col. lected, and should be spread upon the surface of a smoothed fragment of porous sandstone; the stone being placed in a saucer, the bottom of which is covered with water, and a glass tumbler being inverted over it, the requisite supply of moisture is insured, and the spores will germinate luxuriantly. Some of the prothallia soon advance beyond the rest; and at the time when the advanced ones have long ceased to produce antheridia, and bear abundance of archegonia, those which have remained behind in their growth are beginning to be covered with antheridia. If the crop be now kept with little moisture for several weeks, and then suddenly watered, a large number of antheridia and archegonia simultaneously open; and in a few hours afterwards, the surface of the larger prothallia will be found almost covered with moving antherozoids. Such prothallia as exhibit freshly opened archegonia, are now to be held by one lobe between the forefinger and thumb of the left hand, so that the upper surface of the prothallium lies upon the thumb; and the thinnest possible sections are then to be made, with a thin narrow-bladed knife, perpendicularly to the surface of the prothallium. Of these sections, which, after much practice, may be made no more than 1-15th of a line in thickness, some will probably lay open the canals of the archegonia; and within these, when examined with a power of 200 or 300 diameters, antherozoids may be occasionally distinguished.

to form the root; and when these organs have been sufficiently developed to absorb and prepare the nutriment which the young plant requires, the prothallium, whose functions as a "nurse" is now discharged, decays away.

220. The little group of Equisetaceae (Horsetails), which seem nearly allied to the Ferns in the type of their generative apparatus, though that of their vegetative portion is very different, affords certain objects of considerable interest to the Microscopist. The whole of their structure is penetrated to so extraordinary a degree by silex, that, even when the organic portion has been destroyed by prolonged maceration in dilute nitric acid, a consistent skeleton still remains. This mineral, in fact, constitutes in some species not less than 13 per cent. of the whole solid matter, and 50 per cent. of their inorganic ash. The cuticle, which is used by cabinet-makers for smoothing the surface of wood, becomes, through the peculiar arrangement of its siliceous particles, an extremely beautiful object under polarized light. Of these particles (each of which has a regular axis of double refraction), some are distributed in two lines, parallel to the axis: others, however, are grouped into oval forms, connected with each other, like the jewels of a necklace, by a chain of particles forming a sort of curvilinear quadrangle; and these (which are, in fact, the particles occupying the cells of the stomata) are arranged in pairs. What is usually designated as the fructification of the Equisetaceæ, forms a cone or spike at the extremity of certain of the stem-like branches (the real stem being a horizontal rhizoma);

and consists of a cluster of shield-like disks, each of which carries a circle of thecæ or spore-cases, that open by longtitudinal slits to set free the spores. Each of these bodies has, attached to it, a pair of elastic filaments (Fig. 148), that are originally formed as spiral fibres on the interior of the wall of the primary cell within which the spore is generated, and are set free by its rupture; these are at first coiled up closely around the spore, in the manner represented at A, though more closely applied to the surface; but, on the slightest application of moisture, they suddenly extend themselves in the manner shown at B; and this motion, like the extension of the spiral elaters of Marchantia, probably serves to assist in the dispersion of the spores. If a number of the spores be spread out on a slip of glass under the field of view, and, whilst the observer watches them, a bystander

breathe gently upon the glass, all the filaments will be instantaneously put in motion, thus presenting an extremely curious spectacle, and will almost as suddenly return to their previous condition, when the effect of the moisture has passed off. These spores are to be regarded in the same light as those of Ferns; namely, as gemmæ or rudimentary buds, not as seeds. They evolve themselves after the like method into a prothallium; and this developes antheridia and archegonia, by the conjoint action of which an embryo is produced.

221. In ascending, as we have now done, from the lower to the higher Cryptogamia, we have seen a gradual change in the general plan of structure, so that the superior forms present a close approximation to the Flowering Plant, which is undoubtedly the highest type of Vegetation. But we have everywhere encountered a mode of Generation, which, whilst essentially the same throughout the series, is essentially distinct from that of the Phanerogamia; the fertilizing material of the "sperm-cell" being embodied, as it were, in self-moving filaments, which find their way to the germ-cells by their own independent movements; and the "embryo-cell" being destitute of that store of prepared nutriment, which surrounds it in the true seed, and serves as the pabulum for its early development. In the lower Cryptogamia, we have seen that the embryo-cell, after fertilization, is thrown at once upon the world (so to speak) to get its own living; but in the Liverworts, Mosses, and Ferns, the embryocell is nurtured by the parent-plant, for a period that varies in each case according to the nature of the fabric into which it evolves itself. While the true reproduction of the species is effected by the proper Generative act, the multiplication of the individual is accomplished by the production and dispersion of spores; and this production, as we have seen, takes place at very different periods of existence in the several groups, dividing the life of each into two separate epochs, in which it presents itself under two very distinct phases that contrast remarkably with each other. Thus, the frond of the Marchantia, bearing its antheridia and archegonia, is that which seems naturally to constitute the plant; but that which represents it in the Ferns, is the minute Marchantia-like prothallium. On the other hand, the product into which the fertilized embryo-cell evolves itself in the Ferns, is that which is commonly regarded as the plant; and this is represented in the Liverworts and Mosses by the spore-capsules alone. We shall notice, in the next Chapter (§ 256), the representation of these two phases in the life of the Flowering Plant, which is traceable by means of the study of the Lycopodiace and Conifere; two groups that form the link of transition. between these two great divisions of the Vegetable kingdom, the former being probably to be regarded as the highest of the Cryptogamia, and the latter as the lowest of the Phanerogamia.i

See the Author's "Principles of Comparative Physiology," Am. ed. 1854, §§ 499--505.

CHAPTER VIII.

OF THE MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS.

222. Elementary Tissues.-In passing from the Cryptogamic division of the Vegetable Kingdom, to that larger and more ostensibly important province which includes the Flowering Plants, we do not meet with so wide a departure from those simple types of structure we have already considered, as the great differences in general aspect and external conformation might naturally lead us to expect. For a very large proportion of the fabric of even the most elaborately formed Tree, is made up of components of the very same kind with those which constitute the entire organisms of the simplest Cryptogamia; and that proportion always includes the parts most actively concerned in the performance of the vegetative functions. For although the stems, branches, and roots, of trees and shrubs, are principally composed of woody tissue, such as we do not meet with in any but the highest Cryptogamia, yet the special office of this is to afford mechanical support; when it is once formed, it takes no further share in the vital economy, than to serve for the conveyance of fluid from the roots, upwards through the stem and branches, to the leaves; and even in these organs, not only the pith and the bark, with the "medullary rays" which serve to connect them, but that "cambium-layer" intervening between the bark and the wood (§ 240), in which the periodical formation of the new layers both of bark and wood takes place, are composed of cellular substance. This tissue is found, in fact, wherever growth is taking place; as, for example, in the spongioles or growing points of the root-fibres, in the leaf-buds and leaves, and in the flower-buds and sexual parts of the flower; it is only when these organs attain an advanced stage of development, that woody structure is found in them,-its purpose (as in the stem) being merely to give support to their softer textures; and the small proportion of their substance which it forms, being at once seen in those beautiful skeletons, which, by a little skill and perseverance, may be made of leaves, flowers, and certain fruits. All the softer and more pulpy tissue of these organs is composed of cells more or less compactly aggregated together,

and having forms which approximate more or less closely to the globular or ovoidal, which may be considered as their original type.

223. As a general rule, the rounded shape is preserved only when the cells are but loosely aggregated, as in the parenchymatous (or fleshy) substance of leaves (Fig. 149); and it is then only, that the distinctness of the walls becomes evident. When the tissue becomes more

solid, the sides of the vesicles are pressed against each other, so as to flatten them and to bring them into close apposition; and they then adhere to one another in such a manner, that the partitions appear, except when carefully examined, to be single, instead of being (as they really are) double. Frequently it happens that the pressure is exerted more in one direction than in an

Section of leaf of Agave, treated with dilute nitric acid,

other, so that the form pre- showing the primordial utricle contracted in the interior sented by the outline of the of the cells:-a, epidermic cells; b, boundary cells of cell varies according to the the stoma; c, cells of parenchyma; d, their primordial direction in which the sec

utricles.

tion is made. This is well shown in the pith of the young shoots of Elder, Lilac, or other rapidly growing trees; the cells of which, when cut transversely, generally exhibit circular outlines, whilst, when the section is made vertically, their borders are straight, so as to make them appear like cubes or elongated prisms, as in Fig. 152. A very good example of such a cellular parenchyma is to be found in the substance known as "Rice-paper;" which is made by cutting the herbaceous stem of a Chinese plant termed Aralia papyrifera, vertically round and round, with a long sharp knife, so that its tissue may be (as it were) unrolled in a sheet. The shape of the cells, as seen in the rice-paper thus prepared, is irregularly prismatical, as shown in Fig. 150, B; but if the stem be cut transversely, their outlines are seen to be circular or nearly so (A). When, as often happens, the cells have a very elongated form, this elongation is in the direction of their growth, which is that, of course, wherein there is least resistance. Hence their greatest length is nearly always in the direction of the axis; but there is one remarkable exception,-that, namely, which is afforded by the "medullary rays" of Exogenous stems ($239), whose cells are greatly elongated in the horizontal direction (Fig. 161, a), their growth being from the centre of the stem

The Eschynomene, which is sometimes named as the source of this article, is an Indian plant, employed for a similar purpose.