88. Arehegonium. — The archegonia were first discovered and figured by Milde* in Equisetum Telmateia, and have been since more completely described by Hofmeisterf and Bischofft in two other species. The pro jecting papilliform portion consists, according to the last-inentioned observer, of eight cells, of which the four lower, in apposition to each other, have the general form of truncated cones, each presenting two flattened surfaces by which it is united to its two neighbours. The upper, in the same relation to each other, are nearly cylindrical, but are slightly rounded at their summits. The axis of the organ is occupied by a quadrilateral intercellular passage. The whole is supported on a base, which consists of two or three rows of cells attached by a pedicle at its centre. Upon the surface of the disc which faces the stem, the spore-cases are formed. Each spore-case originates as a little papilla, and consists of' a large central cell, which is invested by a single layer of others of smaller size. As the organ enlarges, these last are transformed into a capsule consisting of three concentric layers, within which is enclosed a mass of cells exhibiting large central nuclei and grumous contents. In each of these cells the nucleus is afterwards replaced by two others similar to it, which almost fill the cavity. These, however, soon disappear, and now four globular nuclei, much smaller than their predecessors, present themselves, and are arranged, as in the Hepaticw, towards the four angles of a regular tetrahedron. Around each nucleus a tetrahedral cell is formed, within which, after it has become detached from its fellows, there is deposited on the inner surface of its membrane, a gelatinous transparent layer. Within this layer, and immediately surrounding the nucleus, may be distinguished the primordial vesicle, on the surface of which the cellulose membrane of the future spore is secreted, as well as the two parallel, elastic fibres by which it is surrounded. When the spore is ripe, these last, which are external to the spore tnernbrane, and consequently formed before it, line the inner surface of the parent are originally' developed in situ, from a single parent, or existed before, lying free in the cavity of the spore, is uncertain. Hofineister inclines to the latter opinion. At first the prothallium is a cellular expansion of circular form,which enlarges by growth at its periphery, arid lines the upper part of the proper mem brane of the spore. At its centre it is of considerable thickness, and is composed of several layers of cells. Towards its margin it becomes gradually, thinner and thinner, its two surfaces at last converging at a very acute angle, so as to become continuous with those of the spore membrane.
92. Arehegonia.—The first-formed arche gonium is always found to occupy the centre of the upper surface of the prothallium ; its successors surrounding it at various distances. A superficial cell, distinguished from its neighbours by the quantity of granules which it contains, is divided into two by a trans verse septum. From the upper of the resulting compartments is developed a papilliforrn projection, which is composed, as in the Equi setaceae, of two double pairs of cylindrical cells, surrounding an axial intercellular canal. In cell, from which the gelatinous thickening has now disappeared. Soon after, springing asunder from each other, they tear the membrane of the parent cell, retaining, how ever, their central attachment to the surface of the spore.
90. Lpeopodiace.T.#—The large spore (ma crospore) of Selaginella, consists, when ripe, of an internal spherical vesicle of delicate structure (endospore), which is enclosed in a resistant epispore. The endospore contains a fluid, in which float raucous and oleaginous granules only, its nucleus having disappeared. On its surface are observed three linear projections, all of which converge towards one point, the summit of the spore. The epispore, a structure of later fortnation, is composed of two layers, the internal of which is distinguished from the other by its remarkable transparency. The external sur face is scattered over with acuminated projec tions, which are connected with each other by a network of minute ridges.
91. The developtnent of the prothallium commences (usually several months after the macrospore has been sown) by the deposi tion of several cells on the internal surface of the proper spore membrane, at a point subjacent to that towards which the three ex ternal ridges converge. Whether these cells the lower is contained a vesicular nucleus, the germ-cell of the fully formed archegonium. Its cavity becomes continuous with the axial canal by the solution of its membrane.: 93. The embryo.— About the time that the formation of' archegonia is completed on the upper surface of the prothallium, there is developed, on its inferior aspect, a tissue composed of cells much larger than any of those previously existing. This tissue pro jects, in the form of a cushion, into the cavity of the spore. In general one only of the many archegonia receives the necessary fructifying influence. In this the germ-cell divides re peatedly by transverse septa, as the result of which a structure is formed composed of a series of cylindrical cells placed end to end. The growing extremity of this body, the so-called suspensor, penetrates the single layer of cells which separate it from the in ferior surface of the prothallium, and buries itself in the cushion-like mass. A new deve lopment now commences in the terminal cell, which is divided by a succession of septa inclined alternately in opposite directions.
is occupied by a large cell, which is enclosed by a single layer of others, and supported From this results an egg-shaped body, the primary axis of the embryo, which, as it en larges, causes the absorption of the cells by which it is immediately surrounded. Soon after a new (secondary) axis is developed, the direction of which (obliquely upward), is nearly opposite to that of its predecessor. It finally, makes its escape from the cavity of the spore by penetrating the prothallium near its centre, bearing upon its summit the first pair of leaves of the young plant.
94. Sporangia and spores.— The sporangia of Selaginella denticulate are formed in the axils of the leaves of the fertile branch, in the following manner. A superficial cell of the stem, the position of which is always imme diately above the middle of the line of in sertion of the leaf, is developed to a nipple shaped projection. The centre of this body on a short pedicle. As it advances towards maturity, the spore-case consists of a capsule of three layers. Of these the external or epidermic, is composed of narrow prismatic cellules containing only a transparent fluid. The cells of the middle layer are tabular, and contain starch granules, while those most internal are narrow and somewhat columnar, with very delicate walls. Within this capsule is enclosed a central mass of larger cells, which exhibit central nuclei and granular contents. These, which are the parent cells of the spores, are at first intimately united, but afterwards lie loose in the cavity of the spore-case. Up to this point the development of all the sporangia is uniform. In those in which macrospores are to be produced (oophoridia), one of the parent cells, in no respect different from its fellows in struc ture, continues to increase in size while they disappear. Its nucleus is soon replaced by four others, which arrange themselves, as in Equisetum, towards the four angles of a regular tetrahedron. Septa are afterwards formed, which divide the cell into four com partments, in each of which a spore is de veloped. The spore at first exhibits only a delicate membrane, but as it approaches maturity the three converging ridges, and, finally, the external tegument, the structure of which has been already' described, are formed upon its surface. No sooner is this process completed than the membrane of the parent cell disappears, the four spores retaining their relative position, however, to each other, apparently attached by the remainder of the septa. It is at the point at which the spores are in relation with the centre of the mother cells, that the three ridges converge, as well as the three lines by which the valves of the external tegument give way to allow of the growth of the pro thallium.