77. Development of the The upper portion of the cylindrical fruit, which is des tined to become the capsule, begins, some time after the calyptra has given way, to dilate rapidly. Soon after there is formed, by the separation of the external and superficial lay ers of cells from the central portion, a cavity of the form of a hollow cylinder, the axis of which coincides Nvith that of the fruit. At this stage, the central portion consists of an axile column of large cells, closely invested by a single layer of smaller ones (the coluinella) ; a superficial layer of cells, about four times as large as those last mentioned ; and lastly, between the two, a layer of nucleated cells, with granular contents, the primary parent cells of the spores. The development of these last consists in the disappearance of the nu cleus of each, and the substitution for it of two others ; this bein,g accompanied or fol lowed by the division of the primordial mem brane into two new vesicles, each of which encloses a nucleus. A cellulose membrane is now formed at the surface of contact of the two vesicles by which the original cavity is bisected. In the cavity of each of the result ing nucleated cells, two new ones rnake their appearance, apparently by contraction of the primordial membrane, either before or imme diately after its division into two halves. On the surface of each half cellulose is secreted, so that the spherical cells which are thus formed possess a delicate cellulose external, and a very distinct inner membrane (primor dial vesicle.) This last divides into four por tions (the young spores), each of which becomes invested with a layer of cellulose. The ripe spore has been already described. The capsule now gives way at the line of its insertion on the pedicle which supports it. It is by the opening thus produced that the spores make their escape after the dislocation of the layers of cells immediately surrounding them. Phascum differs from all other genera in the absence of all trace of an operculum.
78. Fern.s.—tio two plants could be found which differ more completely from each other in the appearance which they present to the ordinary. observer, than a Hepatica and a Fern, at the moment that the spores of each arrive at maturity ; yet, in the history of their organ isation and development a very close corre spondence exists. The immediate result of the germination of the spore of a Fern is a frond similar to that ot the s.impler forms of Hepatica ; on this frond antheridia and arche gonia are formed. In each fructified arche gonium, a central germ-cell is developed to a new indiNidual, widely different in organisation from the parent. It, in its turn, produces spores, the germination of each of which is the commencement of a new circle of phenomena similar to the one which precedes it. Dividing this circle into two periods, as before, we have the following stages in the development.
79. First period.* ofthe spore. — The mature fern-spore consists of a delicate transparent vesicle, which is invested in a brown resistant external membrane. Germi nation consists in the budding out of the trans parent vesicle so as to form a nipple-shaped projection, which penetrates the external mem brane. The ptojecting part divides repeatedly by transverse septa. About the same time a second budding out takes place in the oppo site direction, which is destined to the forma tion of a root. By the further growth of new cells, a flattened two-lobed organ is formed — the Prothallium.
and take their origin as follows. A hemisphe rical projecting portion of one of the superfi cial cells is cut off from the rest by a horizon tal septum as in Anthoceros. This is divided by a single transverse septum. In the result ing terminal cell a second septum is formed, inclined to the horizon at a small angle, which is followed by a third, inclined in the opposite direction. Both of the cells resulting from these divisions, and subjacent to the last-formed sevta, are again divided by perpendicular septa coinciding with the axis of the papilliform rudiment. In one oF the resulting cells there
is then formed a perpendicular septum, which meets its predecessor at an angle of 45.° Hence results a club-shaped body, consisting of a four-sided central cell, filled with granular mucus, and enclosed by six others, having the following arrangement. Four of the form of segments of a hollow cylinder, which are in contact by their edges, surround the central cell on all sides. It is surmounted by a fifth, which is hemispherical (the terminal cell last formed). A sixth, the cell resulting from the first division by a hotizontal septum, is cylin drical, and serves as a pedicle on which the whole is supported. The central cell is con verted by a successive division into a round group of dice-shaped cellules, in the interior of each of which a delicate lenticular vesicle is formed, which contains, rolled up in its in terior, a spiral filament. The ripe anthericlium bursts at its summit, and the escape of its contents is, as in the preceding cases, followed by the bursting of the vesicles, and the com mencement of the active motions of the spiral the antherozoid is of two kinds—of progression and of revolution round the axis of the spiral.
81. The arehegonia.— At a period somewhat later than that at which the rudiments of the antheridia begin to appear, there commences on the inferior aspect of the prothallium, and in the immediate neighbourhood of the notch by which its anterior margin is bisected so as to form two lobes, an active development of new cells. The result of this is the formation of a cushion-like projection of the surface bor dering the notch above mentioned, upon the anterior aspect of which the archegonia are formed.
82. Each archegonium takes its origin from a cell, which is distinguished from those sur rounding it by the comparative abundance of granular mucus which it contains, and by the presence of a distinct central vesicular nucleus. This cell divides by a horizontal septum into a superior and an inferior portion. It is from the latter, which is hemispherical, that the papilla which forms the rudiment of the pro jecting portion of the organs is formed. It consists, as in the Hepaticm and Mosses,of four contiguous columns of cells, each of which is a half segment of a cylinder, the whole being surmounted by a hemispherical terminal cell. In the further development, varieties are often observed, even on the same prothallium. This is dependent on the mode in which the canal occupying the axis of the mature archegonium is produced. Most frequently a central column of cells is formed in exactly the same manner as an Anthoceros. The cells forming it are afterwards absorbed and dis appear, leaving a four-sided canal. In the other case, the canal results simply from the separation of the four piles of cells along their common line of contact. This is the arrange filaments (antherozoids.) In each filament the extremity which is directed forwards du ring motion, is broader than any other part, while the opposite extremity (posterior) tapers ofi into a long filament. The anterior coil of the spiral bears on the surface furthest from its axis a number of delicate cilia. The motion of ment which occurs constantly among the Equisetaceoe, Ilcopotliacem, and Rhizocar pere. In reference to the mode of origin of the germ-cell, there is some difference of opi nion. According to Hofmeister,* the cell which contains it originates by the formation of a tangental septum in the lowest of the cells, constituting one of the four columns of which the rudiment is composed. According to Mr. Henfrey*, on the other hand, the germ cell is contained in the superior, and conse quently deeper, of the two portions into which the primary nucleated parent cell of the organ divides by a horizontal septum ; and is dis tinguishable before the formation of the pa pilla-like structure has commenced. This account of the matter is not only supported by analogy, but, as it appears to us, in a very marked manner by Hofmeister's own drawings.