114. In the Ilepaticm and Mosses one sep tum is formed, the inferior of the two result ing cells undergoing no further development, while the superior is transformed into the pri mary axis of the fruit. This fruit-axis, the apex of which is converted into a sporangium, is normally a leafless one. In the Mosses, how ever, examples frequently occur, in which its development is changed, under the influence of peculiar external circumstances, in such a manner that, instead of producing a sporan gium it lengthens considerably, and bears symmetrically arranged leaves. Such a con dition makes it more easy to compare the fruit of the 'Mosses and the leafy stems of the higher plants. In the Ferns and Equisetacem, again, only one transverse septum is formed; but here, it is the inferior secondary cell which is developed to the embryo, the direction of the first axis of growth being opposite to that of the archegonium. In Selaginella, a succession of transverse septa are formed, whence results a conferva-like fila inent, which lengthens downwards by repeated division of a terminal cell. At length the youngest cell is transformed into an embryo. Among the Conifer, the same process pre sents itself, with this important difference, that before it commences, the germ divides by two crucial vertical septa into four cells, which correspond to the four embryos which are afterwards formed. In all the Phanerogamia, probably without exception, the germ-cell divides, in the first instance, by a trans verse septum into two cells, of which the upper is the larger. In some cases the lower cell is developed directly to a spherical cel lular mass (as in Hippuris and Orchis Morio). Much more frequently, however, it is trans formed into a conferva-like filament (sus pensor) w Elicit lengthens by repeated division of an inferior terminal cell. At length the youngest cell, instead of lengthening, becomes spherical, and gives rise to the embryo by a process sirnilar to that described above in Hip purls.
115. The org,an to which the name suspen sor is applied by Mr. Henfrey in Orchis Morio, differs materially from that of Selaginella, the Coniferm, or from that described in the pre ceding paragraph. Its formation does not, like that of the true suspensor, precede, but follows the origin of the embryo. In Hippuris, it appears to result from endogenous cell-forma tion in the lengthened upper compartment of the original germ-cell.
116. The difference between the development of the pollen grain, and that of the microspore of Selaginella and of the Rhizocarpew, is no less remarkable. Among the Phanerogamia, after the pollen grain has remained for some time in contact with the stigma, its inner mem brane grows out at one point of its periphery .into a filiform cell ; this lengthens more or less rapidly until it reaches the micropyle of the ovule, which it enters, and at last comes into contact with the embryo sac. The sac usually resists it strongly ; sometimes it is bulged in, but is very rarely perforated. In consequence of this act the transformation of the germ-cell commences. The absence of moving filaments among the higher plants stands connected with the intervention of a second membrane (that of the embryo sac) between the two fluids, the union of which seems to constitute the essential condition of fecundation.
117. In comparing the development of the microspore with that of the spore of the Ferns with which the plants among which it presents itself are so closely allied, the difference is even more striking. In Selaginella all the steps intervening in the Fern between the spore and the tessellar cells of the antheri dium have disappeared.
118. Direct observations relating to the act of impregnation among the Cryptogamia, are for the most part wanting. The presence of antherozoids in the cavities of the archegonia of the Ferns has been witnessed only by Su minski and Mercklin. Among the Hepaticx and Mosses, Hofmeister observed within the involucre of Jungermannia bivaricata,anthero zoa " which moved rapidly and played livelily round the archegonia."# In this species, as well as in J. bicrenata and bicuspidata, the same observer found a mucous substance of glass-like transparency, occupying the mouths of the archegonia. In this substance were embedded numerous curled fibres, which he considered to be dead antherozoids. Evi dence more to be depended upon is that of the concurrent testimony of all observers that, among the dicecious mosses and liver worts, wherever plants bearing archegonia grow in the neighbourhood of those bearing antheridia, fruits are almost always produced ; while in the contrary case, the archegonia are abortive.
119. Origin and development of germ-cells in special organs destined for their reception, which are capable of transfirmation into rudiments of new plants, without the concurrence of two organs of opposite functions. — Of this, distinct ex amples occur only among the Hepaticx ; viz. among the leafy Jungermanniw and the Marchantim. In on e of the latter, the Lunularia vulgaris, there is formed by the doubling in of the epidermal layer of the upper surface of the frond, immediately behind the notch in the anterior margin, a crescentic pouch, which extends backwards for about a line under the surface. Its cavity is bounded by an inferior and a superior wall, whose concave surfaces unite in a sharp maroin, the plane of which inclines slightly back%ards and downwards. The upper wall is formed by the double epi dermal membrane ; the lower by a membrane which is intimately united with the parenchyma of the frond, in its relations to which it re sembles the tissue which lines the subepider mal air cavities. It consists originally of a single layer of tessellar cells, much smaller than those upon which they are supported. A number of these grow out into papilliform projections, in each of which the projecting hemispherical portion is soon separated by a transverse septum. A second is then formed above the first, and parallel to it. The highest cell next divides by a vertical septum parallel face grow out into nipple-shaped projections, which soon become filamentous roots,and the whole is transformed i n to a riband-sh aped frond. The organ, the development of which is de to the axis of the frond, This is followed on each side by transverse, and afterwards by vertical septa, which last are parallel to it. Hence results a bilateral organ, the surfaces of which are at right angles to the axis of the frond. Its form is at first that of a flattened club : afterwards, as it becomes larger, two notches are formed on each of its lateral margins, which exactly resemble those of the anterior margin of the young frond. As soon as its development is completed—that is, when it has attained a length of about of a line, it is pushed out of the receptacle by its rapidly growing successors. If, after its expulsion, it is sown on a damp surface, a new growth at once commences in two opposite directions, in a line which is at right angles to its axis. At the same time the cells of the inferior sur scribed above, receives, in common with others of a different nature, the name of " gem." The whole process differs widely from that of true gemmation or " rejuvenescence " of an old cell, in order that its primordial vesicle may be transformed into an embryo. This distinction is well illustrated in the getnniation of Anthoceros ; the primordial sac of a cell of the parenchyma of the frond, the position of which is undetermined, contracts and secretes on its surface a new cellulose membrane. The new cell is converted by repeated division into the rudiment of a young frond, which, as it grows, breaks through the tissue of the parent. In Jungermannia bivaricata, we have observed a similar process. A sin,gle cell of the leaf of a marcescent, last year's stem, is seen still to contain a primordial vesicle, lined with green protoplasm. This forms around itself a new cell, which divides by a septum, the direc tion of which is transverse in relation to that of the first gro wth. One of the resulting cells gro ws out so as to project through, or carry before it, the membrane of the old cell. This divides by a septum inclined obliquely to the former, which is succeeded by another, inclined in the opposite direction, and so on alternately. Hence results the rudimentary stem of a new plant. Both of the preceding are examples of gemmation.
The very distinct analogies in development (homologies) which present themselves among the higher plants, are exhibitedin the following table. The six last vertical columns represent the principal groups, which follow each other in the same order as in the preceding pages. In the two first columns are indicated those more partial analogies which may be traced between the higher plants, on the one hand, and the Algae, Fungi, and Lichens on the other.