95. In those sporangia in which microspores are to be formed, all of the original parent cells exhibit a development which corresponds with that which is above described as occur ring in one only in the oophoridiunt, with this exception, that they do not attain the same dimensions. Hence results a large number of microspores which resemble the macrospores in the strurture of their internal membiane, and external three-valved tegument, but differ completely in their mode of germination. After lying a certain time on a damp surface, their inner cavities are found to be occupied by a number of small spherical cellules, each of which contains in its interior a spirally coiled fibre (antherozoid). By the dehis cence of the valves of the external tegument, the antherozoa are set free; and it is presumed, that it is by their agency that the archegonium is fructified, after the prothallium has been laid bare by the bursting of the macrospore at its apex.
96. Rhizocarpee.—In describing the mode of reproduction of the Rhizocarpex, we shall confine our attention to the genus Pilularia, recpecting which the most exact researches have been made.
97. The macrospore of Pilularia is an egg shaped body, presenting an equatorial constric tion. It consists of an internal proper mem brane (endospore), the so-called embryo sac, 98. The prothalliunz.—The first indication of the commencement of the germination of the macrospore is the formation of a lenticular accumulation of granular plasma, at the sum mit of the endospore, which had previously contained only starchy, mucous, or oleaginous granules. Soon after there appears in the same position a delicate cell of similar form, the upper surface of which is in contact with the endosporal membrane, and is immediately subjacent to the aperture in the exospore. It is in all probability from this cell, although the earlier stages of the development have not been clearly made out, that the prothallimn takes its origin. A day or two after germina tion it consists of a central cell, which is sur rounded by a single layer of others of smaller dimensions. Four of these last are invariably found interposed between the upper surface of the large cell and the spore membrane ; the septa by which they are separated being per pendicular, and at right angles to each other. Soon after the central cell itself divides by a transverse septum into two ; of these the upper, of globular form, contains a large ve sicular nucleus, the future germ-cell. The
lower, which is tabular, divides repeatedly by vertical septa, so as to form a single layer of cells which intervenes between the cavity of the archegonium and that of the spore. In the meantime the four cells which surmount which is surrounded by a white coriaceous exospore. This last exhibits two distinct layers, of which the intertral is colourless and vitreous, without trace of structure ; while the external appears to be formed of prismatic columns fitting closely together, which are more distinct at the lower end of the spore, yank they disappear entirely towards its smaller end or apex, at which point the exo spore forms a papilliform projection open at its summit. From this arrangement there results a canal, which is immediately sur rounded by the thickened and dentate margin of the vitreous layer, and leads to the apex of the endospore.
the germ-cell extend upwards in the form of four papillm, separated from each other by an axial canal, which burst through the proper spore membrane, and finally project beyond the aperture of the exospore. By the absorp tion of the central cell, its cavity becomes continuous with that of the vertical quadri lateral canal above mentioned.
99. The embryo.—In consequence of the presumed entrance of the antherozoids in to the cavity of the archegonium, the germ-cell en larges, and is transformed by repeated division into an embryo, which is at first a somewhat meniscus-shaped body, formed of minute cel lules. Soon after, a conical projection of its upper surface presents itself, which rapidly increases by the repeated division of a minal cell by alternately inclined septa. The direction of growth of this structure, which is the first leaf of the embryo, is obliquely wards. In its axil is formed the primary axis, of the vertical series of sporangia which cor responds to each valve, one only of the ori ginal central cells continues its development, the rest becoming abortive, and finally disap pearing. The four spores, which are formed just as in Selaginella, at length become free by the absorption of the cell in which they are enclosed, and for a time continue to en large equally, while their walls are thickened by internal gelatinous deposition. Soon, how ever, one begins to exceed the rest in growth, and finally occupies the whole cavity of the sporangium, which is subsequently burst by the swelling of the exospore, which is pro duced when it is subjected to the influence of moisture.