10 1. The microspores are developed precisely as in Selaginella. The exosporal membrane and soon after, us a lateral development from this last, the second leaf. In the meantime the first root makes its appearance as a rounded projection, which grows from the upper sur face of the embryo, in a direction opposite to that of the first leaf. Both of the last-men tioned organs filially burst through the remains of the prothallium, and become free.
100. Sporangia and spores.—The organ in which the sporangia of Pilularia are contained is an egg-shaped body, supported on a short, curved pedicle, which springs directly from the creeping stem, in the axil of one of the awl-shaped leaves. It presents a tough, cori aceous, cellular coat, which encloses a cavity, which is divided into four compartments by vertical septa, and subsequently dehisces in four valves. The middle of the internal sur face of each valve is, from the first, marked by a ridge of gelatinous cellular tissue, from which the sporangia take their origin as a vertical series of projections. Their development re mains up to a certain time the same, w hether they are to produce large or small spores. All are found to exhibit at this period a central mass of cells, containing nuclei and grumous fluid, which is surrounded by a double capsular layer. In each of the central cells, the nucleus soon after is replaced by four others of smaller size, around which are formed four tetrahedral secondary cells, which are the im mediate parents of the spores. In the lowest dehisces in-th-ree valves, the proper membrane of the spore at the same time giving way irre gularly, to allow the escape of numerous little globular cellules. These cellules contain, in addition to starchy and mucous granules, pa rietal lenticular vesicles, each of which en closes a delicate, spirally coiled antherozoid, which moves actively in water.
102. Phaneroganda.—Between the higher vascular Cryptogamia, and the simplest forms of flowering plants, there exists, as has been already noticed, a wide chasm of obscurity. The researches, however, of Hofmeister, have shown that in the Coniferm the embryo is formed upon a plan which presents the most striking analogies to what is observed among the Ithizocarpem and Lycopodiacem ; and that, in fact, their development stands intermediate between that of the plants just rnentioned and the angiospermous Phancrogamia.
103. Phanerogamia gymnospermia.—Follow ing the same plan of description that we have adopted in the previous section, w e shall confine our attention to the Abietinece, of the deve lopment of which Hofincister has furnished us with a most complete account. The so-called ovule consists, at the time of the scattering of the pollen, of a short and thick nucleus of delicate cellular tissue, which is enclosed in a single, somewhat fleshy integument, leaving open a wide micropyle canal.* In the centre of the nucleus there exists an ovoidal embryo sac, which owes it origin to the coalescence of a vertical and axial series of cells. At this
period it contains only granules of starch and cells contained previously in its cavity. to its membrane. By the continuation of this pro. cess, the sac becomes a second time filled with cellular tissue.* Two or three of the cells sub. jacent to the micropyle end of the embryo mucus, the nucleus which it at first contained having disappeared. It corresponds, as will be seen as we proceed, to the internal mem brane of the ripe macrospore of the Rhizocar pew and Lycopodiacem. The pollen grain in the Co nifermgenerall3 itself reaches the summit of the nucleus by means of the wide micropyle. From each grain emanates a tube, which pene trates for a short distance into the tissue of the nucleus ; not, however, until it has re mained for some time upon its stunmit. the meantime numerous free nuclei have be come visible in the embryo-sac, which imme diately afterwards " presents itself filled with a large nutnber of radially elongated cells, which are arranged in a concentric layer." These continue to multiply by septa in all three directions, until the beginning of winter, at which period the wall of the embryo-sac is so delicate as to be indistinguishable, During the winter months these cells undergo no further change, except that their walls are thickened by internal gelatinous deposition. In the beginning of March of the second year, both the gelatinous material and the cell-wall disappear, the primordial sacs lying free in the cavity of the embryo-sae, each containing a large globular nucleus. Shortly after, the nucleus of each cell disappears, and is re placed by two or four smaller ones, round each of which new spherical secondary cells are formed. The parent cell is dissolved, -and immediately after, the same process is re peated in the secondary cells. While this is taking place, the embryo-sac has increased to twenty times its former volurae ; its membrane has become resistant and vitreous, while throughout the whole ovule, with the excep tion of its summit, an active cell growth has taken place. Towards the middle of May the perrnanent cellular body which after wards fills the whole embryo-sac, originates by the application, in successive layers, of the sac now become larger than the rest, and are destined to contain the germs of the future embryos. As their development proceeds, these bodies, the so-called corpuscula, assume an elongated, oval form, and the space in tervening between their summits and the membrane of the embryo-sac is occupied by four small cells on the same level, which are separated from each other by as many vertical septa, meeting at right angles. Each corpus culum is likewise surrounded on all sides by a single layer of cellules resembling pavement epithelium, and exhibits in its interior a nu cleus which is usually placed at its superior extremity. After some time the nucleus dis appears, and now a number of transparent vesicles become visible, which accumulate for the most part towards the extremities of the corpusculum.