before the rupture of the exospore, which eventually splits at the apex and exposes the prothallus. The archegonia originate from a superficial cell of the prothallus that becomes divided by a wall parallel to the surface, the upper

Fig. 38.-Median longitudinal section through an ovule of the pine (Picea vulgaris). e, embryo-sac filled with endosperm; a, ventral portion; c, neck of an archegonium; n, nucleus of the oosphere; nc, the nucellus of the ovule; p, pollen-grains upon and in the apex of the nucellus; t, pollen-tubes, traversing the nucellus; i, integument; s, portion of wing of seed (x 9). (From Strasburger.)

cell forms the neck, the lower one the ventral portion of the archegonium. The first archegonium originates at the apex

of the prothallus, others following in centrifugal succession. The microspores after remaining in a unicellular condition. during the winter are divided by a septum into two very unequal cells the smaller of the two undergoes no further. change, and represents the last vestige of the male prothallus ; the larger cell divides into from four to eight cells, each of which further divides into four mother-cells of antherozoids. The antherozoids are spirally coiled and furnished at the anterior end with two cilia. After fertilization the oospore divides into two cells by a septum formed at right angles to the axis of the archegonium; from the upper cell is formed the suspensor, the lower cell forming the embryo, which becomes highly differentiated before it leaves the megaspore, resembling in this respect the embryo in Phanerogams. By increase in length of the suspensor the lower portion of the embryo is forced down into the endosperm, from which its foot absorbs food.

In the only other genus, Isöetes, the stem is very short and unbranched, and completely covered by the sheathingbases of the long, subulate or filiform leaves. The sporangia are produced singly in a depression in the sheathingbase of the leaf called the fovea. The outer leaves bear megasporanges only, the inner ones microsporanges only. The megasporangia frequently contain more than four

megaspores.

The species of Selaginella are terrestrial, and most abundant in the tropics. Most species of Isöetes are aquatic, a very few terrestrial.

Rhizocarpeæ.

The species grow either entirely submerged or floating on water, and are mostly natives of warm regions, although one

Q

genus, Pilularia, is not uncommon in Britain. The name is derived from the fact that the sexual reproductive organs are produced near the root or the base of the leaves. The stem, as also the root, grows by the repeated division of a single apical cell. The stem is very short in Azolla and Salvinia, elongated and prostrate in Marsilea and Pilularia; its vascular bundles are concentric and closed, and each bundle is surrounded by a bundle-sheath. Salvinia is rootless, the other genera are furnished with true fibrous roots. The leaves vary considerably in the four genera. In Marsilea and Pilularia the vernation is circinate, in the latter genus the petiole alone is present and grows erect and quill-like, hence the popular name of "quillworts." In Marsilea there is an elongated slender petiole bearing four leaflets at the apex. In Azolla the leaves are placed in two rows on the stem, each leaf is deeply divided, one lobe floating, the other submerged; finally in Salvinia the leaves are arranged in whorls of three, two of which are flattened and float, the third is submerged and divided into many long slender filaments that perform the functions of roots, the latter being absent in this genus. The two kinds of sporangia-megasporangia and microsporangia-occur in clusters, and are always enclosed in a closed capsule, the whole forming a sorus or sporocarp. The sporocarp is by some considered homologous with the ovule and its coats in Phanerogams. In Salvinia and Azolla the sporocarp is unilocular, in Marsilea and Pilularia plurilocular. Up to the formation of the spore mother-cells no difference is observable between the development of megaspores and microspores. In both a tapetum is formed, enclosing a central tetrahedral archespore which divides into fourteen spore mother-cells; each of these divides again into four

genus, Pilularia, is not uncommon in Britain. The name is derived from the fact that the sexual reproductive organs. are produced near the root or the base of the leaves. The stem, as also the root, grows by the repeated division of a single apical cell. The stem is very short in Azolla and Salvinia, elongated and prostrate in Marsilea and Pilularia ; its vascular bundles are concentric and closed, and each bundle is surrounded by a bundle-sheath. Salvinia is rootless, the other genera are furnished with true fibrous roots. The leaves vary considerably in the four genera. In Marsilea and Pilularia the vernation is circinate, in the latter genus the petiole alone is present and grows erect and quill-like, hence the popular name of "quillworts." In Marsilea there is an elongated slender petiole bearing four leaflets at the apex. In Azolla the leaves are placed in two rows on the stem, each leaf is deeply divided, one lobe floating, the other submerged; finally in Salvinia the leaves are arranged in whorls of three, two of which are flattened and float, the third is submerged and divided into many. long slender filaments that perform the functions of roots, the latter being absent in this genus. The two kinds of sporangia-megasporangia and microsporangia-occur in clusters, and are always enclosed in a closed capsule, the whole forming a sorus or sporocarp. The sporocarp is by some considered homologous with the ovule and its coats in Phanerogams. In Salvinia and Azolla the sporocarp is unilocular, in Marsilea and Pilularia plurilocular. Up to the formation of the spore mother-cells no difference is observable between the development of megaspores and microspores. In both a tapetum is formed, enclosing a central tetrahedral archespore which divides into fourteen spore mother-cells; each of these divides again into four.