Each sorus, when magnified, is seen to consist of numerous capsules (thecae); these capsules contain the spores. The spores are angular-shaped bodies (Fig. 125, a), with an external coat of a brownish color, which is variously marked, like the pollen of higher plants. The spores, when placed in a damp surface and exposed to the proper influences of heat and light, germinate; that is, the angles of the spore are rounded off, the internal coat of the spore is then protruded, becoming the root-fibre (Fig. 125, b); the outer coat of the spore bursting, the inner coat grows in an opposite direction to that of the root-fibre as an elongated filament (Fig. 125, c); cell after cell is added in a longitudinal direction, the plant soon resembling an Alga. After a time, however, the cells are produced transversely as well as longitudinally, resulting in the formation of a flattened leaf-like expansion (Fig. 125, d), a cellular structure, the so-called Prothallus, which can scarcely be distinguished from a young Marchantia. In this Prothallus are developed Archegonia and Antheridia: the union of the embryo-cell of the Archegonia and the spiral filament of the Antheridia gives rise to the new Fern, which may be seen growing out of the Prothallus (Fig. 126), which soon passes away. These two stages in the life of a Fern represent two distinct plants. The Prothallus stage is a cellular plant closely resembling a young Marchantia, which is later transformed into a stem- and leaf-bearing plant. The growth of the Horse-tail offers the same metamorphosis; spores producing a Prothallus from which the Horse-tail is developed. While the Mosses are probably the posterity of Jungermannia-like plants, the Ferns have most likely descended from forms allied to Marchantia; this view being based on the fact of the Fern passing through a Marchantia-like stage, with similar reproductive organs.
