The true mode of development of ferns from their spores was that furnished by Nägeli, who announced the existence of antheridia. On the spore starting into life it sends out from the cell-wall of its outer coat a white tubular projection, or root fibre ([Fig. 305], A, B, and C), which passes through the cell-wall of its outer coat. This attracts sufficient moisture to burst open the outer, and then it begins to increase by the subdivision of its cells, until the primary green prothallus D is formed. This falls to the ground, and, being furnished on its under side with thread-like fibres, fixes itself to the earth, and thus is developed the rhizome, or root of the future plant. In each of the antheridia, which are numerous, a cell is formed, chiefly filled with albuminous matter and free spores, each having attached a flat ribbon-like filament, or stermatoid, curled in a spiral manner. These are ultimately set free by the rupture of the cell-wall, and commence revolving rapidly by the agency of the whip-like appendage at the larger end.
Fig. 305.—Development of the Globular Antheridium and Spermatoids of Pteris serrulata.
A. Spores; B, C. Early stages of development; D. Prothallus with radial fibres; a, a and a, b are stermatoids; and h, h. Enclosed antheridia.
The sporangia, or spore-cases, are, for the most part, globular in form, and are nearly or quite surrounded by a strong elastic ring, which in some cases is continued to form a stalk. When the spores are ripe, this ring, by its elastic force, tears open the sporangia and gives exit to a quantity of microscopic filaments, curled in corkscrew-like fashion (Figs. 305 and 307). The ring assumes various forms; in one group it passes vertically up the back of the sporangium, and is continued to a point termed the stomata, where the horizontal bursting takes place. This form is seen in [Fig. 306], a, b. In other groups it is vertical, as in c, c; in others transverse, as in d; or apical, as at e; and in a few instances it is obsolete, as in f. These are the true ferns, and their systematic arrangement is chiefly founded on the peculiarity of the sori and sporangia, characters which become quite intelligible by the aid of the microscope.
Fig. 306.—Sporangia of Polypodiaceous Ferns.
a, b. Polypodiaceæ; c. Cyantheineæ; d. Gleichenineæ; e. Schizeineæ; f. Osmundineæ.
Fig. 307.—Spores of Deparia prolifera.