Whatever the size of a fern may be, its spores (fig. 51 A) are microscopic. They are produced within the sporangium by cell division, and are therefore free and variously shaped. They consist of a grumous mass enclosed in a double coat; and when the spore begins to grow, it sends out from the cell wall of its inner coat a white tubular projection or root fibre (fig. 51 B), which passes through the cell wall of its outer coat. This root sucks up liquid till it expands the inner coat sufficiently to burst open the outer one, and then it begins to increase by the subdivision of its cells, till the primary green leaf or marchantioid prothallus (fig. 51 D) is formed. This prothallus lies flat on the ground, and is furnished on the under-side with fibrous roots to fix it, and supply it with food. [Illustration: Fig. 51. Development of spores of Pteris serrulata:—A, spore; B, C, early stages of development; D, the prothallus with radical fibres (_a_, _b_) and antheridia (_h_, _h_).] Two sets of organs are subsequently developed on the under-side of this prothallus; one of these is a stalked cell called an antheridium, and is situated near the roots; the other is an archegonium containing a germ cell, which is sunk in the cellular tissue. In each of the antheridia, which are numerous, a cell is formed, which becomes filled with a mass of mucilage mixed with a number of free cells, containing a flat ribbon-shaped filament, or spermatozoid, coiled in a spiral manner, which, as soon as set free by the rupture of the cell, revolves rapidly by means of several long cilia placed close to the large end. Fig. 52 shows the globular antheridium and the spermatozoids of Pteris serrulata. The archegonia are fewer in number, and contain the germ cell, represented in fig. 53, as viewed from above, and sidewise; it is placed at a little distance from the antheridia, and after being fertilized by the active filaments or spermatozoids, and matured, it contains the primordial cell of the young fern, which soon sends forth leaves rolled up and curled inwards previous to expansion, and a root, which being sufficient to feed the plant, the flat pro-embryo, which was the first stage of development, perishes. Thus in the family of ferns there are two distinct periods of growth, and one only of fertilization. In flowering plants, fertilization and its products are the final result of vegetation, and the maturation of the fructification is frequently followed by death. In the ferns, on the contrary, fertilization precedes the development of the plant, which, if perennial, continues to bear fertilized fruit year after year. [Illustration: Fig. 52. Antheridium and Spermatozoids of Pteris serrulata:—A, projection of one of the cells of the prothallus showing the antheridial cell (_b_); B, antheridium fully developed, containing sperm-cells, each enclosing a spermatozoid; C, one of the spermatozoids magnified, showing the cilia.]