The apices of the stem and leaves, being otherwise protected, develop segments only from the sides of the apical cell, the outer face never having segments cut off from it. CHAPTER XIII. CLASSIFICATION OF THE PTERIDOPHYTES. There are three well-marked classes of the Pteridophytes: the ferns (_Filicinæ_); horse-tails (_Equisetinæ_); and the club mosses (_Lycopodinæ_). CLASS I.--FERNS (_Filicinæ_). The ferns constitute by far the greater number of pteridophytes, and their general structure corresponds with that of the maiden-hair fern described. There are three orders, of which two, the true ferns (_Filices_) and the adder-tongues (_Ophioglossaceæ_), are represented in the United States. A third order, intermediate in some respects between these two, and called the ringless ferns (_Marattiaceæ_), has no representatives within our territory. The classification is at present based largely upon the characters of the sporophyte, the sexual plants being still very imperfectly known in many forms. The adder-tongues (_Ophioglossaceæ_) are mostly plants of rather small size, ranging from about ten to fifty centimetres in height. There are two genera in the United States, the true adder-tongues (_Ophioglossum_) and the grape ferns (_Botrychium_). They send up but one leaf each year, and this in fruiting specimens (Fig. 70, _A_) is divided into two portions, the spore bearing (_x_) and the green vegetative part. In _Botrychium_ the leaves are more or less deeply divided, and the sporangia distinct (Fig. 71, _B_). In _Ophioglossum_ the sterile division of the leaf is usually smooth and undivided, and the spore-bearing division forms a sort of spike, and the sporangia are much less distinct. The sporangia in both differ essentially from those of the true ferns in not being derived from a single epidermal cell, but are developed in part from the ground tissue of the leaf. [Illustration: FIG. 70.--Forms of ferns. _A_, grape fern (_Botrychium_), × ½. _x_, fertile part of the leaf. _B_, sporangia of _Botrychium_, × 3. _C_, flowering fern (_Osmunda_). _x_, spore-bearing leaflets, × ½. _D_, a sporangium of _Osmunda_, × 25. _r_, ring. _E_, _Polypodium_, × 1. _F_, brake (_Pteris_), × 1. _G_, shield fern (_Aspidium_), × 2. _H_, spleen-wort (_Asplenium_), × 2. _I_, ostrich fern (_Onoclea_), × 1. _J_, the same, with the incurved edges of the leaflet partially raised so as to show the masses of sporangia beneath, × 2.] In the true ferns (_Filices_), the sporangia resemble those already described, arising in all (unless possibly _Osmunda_) from a single epidermal cell.