=Systematic Division.= The Ferns may be divided into two groups, characterized by the structure and development of the sporangia. The sporangia in the EUSPORANGIATÆ take their origin from a group of epidermal cells, and their walls are formed by several layers of cells. The archesporium is the (not tetrahedric) hypodermal terminal cell of the axial row of cells which give rise to the sporangium. In the LEPTOSPORANGIATÆ the sporangia are developed from single epidermal cells, and their walls are uni-layered. The archesporium is a central, often tetrahedric cell, from which sixteen spore-mother-cells are developed.[19] It is difficult to say which form is the oldest (according to Prantl, those which have the sori on the nerve-endings); however, the Eusporangiatæ would seem to have made their appearance long before the others, and also well defined Marattiaceæ and Ophioglossaceæ occur in the Kulm and Coal period, before the true Polypodiaceæ. About 4,000 species of Ferns are now existing, and they are found especially in tropical and sub-tropical forests. Family 1. =Eusporangiatæ.= Order 1. =Ophioglossaceæ.= The prothallium differs from that of all other Ferns in being _subterranean_, _free from chlorophyll_, _pale_ and _tuberous_. The stem is extremely short, with short internodes, most frequently unbranched, vertical, and entirely buried in the ground (Fig. 208 _st_). In several species (among which are the native ones) one leaf is produced every year, which has taken three to four years for its development. In _Botrychium_ a closed, sheath-like basal part of each leaf covers the subsequent leaves during their development. In _Ophioglossum_ and others each leaf has at its base an intrapetiolar, cap-like sheath, which protects the succeeding leaf. The leaves are of two kinds: (_a_) foliage, which in _Ophioglossum vulgatum_ are lanceolate and entire, but in _Botrychium_ however, are pinnate (_b_ in Fig. 208 _A_, _B_); and (_b_) fertile, which are found facing the upper side of the foliage-leaves. These latter in _Ophioglossum_ are undivided and spike-like (Fig. 209 _A_), but pinnate in _Botrychium_ (Fig. 208 _B_). Each foliage and fertile leaf are branches from the same petiole. The large sporangia are placed laterally, and open by two valves. No annulus is formed (Fig. 209).--_Ophioglossum_ reproduces vegetatively by adventitious buds on the roots. [Illustration: FIG. 208.--_A Ophioglossum vulgatum_ (Adder’s-tongue); _B Botrychium lunaria_ (Moonwort), both natural size; _r-r_ roots; _bs_ leaf-stalk; _st_ stem; _b_ foliage-leaf; _f_ fertile leaf.] [Illustration: FIG. 209.--Fertile leaf of _Ophioglossum_.] Three genera with about twelve species.