Order 2. =Marattiaceæ= are tropical Ferns, whose gigantic leaves resemble those of the Polypodiaceæ, but have stipules in addition. The sporangia are grouped in sori, situated on the lower side of the leaves, the sporangia in each sorus being arranged either in two rows or in a ring. In _Angiopteris_ they are isolated (Fig. 210 _A_), but in the other species (_Kaulfussia_, _Danæa_, _Marattia_), they are united, and form “synangia” divided into a number of chambers corresponding to the sporangia. These open by clefts or pores. _Marattia_ presents the highest development, as its sporangia are completely united in a capsule-like synangium, which is closed until maturity, and then opens by two valves. In each valve there is a row of three to eleven sporangia, each opening by a slit towards the inside (Fig. 210 _B_, _C_). An indusium encloses the sorus, except in _Kaulfussia_; it is formed of flat and lobed hairs, which resemble the hairs of the other portions of the leaves. In _Angiopteris_ and _Marattia_ the indusium is very rudimentary; in _Danæa_ it forms a kind of cupule. The numerous fossil Marattiaceæ (15 genera, with 98 species) present similar differences to those now living, but more various forms are found, for example, with solitary free sporangia. Those now living are the last small remnant (4 genera with only 23 species) of a once dominant family, which existed from very early times, and whose culminating point was reached in the Kulm and Coal periods. The Ophioglossaceæ appear also in the Kulm and Coal periods, and were about as numerous as at the present time (presumably 2 genera, with 19 species). Leptosporangiate Ferns appear however to have occurred first of all in the Trias-formation. [Illustration: FIG. 210.--Sporangia of the Marattiaceæ: _A_ _Angiopteris_; _B_ and _C Marattia_; _C_ is a half sorus with nine sporangia, each of which has opened by a longitudinal cleft.] Family 2. =Leptosporangiatæ.= Order 1. =Polypodiaceæ.= Sporangia on the lower side of the leaves, _stalked_ and provided with a _vertical_, incomplete annulus; dehiscing by a transverse cleft (Fig. 211 _D_).--The genera are distinguished by the form of the indusium and the position of the sori, etc. 1. The sporangia cover the entire lower surface of the leaf (Tropical America and Asia). _Acrostichum_, _Platycerium._ 2. Sori without indusia, circular or oval. _Polypodium_ (Fig. 211 _A_). The leaves are most frequently situated in two rows on the dorsal side of the creeping rhizome, and fall off leaving a smooth scar behind.--_P. vulgare_, common in woods, on stones. (_Phegopteris_ also has no indusium; see page 214).