3. The sporangia are situated in continuous lines just inside the margin of the leaf.--_Pteris_[20]: the sporangia form a continuous line along the entire margin of the leaf (Fig. 211 _C_), which bends over and covers the sporangia, forming a “false-indusium.” _Pteridium_ has linear sori situated on a marginal vascular bundle, covered by two linear basal indusia, of which the outer is bent over like the edge of a leaf.--_P. aquilinum_ (Bracken) has a wide-spreading rhizome with large alternate leaves, placed on opposite sides, at some distance apart. Only one leaf is developed from each branch every year. [Illustration: FIG. 211.--Portions of leaves with sori. _A_ _Polypodium_. _B Aspidium_. _C Pteridium_. _D_ A sporangium of one of the Polypodiaceæ: _r_ the annulus; _s_ spores.] _Adiantum_ (Maiden-hair): sori on the underside of small portions of the edge of the leaf, which are bent over (false indusium). _Cryptogramme_ (_Allosorus_), _Cheilanthes_. 4. The sori are oval or linear, situated on one side of the vascular bundle.--_Asplenium_ (Fig. 212 _A_): sori linear; indusium with one of its edges attached at the external side. _A. ruta muraria_ (Wall-Rue); _A. septentrionale_; _A. trichomanes_.--_Athyrium_: sori linear or curved; _A. filix-fœmina_ (Lady-Fern).--_Scolopendrium_ (Fig. 212 _B_): sori as in _Asplenium_, but situated in pairs across the lanceolate, entire leaves. Each sorus is covered on the external side by an indusium, whose free edges are parallel and approach each other. _S. vulgare_ (Hart’s-tongue).--~_Blechnum_ (_B. spicant_, Hard Fern; the fertile leaves differ from the barren, the pinnæ being narrower, while the underside is almost entirely covered with sori, and hence they are of a much darker brownish hue than the barren ones).--_Ceterach_: indusium rudimentary or absent.~ 5. Sori circular and covered by a shield-like, or reniform indusium.--_Aspidium_ (Fig. 211 _B_); the leaves wither away and leave no scar upon the root-stock. _A. filix-mas_ (Male-Fern); _A. spinulosum_.--_Phegopteris_ has no indusium, the withered bases of the leaf-stalks are persistent; _P. dryopteris_ and _P. polypodioides_. 6. The indusium is situated below the sori, and has the shape of a one-sided scale (_Cystopteris_, _Struthiopteris_), or of a cup or cupule, which in _Woodsia_ is sometimes fimbriate (Fig. 212 _C_, _D_). [Illustration: FIG. 212.--_A Asplenium_. _B Scolopendrium_. _C Woodsia_; _D_ single sorus of the same. _E Cyathea_: the sporangia have fallen off in the upper sori. (All magnified.)] 7. The sori are situated on the margin of the leaf, and at the end of a vascular bundle. Indusium, semi-cupular. _Davallia._ Principally tropical species. 1 in S. Europe. This order is the greatest, comprising about 2,800 species, the majority being perennial plants. A few are large, and known as Tree-Ferns.