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]).
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.
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.