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).
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.
As plants in conservatories and rooms the following are cultivated: species of Gymnogramme (tropical America), Lomaria, Nephrolepis, Pteris (P. serrulata, cretica).
Officinal. Aspidium filix-mas, rhizome and the withered petioles.—Species of Alsophila and Cibotium give Penghawar Djambi. The rhizome of Pteridium aquilinum, var. esculentum, contains so much starch that it is used as food.
The other orders of true Ferns deviate from the Polypodiaceæ, especially in the formation of the annulus, the bursting of the sporangium and its mode of attachment and development, and in the differences in the formation of the prothallium, etc. The principal are:—
Order 2. Hymenophyllaceæ. To this order belong the lowest and most Moss-like Ferns; the leaves, with the exception of the veins, are most frequently formed of only one layer of cells, and consequently stomata are wanting; the formation of the prothallium also somewhat resembles the Mosses. Sori marginal, on the extremities of the vascular bundles, and surrounded by a cupular indusium. The sporangia are sessile, with equatorial annulus. Hymenophyllum (H. tunbridgense, European). Trichomanes (T. speciosum, European). Species about 200, which live especially on rocks and trees in damp and shady tropical forests. Some have no roots.