Order 3. Cyatheaceæ. Annulus complete and oblique. To this order belong, principally, the tree-like Ferns with palm-like habit. The number of species is about 200, they are all tropical and form forests in some regions of Australia. Cibotium and Dicksonia have marginal sori, with cupular, basal indusium. (The stem of D. antarctica is covered with aerial roots.) Alsophila (without indusium); Cyathea with cupular, inferior indusium (Fig. [212] E).
Fig. 213.—Gleichenia: A part of a leaf with sori; B a single sorus.
Order 4. Gleicheniaceæ. Sporangia with equatorial annulus, and longitudinal dehiscence, most frequently groups of 3–4 in sori without indusium (Fig. [213]). Gleichenia: the apical growth of the leaves continues for a long time.
Order 5. Schizæaceæ. Annulus apical. To this order belongs Aneimia, which is so commonly cultivated in conservatories. The two lowest pinnæ are metamorphosed, having no leaf parenchyma and being covered with sporangia. Schizæa. Mohria. Lygodium, a climber, whose leaves have unlimited growth and attain a length of several metres. About 70 species. Tropical.
Order 6. Osmundaceæ. The sporangia have at the apex a lateral group of strongly thickened cells, which gradually pass over into the ordinary cells. The sporangia open by a longitudinal cleft. Indusium wanting. Osmunda bears the sporangia upon peculiar, branched pinnæ, without parenchyma (the uppermost in the leaf). O. regalis (Royal-Fern): European.
Sub-Class 2. Hydropterideæ (formerly Rhizocarpeæ), Water Ferns.
The following further characteristics must be added to those given on page 205:—
Fig. 214.—Salvinia natans: A microsporangium with germinating microspores and protruding prothallia (s); B a prothallium with the bicellular antheridium (s) growing out of the microsporangium; C the two cells of the antheridium have opened by transverse clefts; beneath is seen the microspores enclosed by the hardened mucilage; D spermatozoids still enclosed in the mother-cells.