I. Leptosporangiate Filicales.
In these homosporous and heterosporous plants the sporangia are developed from single epidermal cells.
Fig. 220. Young fronds of (A) Angiopteris evecta and (B) Cycas revoluta. (Reduced.)
(a) Eufilicineae. The sporangia bear spores of one kind only; the wall of a sporangium consists of one layer of cells. In the great majority of cases the sporangia are characterised by the possession of a conspicuous row of thick-walled brown cells, the annulus[683], which serves as a mechanism for dehiscence and spore-dispersal. The fertile leaves, identical in form with the sterile, or more or less sharply contrasted, usually bear the sporangia on the under surface of the lamina in definite groups or sori, and not on the upper surface or grouped in strobili as in the Lycopodiales. The stem is dorsiventral or radial in structure, creeping or erect, frequently clothed with chaffy scales (ramenta) and less often with multicellular hairs. The sexual generation is represented by a small green prothallus which lives for a short period only and dies after nursing the fern-plant through its earliest stages.
(b) Hydropterideae. Heterosporous water-ferns differing considerably in habit from the true ferns. Each megasporangium contains a single megaspore and several microspores are produced in each microsporangium. The gametophyte is represented by tissue more or less enclosed in the spore. [Genera Salvinia, Azolla, Marsilia, Regnellidium, Pilularia. See Chapter XXVI.]
Eufilicineae.
The classification of the true ferns in common use is based almost exclusively on the structure of the sporangium, the form and position of the sori, and on the presence or absence of an indusium (the tissue which in some ferns partially or completely covers each sorus). In recent years there has been considerable activity in the investigation of fern anatomy with a view to elucidating the natural relationship between recent families or genera. The results of these researches are on the whole consistent with the scheme and grouping adopted in the Synopsis Filicum of Hooker and Baker and in general harmony with the main conclusions arrived at by Bower from an intensive study of the development of fern sporangia. The following classification is based on that of Bower who takes as a basis (i) the relative time of appearance of the sporangia in a single sorus, (ii) the structure of the sporangia and their orientation relative to the whole sorus, (iii) the productiveness of sporangia (spore-output).
| Osmundaceae Schizaeaceae Gleicheniaceae Matonineae | Simplices (Bower). The sporangia are relatively large and all the sporangia in a sorus have a simultaneous origin: the annulus is oblique. | |
| Loxsomaceae Hymenophyllaceae Cyatheaceae Dennstaedtiinae | Gradatae (Bower). Sporangia arise in basipetal succession on a more or less elongated receptacle (portion of the leaf lamina which projects as a cushion or column on which the sporangia are borne); annulus oblique; indusium, if present, in the form of a cup or flap of tissue arising from the base of the sorus. | |
| Polypodiaceae Parkeriaceae | Mixtae (Bower). This division includes the Polypodiaceae, by far the largest family of ferns. The sporangia are characterised by their relatively small size, the presence of a slender stalk, the absence of regular orientation or sequence in development, and by the presence of a vertical annulus. | |
| Dipteridinae | The Dipteridinae include species with the characters of the Mixtae, and one species in which the sporangia develope simultaneously (Simplices). |