(_a_) _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 ╭ _Simplices_ (Bower). The sporangia are relatively Schizaeaceae ┤ large and all the sporangia in a sorus have a Gleicheniaceae │ simultaneous origin: the annulus is oblique. Matonineae ╰ ╭ _Gradatae_ (Bower). Sporangia arise in basipetal Loxsomaceae │ succession on a more or less elongated receptacle Hymenophyllaceae │ (portion of the leaf lamina which projects as a Cyatheaceae ┤ cushion or column on which the sporangia are Dennstaedtiinae │ borne); annulus oblique; indusium, if present, │ in the form of a cup or flap of tissue arising ╰ from the base of the sorus. ╭ _Mixtae_ (Bower). This division includes the │ Polypodiaceae, by far the largest family of ferns. Polypodiaceae │ The sporangia are characterised by their Parkeriaceae ┤ 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. ╭ The Dipteridinae include species with the characters Dipteridinae ┤ of the _Mixtae_, and one species in which the ╰ sporangia develope simultaneously (_Simplices_). =Osmundaceae=[684]. (_Osmunda_, _Todea_.)