Each has an almost horizontal annulus resembling that of Gleichenia, but the dehiscence is lateral.
The differences in the form and mode of dehiscence of the sporangia (those of the Simplices having median dehiscence and a horizontal annulus, those of the Gradatae a more or less oblique position of the annulus and of the plane of dehiscence, while in the Mixtae the annulus is vertical and the dehiscence transverse) stand in relation to the position of the sporangia in the sorus relatively to one another.
When the synangia are ripe dehiscence takes place along a median line between the two rows of loculi.
In size, position, arrangement, and manner of dehiscence the sporangia bear a striking resemblance to those of Marattia and Danaea among recent Marattiaceae.
In A the stomium (st) or place of dehiscence is shown.
The cells of the wall of the sporangium are usually so constructed as to determine the dehiscence of the sporangium and the liberation of its spores.
The plants show great diversity in vegetative structure, which together with the character and mode of dehiscence of the fruit afford a basis for the subdivision of the order into tribes, eleven of which are recognized.
The dehiscence of the sporangium is almost transverse, as in the Polypodiaceae, but the annulus is slightly oblique.
The spores, when liberated by the dehiscence of the sporangium, give rise to the prothallus, which is now, owing mainly to the investigations of Treub and Bruchmann, known in a number of tropical and temperate species.
When the anther lobes are rendered horizontal by the enlargement of the connective, then what is really longitudinal dehiscence may appear to be transverse.