The species differ greatly in size and general appearance and in the character of the frond; the sori or groups of spore-cases (sporangia) are borne on the back of the leaf, are globose and naked, that is, are not covered with a membrane (indusium) (see fig.
Even if we admit van Tieghem's interpretation of the integuments to be correct, the diagnostic mark of his unitegminous and bitegminous groups is simply that of the absence or presence of an indusium, not a character of great value elsewhere, and, as we know, the number of the ovular coats is inconstant within the same family.
(A, B X 240; C, D X 540.) 't7 ' each consisting of a single series of large sporangia covered by a coriaceous indusium, which is attached to the central part of the receptacle.
When mature, the sporangia are raised above the margin of the indusium by the elongation of the receptacle, thus facilitating the dispersion of the spores.
The sorus has a somewhat elongated receptacle, on which the sporangia arise basipetally; the indusium may be cup-shaped, bivalve or wanting.