On the ground mainly of an examination of the sorus and sporangium, Bower has shown that the Filicaceae may be divided into three groups - the Simplices, Gradatae and Mixtae - in which the sporangia arise simultaneously, in basipetal succession, or irregularly in the sorus respectively.
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.
Though on this account and because the subdivisions Simplices, Gradatae and Mixtae do not correspond to definite phylogenetic groups, they have not been used in classifying the Ferns above; they are of great importance as an advance towards a natural classification.
