Bower.—Studies in the Phytogeny of the Fi tic ales. VII. 21 
The regular marginal segmentation of the pinna, with a wedge-shaped 
initial and very strong convexity of the adaxial (upper) face, is shown in 
Fig. 18, a-e. The series of segmentations can be clearly followed, so 
that the identity of the wedge in c is plainly that of the margin in a and 
b. In the latter, and more clearly in c , the last segment on the adaxial 
side grows out to form a flap, which is the upper indusium. It is thus clear 
that it is a superficial, not a marginal growth. Stages slightly older are 
Fig. 18. a-e. Sections through the pinnule margin of Pteridium aquilinum , showing how the 
receptacle (x) originates directly from the marginal segmentation, while the indusial flaps are 
superficial in origin. (X150.) 
seen in Fig. 18, d, e. The margin here still retains its identity as the 
cell marked x, as shown by the series of segmentations. But now both 
of the indusial flaps are seen in relation to it. The upper is the stronger, 
as its earlier origin would indicate. The lower is thinner, less constant in 
its occurrence and in its structure. It is usually a continuous flap, but 
sometimes appears as though replaced by a number of distinct hairs. The 
convexity lying between the two flaps becomes the receptacle, which thus 
corresponds to that in Dicksonia , Saccolcma , Lind'saya, and Odontosoria . 
Sections from older pinnules show the sporangia borne on the receptacle. 
Sometimes there is a regular gradation of these, as is seen in Fig. 19, a , 
