Bower.—Studies in the Phytogeny of the Filicales. VII. 
Fig. 20. Vertical section of the fusion-sorus of Pteriaium aquilinum, following the line 
of the commissure, (x 150.) 
Fig. 21. i, ii. Vertical sections of pinnule of Pteridium aquilinum, where the sorus i§ 
merging into the sterile margin. ( x 75.) 
