22 Bower.—Studies in the Phytogeny of the Filicales. VII. 
which is comparable with Fig. 13, iii, of Lindsay a. But usually the 
sorus is clearly a ‘ mixed ’ one, as in Fig. 19, b , with a rather flattened 
receptacle, supplied by a mass of storage tracheides. These are connected 
Fig. 19. a. A more mature sorus showing strong basipetal sequence of sporangia in 
Pteridium aquilinum , var. caudatum. b. A similar section from Scotch Bracken, showing a mixed 
sorus. (x 150.) 
laterally between the veins by the well-known commissures shown in 
Fig. 20, where also the continuous sequence of the sporangia is shown. 
From the fully developed soral state as thus described, various steps lead to 
