44 



PTEROID FERNS 



[CPI. 



strict technical characters this group of species, which differs from the rest 

 of the genus also in habit of growth, has as good a claim to be placed in 

 Lindsayae as Pterideae {Syu. Fil. 1873, p. 162). In both the structure of the 

 sorus suggests the linkage of a number of marginal, two-lipped sori of a 

 Dicksonioid type to form linear coenosori: a view that greatly adds to the 

 phyletic interest of the Pterideae. 



This being so, it becomes a matter of importance to know the development 

 as well as the adult structure of the coenosorus of the Bracken. As seen in 

 A 



Fig. 609. Development of the coenosorus of Pteridium aquiliiium. A, 5 = vertical sections through 

 the margin of the pinnule, showing how the receptacle ( x ) originates directly from the marginal 

 segmentation, while the indusial flaps are of superficial origin ( x i8o). C=a vertical section of an 

 older coenosorus, following the line of the vascular commissure ( x 90). Z) = a vertical section of 

 a similar sorus, for comparison with A and B ( x 90). The sporangia are of mixed ages. 



vertical section, the regular marginal segmentation of the pinnule continues 

 with its row of wedge-shaped initials directly into the receptacle (Fig. 609, 

 A, B). The upper or adaxial indusium is the first to appear, as a superficial 

 outgrowth back from the extreme margin ; the lower or abaxial appears 

 slightly later, and it is also smaller and less constant in structure than the 

 upper. These facts for Pteridium are in accord with what has been seen in 

 Lindsaya (Fig. 603). The order of appearance of the sporangia upon the 

 receptacle shows some variety: sometimes there is a regular basipetal sequence, 

 as in P. aquilinuin var. caudahnn from Jamaica, the oldest sporangia being 

 adaxial (Fig. 6io). But in the native Bracken the succession is less regular, 

 and the sorus soon acquires the mixed condition (Fig. 609, U). A vertical 

 section parallel to the margin of the pinnule, and following the vascular 



