l62 THE MORPHOLOGY OF PTERIDOPHYTES 



The sporangium is relatively small, with a four-rowed 

 stalk, an oblique annulus (Figs. 24O and 24P), and a fairly 

 well marked lateral stomium. The spore output ranges from 

 sixty-four to sixteen, and even eight in some species. 



We now come to the large assemblage of ferns whose sori 

 show the mixed condition and which Bower grouped to- 

 gether in the one big artificial family, the Polypodiaceae. 

 Some, he believed, had affinities with the Dicksoniaceae, 

 some with the Cyatheaecae and some with the Osmundaceae, 

 yet all had achieved the same advanced type of sporangial 

 structure, with a thin stalk, a vertical incomplete annulus, 

 and lateral dehiscence. Figs. 24T and 24U are two views of 

 the sporangium of Adiantum, which demonstrate the small 

 number of cells constituting the capsule, and the way in 

 which the stalk is composed of just one row of cells, in the 

 most highly evolved types. 



In 1949 Holttum^^ suggested a more nearly natural 

 classification of these ferns, by creating a new family, the 

 Dennstaedtiaceae, within which he grouped a number of 

 subfamihes which, he believes, have affinities with the 

 Dicksoniaceae. In this new scheme of classification, the 

 Polypodiaceae constitute a very restricted family, having 

 affinities with the Matoniaceae, the Dipteridaceae being 

 absorbed into it. Within the Dennstaedtiaceae, so many 

 evolutionary processes have taken place that the group is 

 hard to define; indeed, it would almost seem that the sub- 

 families warrant elevation to family status. 



Dennstaedtioideae 



This is the most primitive of the subfamihes of the 

 Dennstaedtiaceae, for some species still retain the gradate 

 arrangement of sporangia in the sorus. Most have creeping 

 rhizomes with solenosteles. The sorus of Dennstaedtia (Fig. 

 25I) is very similar indeed to that of Dicksonia in having two 

 indusia. In Microlepia, however, the upper indusium is 



