12 HYPOLEPIS, ETC. [CH. 



typically mixed state of the sorus. It will be seen later that a similar pro- 

 gression may be traced in other sequences of Ferns, whether with marginal 

 or with superficial sori. (ii) It illustrates a transition of the sorus from the 

 marginal position to the surface of the leaf, culminating in certain forms of 

 Hypolcpis repens that have been ranked as Polypodium. The slide of the sorus 

 from the margin to the surface is again a feature that recurs elsewhere, for 

 instance in the Pteroid Ferns, (iii) The series also demonstrates the partial or 

 complete elimination of the inner or lower indusium: for the inequality of 

 the two lips in Dicksonia {Cibotmm), and more markedly in Bennstaedtia, 

 leads to the complete elimination of the lower lip in Hypolepis nigrescens, 

 while in H. repens various degrees of its abortion are found. Such abortion 

 of the indusium is also illustrated in other Ferns, (iv) Together with these 

 modifications of the sorus the type of the sporangium also changes. That 

 of the Dicksonieae is massive, with a thick stalk, and complete oblique 

 annulus of many cells: that of Dennstaedtia and Hypolepis is of a type usual 

 for advanced Leptosporangiate Ferns with vertical interrupted annulus of 

 few cells. But the spore-output remains relatively uniform. A broad com- 

 parison of Ferns at large shows that all of these changes indicate advance. 

 Here they all run substantially parallel to one another, and are probably 

 correlative. 



The Dennstaedtiinae prefigure in fact a number of those features which 

 appear more deeply stamped upon the typically Leptosporangiate groups 

 that are associated with the genera Davallia and Pteris. The creeping 

 solenostelic rhizome of Dennstaedtia shows the first steps of perforation 

 (Vol. I, Fig. 144, E), moreover the breaking up of the leaf-trace is already 

 suggested in Cibotium and in Saccoloma (Fig. 581): both of these conditions 

 are more fully developed in Davallia and in Pteris. The Dennstaedtiinae 

 give no suggestion of the production of protective scales, except in Saccoloma 

 which is relatively advanced in other ways: but scales are general in the 

 Davallias, though they are not always present in the Pteroid Ferns. On the 

 other hand, in Saccoloma there is a partial linkage of the marginal sori into 

 a linear sequence, which is so outstanding a feature in the Pteroids. The 

 flattening of the receptacle in the mixed sorus is foreshadowed in Denn- 

 staedtia and Hypolepis, also the elimination of the lower indusium: this 

 elimination does not become general in the Davallioid Ferns but it becomes 

 a definite feature among the Pteroids. Further, there is the advance in 

 H. repens to a flattened Polypodioid sorus : but there is in the Dennstaedtiinae 

 no suggestion of any transition to an Acrostichoid state, unless the spread 

 of the receptacle in H. repens can be so regarded. Lastly, the sporangia of 

 Dennstaedtia and Hypolepis are of the advanced Leptosporangiate type, as 

 are also those of the Davallioids and Pteroids. It will thus be seen how 

 interesting is this intermediate Sub-Family in the morphology of those 



