274 SUMMARY OF RESULTS [CH. 



the first of which originate from marginal cells: they are followed by others 

 in gradate sequence. They are short-stalked and massive in Thyrsopteris, a 

 type which is believed to have had a Jurassic origin : but in Dicksonia and 

 Dennstaedtia they are longer stalked, and more delicate. The spore-output 

 is typically 64, and the spores are without perispore. 



Two important modifications of the sorus make their appearance in this 

 Family. One is a definite ontogenetic bias towards the lower surface, so that 

 the mature sorus faces downwards: the other is the departure from the basi- 

 petal sequence, by interpolation of younger sporangia without strict order 

 between those earlier formed. Both of these, together with the interruption 

 of the annulus at the insertion of the stalk, are seen in Dennstaedtia, pointing 

 towards a superficial type, so common in advanced Leptosporangiate Ferns. 

 The most interesting genus in this connection is Hypolepis (Chapter XX XVI). 

 Here with consistently Dennstaedtioid habit, anatomy, and dermal hairs, 

 not only is the sorus deflected to the lower surface and of a fully mixed 

 type, but the lower indusium is partially or completely aborted : in H. repens 

 the upper indusium is even merged in the flattened blade itself; the lower 

 vestigial or absent; the receptacle is flattened, and apparently superficial, 

 with "mixed" sporangia. In fact Hypolepis merges into Polypodiiim, giving 

 a phyletic transition from Ferns with a marginal, gradate, two-lipped sorus 

 to a type with sorus superficial and mixed, and apparently without any indu- 

 sium at all. Thus the Dicksoniaceae have given rise to Ferns which have been 

 ranked under that advanced Leptosporangiate type that has been designated 

 Polypodiiini. 



Two derivative series may be traced from a Dicksonioid source: viz. the 

 Davallioid Ferns, in which the sori for the most part retain their identity and 

 marginal position : and the Pteroid Feinis, which show soral fusion so as 

 to form linear coenosori, while the receptacle tends strongly to become 

 superficial. The first of these centre round Davallia (Chapter XXXVli). They 

 mostly retain a Dennstaedtioid habit, but with dermal scales, and often a 

 highly segregated vascular system. The sorus is mixed, often with signs of 

 a gradate state at first, and a flattened receptacle. But in some the receptacle 

 may slide to the lower surface, and not unfrequently the individuality of the 

 sorus may also be lost by fusion into coenosori, as in NepJirolepis and Lind- 

 say a. These Ferns appear to have led to no further development. 



The Pteroid Ferns illustrate a progressive sweep of greater extent and 

 importance for comparison with other phyla. It may be held to start from 

 Pteridinni and Paesia, which have marginal, bi-indusiate fusion-sori, and it 

 leads by gradual steps to the fully Acrostichoid state seen in Acrostichnni 

 aureuni. This may not be a simple phyletic line, but the comparisons are 

 broad and cogent (Chapter XXXVlll). The sequence involves parallel progres- 

 sion from solenostely to advanced disintegration : from hairs to scales : from 



