Bower.—Studies in the Phytogeny of the Filicales. VII . 49 
intermediate type with marginal sorus, protected by indusial flaps on either 
side. Comparison of them inter se indicates that such a type would have had 
hairs as dermal appendages, a relatively simple vascular system, an isolated 
gradate sorus with convex receptacle, and sporangia with a complete, 
oblique annulus, a lateral dehiscence, and a number of spores not 
exceeding 64. 
This type probably sprang ultimately from some Schizaeoid source, 
in which as in Lygodiutn the vascular system was quite primitive, the 
dermal appendages hairs, and the sporangia actually marginal. Sorally 
the sequence probably proceeded, through characters now exemplified by 
the Dicksonioid and Lindsayoid types, to the ultimate Pterid characters. 
It is not suggested that the species now constituting such genera were 
themselves the progenitors of the Pterids, but that they illustrate their 
probable phyletic origin. There would thus have been a transition from 
the isolated marginal sorus, with double indusium and marginal receptacle, 
as seen in Dicksonia , to the superficial fusion-sorus, with vascular commis¬ 
sure and a single indusium. This would have involved not only a lateral 
fusion of sori, but also a ‘ phyletic slide 1 of the receptacle to a superficial 
position, in which case, as the sorus becomes deflected to the lower surface, 
the inner (abaxial) indusium becomes non-functional and obsolete. This 
transition has been illustrated developmentally in various species, but in 
none so convincingly as in Histiopteris incisa> which is linked so directly 
with Pteridium by similarity of habit. It thus appears that the genus 
Pteris as at present defined by Christensen, together with Histiopteris and 
Lonchitis , has arrived at the present uni-indusiate state by loss of the inner 
indusium, which is still present in Pteridium and Paesia. It may, however, 
turn out later that some species now ranked as Pteris may have to be 
referred to some other source. Till decisive evidence is available, the usual 
systematic grouping of the species should be followed; and the conclusion 
may accordingly apply provisionally to the whole genus as accepted by 
Christensen. 
Once the inner indusium is obsolete, the fusion-sorus may widen its 
receptacle, or it may extend inwards. Various degrees of widening of 
the receptacle may be seen, and an extreme case has been demonstrated 
in Pterispodophylla . Or it may extend towards the midrib, covering the 
surface in part (Acrostickum praestantissimuni) or wholly (A. aureum , 
L.). On the other hand, the converse progression is illustrated by Dictyo - 
xiphium , in which, as has been shown above, the upper indusium is 
abortive, and the receptacle is tilted towards the upper surface, with some 
indication of its spreading upon that area, somewhat after the manner of 
Acrostickumpraestantissimum upon the lower. Thus along steps individually 
slight, states may be attained apparently very different from the initial 
condition of the isolated, marginal, bilabiate sorus of the Dicksonioid type. 
E 
