XLV] COMPARISON 227 



though the upper might expand to the three rows, bearing distally the usual 

 type of sporangia! head. Thus comparatively the sporangium of Phlebodiwn 

 would provide the most advanced type of the series, the steps of reduction in 

 complexity of the stalk following roughly the course of advance in other 

 features disclosed in the preceding pages. 



A parallel series culminates similarl)' in the one-rowed stalk of Aspleniinn 

 (compare Fig. 670). 



Fig. 733. \ owwg soxus oi Polypodium [Phlebodintu) 

 aureinn L. Cut vertically. 



Comparison 



The external features upon which the comparison of the Ferns here 

 attributed to a Matonioid-Dipteroid affinity has been based have been taken 

 up as the description proceeded. Progress has also been remarked from 

 dermal hairs to scales, from protostely (so curiously retained in C heir cp leu ria), 

 through solenostely, to a highly perforated state in Platycerium : this is seen 

 particularly also in Neocheiropteris (see Studies III, Ann. of Bot. 1913, p. 474, 

 Text-fig. B). Nevertheless the venation of the expanded blade shows a high 

 degree of uniformity from Dipteris onwards. But it is the comparison in 

 respect of the sorus, and of the sub-soral vascular supply and of the 

 sporangium, that gives the most striking results; these may now be sum- 

 marised for the whole series. 



If we compare the constitution of the superficial sorus, as it is seen in 

 Gleichenia, Matonia, Lophosoria, Dipteris, Cheiroplenria, Christopteris, Platy- 

 cerium, Hynienolepis, and again in Neocheiropteris and related forms included 

 under " Polypodiunz' — such Ferns, akin in other characters, illustrate progres- 

 sive changes in its elaboration. Though they may not themselves constitute 

 a simple phyletic series, yet these Ferns may be held as being so far related 

 that they indicate probable steps of transition from a simple radial sorus, of 



15-2 



