XLiii] COMPARISON 159 



are of like texture. The Asplenioid sorus with its dehcate indusium is also in 

 keeping with this hygrophytic habit. 



The Onocleoid and Blechnoid Ferns present as a whole a strong contrast, 

 which appears in the leathery texture of their leaves. These as a rule are 

 not highly subdivided, and often the plants are markedly heterophyllous, 

 while the young sori are closely enfolded in the narrow segments of the 

 sporophylls: the lateral flaps of these are strongly recurved, and not 

 unfrequently brown and almost woody in texture. Such features are clearly 

 xerophytic adaptations : they are characteristic generally of the Onocleoid 

 and frequently of the Blechnoid Ferns. Thus while the Asplenioids may be 

 held to have sprung from types adapted to a shade-habit, the Onocleoids 

 and Blechnoids appear to have been from the first more or less xerophytic, 

 and stand in marked antithesis to the hygrophytic origin of the Asplenioids. 



This is the hypothetical position from which the comparative study of the 

 Onocleoid and Blechnoid Ferns may start. Both they and the Asplenioids 

 have been very successful t}'pes, as shown by the large number of living 

 species. They have spread into the most varied areas, even into those 

 climatically resembling one another, and conversely differing from their 

 hypothetical sources. Accordingly it need be no cause of surprise that they 

 should have developed along parallel lines, and that their later derivatives 

 should present a high degree of homoplastic resemblance. This has led to 

 systematic confusion of the living representatives of the two stocks, which a 

 more strict comparison may tend to resolve if the biological aspect of the 

 problem, as here sketched, be kept clearly in view from the first. 



The immediate phase of our problem deals with the Onocleoid Ferns, and 

 more particularly with their relations dow^nwards in the scale. The most 

 striking differences from the Cyatheoids are: (i) the smaller size, the absence 

 of a tall dendroid habit, and consequently their simpler anatomy: (ii) the 

 heterophylly : and (iii) the seasonal leaf-fall of the Onocleoids. The first of 

 these is in no way distinctive, for there are Cyatheoids of low stature, while 

 some Alsophilas, and in particular LopJwsoria itself, bear creeping runners 

 of the same nature as those of Matteiiccia, with a relatively simple vascular 

 system. Metaxya, a Protocyatheoid, has actually a creeping solenostelic 

 rhizome with an undivided leaf-trace. It is worthy of remark that the nearest 

 parallels are found with those Cyatheoid Ferns, which are held to be the least 

 advanced (Chapter xxxil). Heterophylly is a feature strongly impressed upon 

 both the Onocleoid and the Blechnoid Ferns: but it does not occur in the 

 Cyatheaceae : nor is a seasonal leaf-fall a feature with them, though there 

 is a seasonal change of leaves. Both of these are, however, characters 

 adaptive to season and station. 



The Onocleoid Ferns may be regarded as a series of advance running 

 parallel to that of the Woodsieae, and like them they have carried the more 



