XLiv] COMPARISON i95 



Towards that end each example will need to be subjected to a separate 

 comparative analysis, a line of study that has been advanced of late by 

 Von Goebel {Botanische Abhandlungen, Heft i, Jena, 1922). Sir William 

 Hooker, in constructing his scheme for systematic use rather than embodying 

 any such evolutionary ideas as those here contemplated, placed the simplest- 

 leaved types first, and so they are still disposed in the Synopsis Filicum. 

 But phyletically it seems probable that they should stand last, as the later 

 and it may often be the final result of phyletic advance. This is the general 

 view which will apply in most families of Ferns, and it finds some degree 

 of illustration in the Blechnoids, particularly in those of the family in which 

 soral disintegration is a feature. For with the formation of broad leaf- 

 surfaces modification of the sorus is apt to follow, particularly where such 

 surfaces result chiefly from widening or extension of parts already present. 

 The sori may be expected to follow the expansion of the part that bears 

 them. These general reflections form a proper introduction to the study of 

 that soral disintegration which plays so prominent a part in the morphology 

 of the more advanced Blechnoid Ferns. 



The general hypothesis entertained for these Ferns is that they originated 

 under xerophytic conditions, demanding reduction of the proportion 

 of surface to bulk. This led to a compact and coriaceous type of leaf 

 Commonly they are simply pinnate: but comparison has indicated a more 

 definite origin from Onocleoid and Cyatheoid forms with circular superficial 

 sori borne upon leaves often showing a high degree of pinnation. Evidences of 

 this are still clearly to be seen in the leaves of the Onocleeae, and even of 

 the Blechnoids themselves. It has been seen how the narrow and strongly 

 recurved fertile pinnae of the Loinaria-\.y^^& have freed themselves from a 

 position so extreme as to be unpractical, by the innovation of the flange, 

 giving the Eu-Blechnoid sporophyll. This has resulted not from webbing 

 but from widening of the pinna, which thus takes the lead in the formation 

 of a broad surface. But apical growth may still elongate the pinna indefinitel)^ 

 Thus the soral tracts will be subject to extension either longitudinally or 

 transversely, or both. The forms which they take may be examined with 

 advantage from this point of view^ 



If the growth of the pinnae in length outstrip the capacity of soral 

 development, disintegration of the .linear coenosorus of Bkchniiui would 

 result: the effect of this is seen not unfrequently as an abnormality, and it 

 is illustrated in Blechnum spicant (Fig. 701, 23). It becomes a generic 

 character for Woodzvardia and Doodia (Fig. 704). A wider comparative 

 interest attaches, however, to the changes in outline and continuity of the 

 coenosorus in B. punctulatum var. Krebsii. The normal species belongs to 

 the section Louiaria, and has narrow pinnae, "often not more than one-eighth 

 of an inch broad": but the breadth is variable. In the variety Krebsii the 



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