XLiv] COMPARISON 193 



exacting conditions. The "flange" with its broadening expanse of photo- 

 synthetic tissue secured this. Within the genus BlecJimim all degrees of its 

 origin may be seen, from the state of the highly coriaceous types, such as 

 B. tabulare and discolor, where the convex upper surface of the fertile pinna 

 shows a smooth curve and no flange; through such states as are seen in 

 B. attemiatiivi and UHerminieri, where slight outgrowths appear at the 

 point of strongest convexity, or B. spicaiitwhexQ the outgrowth is larger; to 

 the full type of Eic-Blechnuui as seen in B. brasiliense and occidentale. Here 

 the "flange" has the appearance of a normal blade, with epidermis, mesophyll, 

 and a system of open venation extending outwards from the commissure, 

 but sometimes linked towards the pseudo-margins. The true margins, still 

 strongly recurved over the coenosori, retain their identity throughout the 

 whole series of species that illustrate this new development. The flange, 

 however highly organised, is bounded by a pseudo-margin, and is itself a 

 pseudo-lamina as regards its phyletic origin. By its formation the sporophyll 

 assumes an outline similar to that of a foliage leaf, and Eu-Blechnum appears 

 as though homophyllous. This origin of an innovation at the point of greatest 

 curvature of a reflexed margin is not without parallel among Ferns, for 

 instance in Pellaea intramarginalis (Klf ) J. Sm. (Hooker, Second Century of 

 Ferns, Plate LXXIl); while signs of it are seen also in Cassebeera, and in 

 Cheilanthes lendigera. The point of greatest curvature will naturally be that 

 of least resistance by pressure of the superficial tissues.^ 



A very interesting feature morphologically is the fact that the true margin, 

 after taking the character of an indusium, is not only displaced but is actually 

 delayed in time of its appearance, as the flange increases in proportion to 

 it. If comparison be made between the figures illustrating B. tabulare, 

 B. spicant, B. capense, and B. brasiliense, it is seen that by gradual steps the 

 marginal segmentation is diverted from the "true margin" to the "flange." 

 The former is also delayed in its appearance, so that the marginal segmenta- 

 tion is continued directly into the flange, upon which the morphological 

 margin appears as a secondary development. There has in fact been a 

 "phyletic slide" of the true margin to a superficial position, involving changes 

 both in time and place of its origin. Such a progression is not without its 

 parallels elsewhere among Ferns and particularly in the Pteroids ; while it 

 finds its biological justification in the fact that nutrition must needs precede 

 the formation of propagative organs. 



Some species may be doubly pinnate, or they may show intermediate 

 states between simple and double pinnation. This is seen in Blechnnni 

 Fraseri and divers if alia: it also appears in the leaves of Stenochlaena 



1 A significant homoplastic parallel to this origin byenation of an elaborate photosynthetic organ, 

 with stomata but naturally without any vascular tissue, is seen in the sporogonium of the Moss 

 Splachniim luteiim, described by Vaizey {Ann. of Bot. Vol. v, Plate n, p. 1. 1890). 



BUI 13 



