XLiv] BLECHNUM i73 



undergone a phyletic slide from its originally marginal position towards a 

 position on the lower surface of the pinna. In course of this change the 

 marginal segmentation, typical of Fern-leaves, is transferred from the true 

 margin to that new growth, the flange, as it gradually asserts itself in the 

 series compared. If this be the true history, then the flange, though secondary 

 by descent, gradually assumes the prior place in the ontogeny, in accordance 

 with its increasing importance as a photo-synthetic organ. By its formation 

 the assimilating tissue of the fertile pinna is greatly increased, spongy 

 parenchyma and stomata being produced upon it. The consequence of its 

 appearance is that the nutritive disability of dimorphism is obliterated, and 

 all the leaves, sterile or fertile, take a similar form. The biological advantage 

 of self-nutrition of the sporophylls thus gained is too obvious to need insist- 

 ence. It might of course be possible to invert the thesis, and to suggest 

 that the Lomarioid state is derivative, by abortion of the broad lamina ; but 

 this view would present comparative difficulties in relation to the sorus and 

 the indusial flap, while it would conflict with physiological probability. 



Fig- 695. Scheme of the venation of Blechiium, 

 showing the mid-rib below, and forked veins arising 

 from it. The dotted line indicates the commissure 

 consisting chiefly of storage tracheides. 



On the view thus stated the Blechnoid fusion-sorus arose from a gradate 

 type, with isolated sori like those seen in Mattejiccia. These were all seated 

 on distinct veins. The formation of commissures connecting the veins, and 

 the consequent running together of the separate sori into the Blechnoid 

 fusion-sorus, is a step easily understood when starting from a type like 

 Matteiiccia, where the sori are in very close alignment in the young state 

 (Fig. 683, C). The constitution of the vascular connections is partly by 

 deflection of the anadromic branch of each forked vein, partly from a spread 

 of storage tracheides of the receptacles. It is important to realise the 

 distinctness of these two factors in the formation of the vascular connections 

 (Fig. 695) (see Studies IV, p. 400). The general conclusion of this com- 

 parative study is then that the Blechnoid Ferns as a whole are an up-grade 

 sequence: that they originated from a heterophyllous source, represented 

 among living Ferns most nearly by Matteiiccia intermedia C. Chr., and that 

 the sequence culminated in the homophyllous type of Eu-Blechnnvi. The 

 earlier steps of the sequence were characterised by adaptation to xerophytic 



