622 FILICALES 



the lamina. These are all relatively primitive characters, and unusual in 

 Ferns showing a mixed sorus : they direct the line of comparison down- 

 wards to Matonia and Gleichenia. The former has a vascular system of 

 the same type as Dipteris, but it has run into greater complications, 

 with its concentric solenosteles. Both genera, however, are considerably 

 in advance of the most complex Gleichenias, Yet all these Ferns appear 

 to conform in their various degrees of elaboration to the same vascular 

 type. 



There is, however, no exact parallelism in the soral and vascular 

 characters. Gleichenia is the most primitive in both respects ; while 

 Matonia is the most advanced of all in vascular structure, its sorus is still 

 that of the Simplices, though it has only a small spore-output per sporan- 

 gium, and a protective indusium is present ; but as this is apparently 

 absent in Laccopteris, it has probably been in Matonia a special generic 

 feature. Dipteris, with its vascular system taking a middle place, has the 

 most advanced soral condition, as shown by their distribution on the leaf, 

 by the flat receptacle, and by the mixed aggregation of the sporangia in 

 D. conjugata. But still it proclaims its origin by the absence of indusium, 

 the oblique annulus, and the imperfectly differentiated stomium. The sum 

 of characters justifies the conclusion that in Dipteris we see a genus of 

 origin from a stock included in the Simplices, in which at least one species 

 has passed, apparently without the intermediate state of a basipetal sorus, 

 directly to the condition of the Mixtae. There is, moreover, good reason 

 for holding that this phyletic line has proceeded quite independently of the 

 other progressions to a mixed sorus which have been traced elsewhere. 



Finally, the palaeophytological data harmonise with this conclusion ; 

 for representatives of the Dipteridinae figured largely in the Mesozoic 

 Flora, as far back as the Rhaetic, with sori agreeing in form and distribution 

 with these of Dipteris; but the annulus is described as probably complete. 1 

 This point may be considered doubtful ; but if it were confirmed it would 

 fall in readily with the phyletic position suggested for the Dipteridinae. 

 The conclusion of Seward seems fully justified that Matonia and Dipteris 

 are linked together as remnants from a bygone age. They have advanced 

 independently, the one to higher vascular complexity, the other to a distri- 

 bution and construction of the sori characteristic rather of the more recent 

 Ferns than of its own progenitors. 



It has now been seen that the condition of sorus characteristic of the 

 Mixtae is absent from the Ferns which Palaeophytology tells us were the 

 most primitive, but that it is the prevailing feature in the Ferns of the 

 present day. It has also been seen that steps leading from the more 

 primitive condition of the Simplices and Gradatae to the mixed type of 

 sorus exist in certain Ferns : and further, that there is a probability that this 

 end has been achieved by progression along more than one phyletic line : 



1 Seward, I.e., p. 507. 



