XLiv] COMPARISON 197 



oi Phyllitis to Asplenium, with which genus it has usually been ranked, or 

 even included? This question must be decided not by the comparison of 

 extreme types of either, but by the comparative study of their probable 

 origins. The preceding paragraphs indicate that in the history of the soral 

 tracts of Phyllitis there was in the first instance the formation of a Blech- 

 noid coenosorus with an underlying vascular commissure, which is itself a 

 secondary development from the original venation: and that each soral tract 

 is an isolated fragment of that coeitosoriis seated upon a portion of that 

 commissure^ and covered by a length of the original leaf margin. In origin it 

 is not a single sorus at all, but a highly derivative body. On the other hand, 

 it has been shown in Chapter XLII that the Asplenioid sorus is referable in 

 origin to a single sorus of the Dryopteroid type, by exte?tsion of the fertile tract 

 along the course of the vein that bears it. The clue to this is seen in Diplazium 

 lancemn (Fig. 672), in which the entire leaf is "attenuated gradually upwards 

 and downwards", while the sori are "irregular and linear" i^Syn. Fil. p. 229). 

 This is a type which gives the same opportunity, for studying the effect of 

 increasing width of the leaf-expanse as that afforded by B. punctulatiim var. 

 Krebsii. Here, however, it appears that definite sori extend along the regular 

 veins of the widening leaf, sometimes on one side sometimes on the other, 

 sometimes on both sides of it, and usually (but not always) with obliteration 

 of the median, that is, the marginal region of the sorus. Thus are produced 

 the types oi Eu- Asplenium and o{ Diplazium. Here the indusiuni is through- 

 out the homologue of a part of that seen in Dryopteris, in fact it is of the 

 nature of a true indusium. 



It thus appears that however similar the soral tracts may seem to be in 

 Asplenium and Phyllitis, their origin has been quite distinct. The former is 

 a highly specialised single sorus extended along the line of a normal vein, 

 and covered by a true indusium : the latter is a tract segregated from a 

 coenosorus, extended along a portion of a secondary vascular commissure, 

 and covered by a segment of an original leaf-margin. The two types provide 

 one of the most remarkable instances of homoplastic development. As the 

 leaves of the Ferns in question become widened and their form condensed 

 — that is, as they depart farther from the original highly branched types — 

 these quite different structures become more and more alike; and it is this 

 which has led to the systematic confusion of two phyletically distinct series. 

 The only really homogenetic factors in these soral tracts are the sporangia, 

 and the receptacle that bears them. 



It will probably be objected that the well-known upward fusion of the 

 paired vascular strands of the petiole, and the formation of those peculiar 

 X-shaped conducting tracts seen in Asplenium and in Phyllitis are real 

 indications of affinity (see Vol. I, p. 166, Fig. 157. Also Luerssen, Rab. 

 Krypt. Fl. Ill, pp. 120, 150). But these anatomical features may fairly be 



