146 ASPLENIOID FERNS [CPI. 



the sorus appears unprotected, the inclusium having shrivelled (Luerssen, 

 I.e. p. 143). Such reduction or abortion of the indusium may be found also 

 in some of the larger forms of Diplasiiim, and it becomes a distinctive 

 feature in Ceterach and in Pleurosorus. 



Allantodia Wall. 



This problematical genus, styled DipLaziopsis by C. Christensen, contains 

 a single large Eastern species. It will have to be examined in fresh and 

 developmental material before it can be properly understood. Provisionalh' 

 it would seem probable that its single species is a relatively advanced 

 Asplenioid type, as its reticulate venation indicates. The sorus may probably 

 be structurally like that of Aspleniuvi, but the spores are liberated b\- 

 rupture of the thin membranous indusium, not by inversion and shrivelling, 

 as in Aspleiiutiii. 



DiPLAZiUM Swartz 



The distinction of this genus from Asplenium or AtJiyriuni is not a sharp 

 one. In Diplaziinn the veins are as a rule free, and the sori and indusia 

 extend along both sides of some of them (Fig. 669, 35). This is exactly what 

 might be expected to occur at times, if the Asplenioid Ferns were derived 

 from the Dryopteroid type, along lines already suggested in the case of 

 Asplenium {Diplasmm) lanceum. As Diels remarks (I.e. p. 225), the character 

 of the Diplazioid sorus, used by Presl to delimit the genus from Asplenium, 

 has only this importance, that in Asplenium it appears exceptionally, or at 

 least much more rarely than in Diplazium. Both may in fact be held as 

 derivatives of the Dryopteroid type. There are about 200 species in this 

 genus, mostly inhabitants of moist tropical countries, and often of large size. 

 This combined with the suggested origin points to a nutritional explanation 

 of the Diplazioid state. The easy nutrition of the moist tropics would 

 encourage a full habit, with the development of both sides of the originally 

 horse-shoe-shaped sorus, whereas onl)' one side is fully developed in typical 

 Asplenium, or in Athyrium. (Compare Fig. 672, p. 143.) 



The vascular system of Diplazium resembles that of Asplenium (Fig. 671). 

 But in a large Diplazium such as D. marginatum the two straps of the leaf- 

 trace may take each a separate course far up into the petiole ia), while in 

 a small Asplejtium such as A. alatum they may fuse almost immediately on 

 their departure, after the manner frequent in these Ferns {}}). 



In a sub-dendroid Asiatic species, Diplazium esciUentum (Retz) Sw., a 

 medullary system is found within the dictyostele; strands may originate 

 independently in the pith, or as internal thickenings of the meristeles. It 

 may be held as a cfoncomitant of large size (Y. Ogura, Reprint from 

 Botanical Magazine, Tokio, Vol. XLI, No. 483, p. 172). The detail of the 



