XLIl] 



CETERACH. PLEUROSORUS 



147 



sorus o{ Dipla::iu]ii, compares on the one hand with that o{ Aspleniiivi, on the 

 other with Dryopteris. A section traversing the vein of Diplaziuiu celtidifolhiui 

 Kze. transversely shows the indusium with 

 its arching flaps right and left, as they 

 appear in Dryopteris when cut in a similar 

 direction (Vol. I, Fig. 14): while either side 

 of it corresponds to what is seen in As- 

 pleniiim (Fig. 669, 36 a-c). 



Some of the Ferns of this affinity attain 

 a very considerable size, and a high com- 

 plexity of leaf-architecture. Christ quotes 

 D. ceratolepis Christ as being several metres 

 high, with dendroid habit. The leaf-seg- 

 ments of the larger species often appear 

 as broad expanses in which occasiona 

 vein-fusion or even reticulation may occur 

 as a secondary consequence. Such de- 

 velopment is sometimes accompanied by a ^f f' "^'"'"''^l ^.^^^' ^\°'^'"? vein-fusion and 



\ ... elongated son, without indusium. (After 



partial orcomplete abortion of theindusium, Christ.) 



a condition consistent with growth in moist forest shade (Fig. 675). 



Ceterach Adanson 

 A similar abortion of the indusium is seen as a regular feature in Adanson's 

 genus Ceterach, though it is probably to be ascribed to different biological 

 circumstances. The genus comprises small xerophytic plants with upright 

 rhizome and simply pinnatifid leaves, their lower surface being covered by 

 brown scales. The veins anastomose towards the margin. The sori which 

 are of the type of Asplenmm have the indusium partially or completel}- 

 abortive (Fig. 676). Biologically it may well be that the effective protection 

 by scale armour has made the indusium superfluous. States of partial 

 abortion shown by the detailed drawings of Luerssen clearly demonstrate 

 the Asplenioid origin, while the vein-fusions indicate a derivative position 

 i-n that series. 



g. 675. Diplaziuiu ceratolepis Christ. 

 wo ultimate segments from the very large 



PLEUROSORUS Fee 

 With this may probably be ranked the small genus Pienrosortis, which 

 includes mountain Ferns of wide distribution, in which the indusium is 

 completely absent (Fig. Gyy). The condition of these is very nearly that of 

 Gyiniiogramme, and it is quite likely that some of the Ferns that have been 

 ascribed to that affinity may ultimately find their natural place here. This 

 has been broadly suggested for Aspleniopsis Mett. & Kuhn, in which the 

 habit clearly supports the affinity (E. & P. I, 4, p. 272, Fig. 145, A). There 



