XX] CYATHEACEAE 295 



pound, but Gyathea sinnata Hook, a rare Ceylon species, bears 

 simple narrow linear leaves. This family includes, with few 

 exceptions, all the tree ferns ^. The sori of Dicksonia are 

 enclosed in a two-valved indusium (fig. 229, F, G) ; in the 

 species represented in fig. 230 the fertile segments, which 

 terminate in cup-like indusia, are characterised by the absence of 

 a lamina and closely resemble those of Thyrsopteris (fig. 229, A). 



Fio. 230. Dicksonia Bertercana Hook. Fertile aud sterile pinnae. (Nat. size. 

 British Museum Herbarium.) 



In Gyathea the indusium has the form of a cup which is at first 

 closed and afterwards opens at the apex (fig. 229, B) ; in He- 

 mitelia the indusium is much reduced and in Alsophila the sori 

 are naked. Thyrsopteris is characterised by the reduced fertile 

 pinnules bearing stalked sori in deep cups (fig. 229, A). The 

 appearance of this fern "is very remarkable, for the cup-shaped 

 > Scott, J. (74) ; Hannig (98). 



