294 CYATHEACEAE [CH. 



thickness. In these Ferns, as also in the Dicksonioids, the best has been 

 made of an unpractical scheme of construction, viz. an unlimited upward 

 growth with a constantly increasing head of leaves, but without secondary 

 thickening. Its origin from a creeping habit is clearly seen here, through 

 comparison with Lophosoria. But the dendroid habit has also been adopted 

 by the Osmundaceae and the Blechnineae. It is therefore concluded that 

 it may arise polyphyletically, and is in itself no index of affinity between 

 Dicksonioids and Cyatheoids. 



Fig. 556. Transverse section of a large bifurcated trunk of Cyathea medtillaris 

 showing the small size of the twin stems and the large bulk of the adventitious 

 roots that embed them. (Much reduced.) 



The leaves of the Cyatheaceae are as a rule large and repeatedly pinnate. 

 Many have narrow segments, often of "Pecopterid" type as in Gleichenia, 

 and the venation is usually open. But various degrees of condensation of 

 the leaf-structure may be seen, and some steps towards vein-fusion. For 

 instance, while many Cyatheoids have leaves 3 or 4 times pinnate, there is 

 only a double pinnation in A. Taenitis. A single pinnation appears in 

 A. phegopteroides, or in Hemitelia grandifolia, though in the latter the 

 pinnae are themselves pinnatifid. Lastly, in Cyathea sinuata there is a simple 

 leaf, the origin of which by condensation is suggested by the open pinnate 

 venation (Fig. 557). These may all be held as secondary results of simplifi- 

 cation of a pinnate branching, combined with webbing of the segments. 



