602 FILICALES 



and leaf-form ; but also upon the characters of the marginal sorus, and 

 lastly upon the vascular system ; for even the dendroid Dicksonias show 

 a stelar system but little in advance of the solenostelic Dennstaedtias, 

 allowance being made for the closer leaf-arrangement in their upright 

 shoot. 



It is thus seen, not from one character alone but from several, that 

 the Dennstaedtiinae take a transitional position between certain types 

 of the simpler Gradatae and those Mixtae which have marginal or approxi- 

 mately marginal sori. But lastly, the inequality of the lips of the indusium, 

 and the obvious lopsidedness of the sorus, both in Dicksonia and in the 

 Dennstaedtiinae, has its interest in relation to what is seen in some of 

 the more advanced Ferns. The origin of the receptacle is still strictly 

 marginal, but already there is a leaning towards the lower surface, and it 

 will be seen that, in forms belonging to the Mixtae which appear to be 

 related, this becomes more pronounced, till finally a superficial position 

 of the sorus is fully attained. 



CYATHEAE. 



In all the Gradatae hitherto considered the sorus is of marginal origin, 

 though in some of the most advanced there is a tendency towards the 

 lower surface. But in the Cyatheae, in which the sorus is also basipetal, 

 its position is superficial, being thus comparable with that of the Gleicheni- 

 aceae or Marattiaceae. There is no comparative ground for referring this 

 in the Cyatheae immediately to any transition from a marginal position : 

 there is indeed good reason for believing that the superficial sorus was 

 of very early occurrence, for it is exemplified in some of the most primitive 

 types of Ferns. 



The Cyatheae as now limited include the dendroid genera Alsophila, 

 Hemitelia, and Cyathea, though it will be seen that certain other genera 

 of Ferns of smaller stature are probably related. In habit they are occasion- 

 ally creeping (A. blechnoides), but mostly of tree-like habit, the columnar 

 stem being covered by the scars of the tufted leaves : these may be simple 

 (C. sinuata\ or singly pinnate (C. Brunonis\ but usually repeatedly pinnate. 

 Broad superficial scales are present generally, but hairs commonly accompany 

 the sorus. Thorn-like outgrowths are not uncommon upon the surface, 

 especially about the base of the petiole : these must be held as new 

 formations, by enation from surfaces previously untenanted in descent : 

 they show^hat such origin of new appendages existed among very early 

 vascular Plants. Adventitious roots are numerous, and form a dense felt 

 investing the lower part of the erect trunk to a thickness often far beyond 

 its own bulk. 



The three genera named form a very natural group, separated from 

 one another technically by the character of the indusium, which is absent 

 in Alsophila^ incomplete and scale-like in Hemitelia, while in Cyathea it 



