Studies in the Phytogeny of the Filicales 
VII. The Pteroideae. 
BY 
Professor F. O. BOWER, Sc.D., F.R.S. 
With forty-three Figures in the Text 
I N the third memoir of this series 1 I suggested that the Leptosporangiate 
Ferns, exclusive of the Osmundaceae, may be grouped into two 
phyletically distinct series: the Super"ftdales, in which the origin of the 
sorus is constantly from the surface of the leaf; and the Marginales , in 
which it is as constantly from the margin. But this distinction, important 
as it is, cannot be applied with rigid uniformity. Investigation shows that 
in the course of the evolutionary advance of the Marginales, as also in the 
development of certain individual representatives of that series, the original 
position of the sori is liable to modification. This was illustrated with 
clearness in the third memoir, in the case of the Dicksonioid derivatives. 
Dicksonia itself shows a movement of the marginal sorus during its individual 
development towards the lower surface ; but this result becomes much more 
marked in Davallia , and reaches its climax in Nephrolepis , or perhaps in 
Oleandra. Here the appearance of the mature sorus is as though inserted 
intramarginally, that is, on the lower surface of the leaf. The biological 
reason for the change is readily understood. The protection of the young 
sorus on the lower surface is more efficient than that of a sorus on the 
margin. 
The fact that such modifications actually do occur may dispose the 
critical mind altogether to reject the distinction drawn between the two 
series. But the fact that the receptacle in downward-directed sori such as 
those of Dicksonia , Odontosoria , and Lindsay a is actually marginal in origin, 
though deflected downwards as development proceeds, illustrates how the 
initial steps of the modification may actually be observed in the ontogeny 
of typical Marginales . 2 It is held as probable that a change which can be 
traced in the ontogeny of certain Ferns may have become stamped per¬ 
manently upon the descent of others. Accordingly the distinction between 
the Superficiales and the Marginales would not be absolute. It would be 
1 Ann. of Bot., vol. xxvii, p. 476. 2 See Ann. of Bot., vol. xxvii, pp. 457-63. 
[Annals of Botany, Vol. XXXII. No. CXXV. January, 1918.] 
B 
