3 1 6 Bower. — Studies in the Phytogeny of the Filicales . 
cells, as is the case in the modern Gleicheniaceae, but of several rows. 
This has already been suggested by Dr. Kidston ( £ Les Vegetaux houillers *, 
Bruxelles, 1909, p. 35). 
As regards the remaining genera with superficial sori and basal 
indusium, the details described above have only served to strengthen their 
relation to the Cyatheaceae, a relationship which has never been in doubt 
except in the case of Acrophorus and Cystopteris . The position of the 
former must remain uncertain until it has been properly re-examined. But 
of the Cyatheoid affinity of Cystopteris there can now be little doubt. Its 
sorus is superficial in origin, as it is in all of these Ferns. The demonstra- 
tion of its basipetal sequence of sporangia upon the receptacle, which has 
itself become lop-sided in accordance with the lateral attachment of the 
indusium, combined with the superficial position, is distinctive ; while the 
anatomy of the stock and petiole, so like that of Struthiopteris , gives 
independent and cogent support to the relationship suggested. 
The importance of this conclusion lies, however, in the consequences 
which follow from it. The comparison of Cystopteris with the Aspidieae 
dates back to the time of Swartz and of Presl. The similarity of habit 
is unmistakable ; in point of vascular structure Struthiopteris and Cystopteris 
compare very closely with such a type as Nephrodium Oreopteris. The 
sori of these Ferns are alike in position, and in their indusium, to those of 
the Aspidieae. Both have also a vascular receptacle. The similarity 
of the young sorus of Nephrodium Filix-mas when cut in median section to 
that of Cystopteris appears clearly from a comparison of Figs. 45, 46 of the 
former Fern with Fig. 38, b , of the latter. But while we thus compare 
its sorus with that of Nephrodium , it is to be borne in mind that the one is 
gradate in the sequence of its sporangia, the other mixed. The conclusion 
which naturally follows on such considerations is that there has been 
a further phyletic progression—the recognition of which is facilitated by 
the living genera Struthiopteris and Cystopteris — from Cyatheoid derivatives 
to certain xA.spidieae, by the assumption of a mixed character of the sorus. 
Such a progression has already been recognized within this very sequence, 
in Hypoderris , while Peranema and Diacalpe , which are shown in Mr. Davie’s 
paper (p. 245) to have mixed sori and a vascular system closely resem- 
bling Nephrodium , have in the form of the sorus and in the basal indusium 
clear Cyatheoid characters. As Mr. Davie justly points out, between them 
Peranema and Diacalpe seem to unite all the intermediate features 
between the Cyatheaceae and the Aspidieae. A similar transition from 
gradate to a mixed type of sorus is believed to have occurred in several 
other evolutionary lines. This change appears in the present case to have 
gone along with a flattening of the receptacle between the indusium and the 
leaf-surface. A further step was the lateral extension of the sporangium- 
bearing surface right and left, in the direction of the midrib of the pinnule, 
