64 A. G. Tansley. 
which branching is always strictly in the plane of the frond. 1 What 
is the connexion between the two ? It seems that if a frond of the 
Zygopteris- type lost either its posterior or its anterior set of 
branches on both sides of the rachis, while the remaining set came 
to lie in the horizontal plane (as indeed they apparently tend to 
do, Figs. 12, 13b, 19c), we should have the ordinary type of fern 
frond, while with the disappearance of one set of branches 
the corresponding wings of the vascular strand would likewise 
disappear and the “ double anchor ” would be converted into a 
C. If it were the anterior branches which became aborted the 
concavity of the C would face outwards, aw r ay from the stem-axis, 
as in Tubicaulis, while if the posterior branches disappeared the 
C would lie with its concavity inwards, the normal orientation in 
the modern ferns. The probability of such reduction is rather 
suggested by the inequality of the twin branches on one side of 
Fig. 14. Diplolnbis estiotensis. From Autun. Strand of rachis giving off 
branch traces. Protoxylems as in Fig. 13. This type, which should probably 
be included in Zygopteris, has a strand intermediate in shape between Z. 
duplex and the ordinary types. Univ. Coll, collection, K 21. x 6. 
the rachis in Zygopteris Lncnttii as figured by Williamson, and 
perhaps also by the unequal development of the anterior and 
posterior wings of the rachis-strand often seen in Z. insignis 
(1 corrugata ) (Fig. 9). 
It may be that Stauroptcris oldhamia represents a type of 
frond-branching from which the Zygopteris- type is derived. 
Williamson described this frond as presenting “clusters of minute 
twigs,” and it is easy to see from sections that an axis of the frond 
must have borne quadriseriate branches, which themselves branched 
in a similar manner (Figs. 16 and 17). The whole structure, even if it 
is to be regarded as the fertile frond of a plant with dimorphic 
leaves, must still have been extraordinarily unlike any other known 
type of fern frond. If now in such a case as this the whole frond 
1 A type of branching superficially similar to the Zygopteris -type 
seen in some species of Glcichenia and Lygodtum is clearly 
shown by an investigation of the mode of branching of the 
strand of the rachis to be derived from the normal type by 
twisting of the branches through 90°. 
