The Botryopteridecz. 
65 
tended to become flattened in one plane, and the main axis became 
1 datively important, it is easy to see how the Zygopteris-type of 
structure might result. Schizopteris, the supposed leaf-impression 
of Zygopteris is a bipinnate, apparently normal frond, which has not 
Fig. 15. Stauropteris oldhamia. Strand of an axis of the frond. The 
cruciate xylem is seen with its smallest elements (probably protoxylem) at the 
corners. The three groups of large thin-walled cells in the bays between the 
strands are probably large metaphloem sieve-tubes. Similar though smaller 
elements are seen broken away from the xylem a little to the left of the 
centre at the top of the figure. It is doubtful if the phloem is continuous 
round the angles of the wood. The inner cortex, not apparently very sharply 
separated from the small celled pericyclic tissue, is indicated in places. Scott 
Collection. X 60. 
yet been properly correlated with the structural material of Zygo¬ 
pteris, but it is possible to suppose either that one member of each 
pair became aborted, or that the pair of branches on each side 
became bent from the transverse into the horizontal plane, as 
happens for instance to the spirally arranged leaves of modern 
dorsiventral shoots in the flowering plants, under the influence of 
dorsal illumination. 
It is not intended to suggest that Stauropteris was necessarily 
the ancestor of the Zygopteris- type, nor the latter of the Tubicaulis- 
type and of the C-type found in most ferns. But it seems probable 
that these four types have the homologies indicated and that the 
recent forms present a specialised and stereotyped type of organi¬ 
sation of the axis of the frond, which is only one of a number of 
types of organisation formerly existing, some of which reveal to us 
examples of branching all round the axis of the frond. This tendency 
to radial organisation of the frond may perhaps be regarded as a 
