264 
A. G. Tansley. 
evolution of vascular structure we should regard this as a more 
primitive condition than that in which the internal phloem of the 
petiolar strand is attached to internal phloem of the stem-stele* 
But on Faull’s view we should have to suppose that the internal 
cauline phloem had completely disappeared while the internal 
phloem of the trace remained intact. 
It should be pointed out that even in O. skidegatensis the phloem 
lining the concavity of the leaf-trace is not directly continuous in 
the radial plane with the internal phloem of the stem-stele. Traced 
downwards it dies out, in the concavity of the U-shaped strand of 
xylem, below the point of insertion of the trace. It is the external 
phloem of the stele, on the right and left of the leaf-gap which 
becomes continuous, at a higher level, through the gap with the 
internal phloem of the stele. Here we have, pace Faull’s statement 
that the Osmundaceous stele “is neither primitive nor anomalous 
among the Filicales,”a character which is certainly quite exceptional 
among the Ferns, and which seems to be connected with the “proto- 
stelic” mode of departure of the leaf-traces. In spite of the breaking 
up of the xylem of the Osmundaceous stele into separate strands, this 
“ protostelic” mode of departure is essentially maintained in the 
most advanced forms, inasmuch as the leaf-trace always departs 
from the outer face, and does not “take away” a complete segment 
of the cylinder. For these reasons the suggestion of Kidston and 
Gwynne-Vaughan that the internal phloem and endodermis of O. 
cinncunomea should, perhaps, be regarded as “cladosiphonic,” in the 
sense that their development started from the branch-gaps, not from 
the leaf-gaps, is certainly to be regarded as a plausible one. And 
for all we know to the contrary the stele of O. skidegatensis may 
represent a higher phase of the same type, in which the internal 
and external phloem have become secondarily continuous through 
the leaf-gaps, as they are occasionally in O. cinnaniomea. It 
is no doubt conceivable that O. skidegatensis is reduced from 
a normal dictyostelic type, in which the leaf-traces have secondarily 
come to depart from the outer face of the xylem cylinder, their 
internal phloem has ceased to be connected with the internal phloem 
of the stem, and the leaf-gaps have become very narrow. Some 
such process of reduction as this has very likely occurred, as we 
have already said, in Platyzoma. But when we consider the 
palaeontological evidence connecting Osmundaceae with Botryo- 
pterideae, as well as the other considerations given above, such a 
hypothesis becomes distinctly unlikely. 
