Filicales. 
i5 
mature stem has an internal endodermis, and in Osmunda ciuua- 
moinea a little internal phloem may occur at the branching of the 
stem. Dr. Jeffrey and Mr. Faull assert that this internal phloem 
is vestigial (17), (24), while Mr. Seward and Miss Ford regard it as 
a relatively recent acquisition (39). 
The inward projection of the phloem at the medullary rays is 
quoted in support of the vestigial nature of internal phloem. So 
is the presence, in certain species, of an internal endodermis, 
coupled with the rarity of the ectophloic siphonostele among ferns, 
and the fact that in Platyzoma and some Polypodiaceae, this type 
was clearly derived from a solenostele by loss of internal phloem 
(43). Appeal is also made to an analogy with Scliizcea and the 
Ophioglossaceae (17); but the evidence that these forms ever 
possessed internal phloem is quite insufficient. Again, if internal 
phloem is in statu nascendi, it must have arisen at the branching of 
the stem ; but in those ferns in which its origin has been traced it 
arises in the ontogeny, and presumably in the phylogeny, at the leaf- 
gaps. Dr. Jeffrey claims that the mesarchy of the xylem and the 
concentric structure of the leaf-trace favour the vestigial nature of 
the phloem ; but, as Dr. Scott points out, both these characters 
occur in protostelic forms, “ .... so that it is difficult to see how 
they affect the question ” (37). Mr. Faull claims that if the internal 
phloem of Osmunda cinnamomea is a recent acquisition, the stele of 
this species would be very plastic; this is true, but this plasticity 
would appear to be just as great if the internal phloem were 
vestigial. That every stage, except the complete disappearance of 
internal endodermis, occurs in 0 . cinnamomea shows that the series , 
of forms connecting the extreme types is very complete, but does 
not prove which end of the series is the more primitive. 
Mr. Kidston’s and Mr. Gwynne-Vaughan’s recent researches on 
the geological history of the Osmundaceae, clearly show that the 
older Permian members of the Osmundaceae (included in the genus 
Zalesskya) were protostelic, though the central tracheae of some 
forms had already become short and wide, functioning rather as 
water-storers than as water-conductors (29). They have shown that 
in the Jurassic Osmuudites Dunlopi the centre of the stem is occupied 
by a pith, abutting on a woody ring, unbroken by the departure of 
leaf-traces (28). Their work seems to show that the Osmundaceae 
as a whole are descended from protostelic forms; their results have 
been widely accepted considering how recent is their publication 
(43), (9). But Mr. Kidston and Mr. Gwynne-Vaughan have also 
shown that Osmundites skidegatensis, from the Lower Cretaceous, is 
