384 Chandler . — On the Arrangement of the Vascular Strands 
The periderm presented the appearance of a large patch of thin- 
walled cells in radial rows, the origin of which could be traced to a very 
narrow phellogen just beneath the crushed cells of the wounded area 
(Fig. 308). Very little external tissue seems to have been formed by this 
phellogen, and it was quite evident that the cells of the ground-tissue 
beneath the phellogen also divided directly. This is exactly the state 
of affairs found by Holle to exist in Botrychium. 
The changes undergone by the vascular system in the transitional 
region, as described above, present some striking points of difference from 
the usual course of events. Instead of the transformation of the diarch 
xylem plate of the root into a solid strand, a later appearance of a central 
patch of phloem, and finally the formation of a leaf-gap, we have what 
is practically the root-strand nipping off a simple concentric leaf-trace 
directly, without any preliminary appearance of a central pith of phloem, 
and without the formation of a leaf-gap. 
The absence of gaps in connexion with the formation of early leaf- 
traces is, as pointed out above, a phenomenon for which it is easy to find 
a parallel, but the division of the cauline strand itself, after the exit of the 
first few traces, is so strikingly different from the normal course of events 
as to warrant, at first sight, the assumption that in Polypodium a7ireum 
a type of transition obtains which is fundamentally distinct from that 
occurring in the majority of Polypodiaceae. It has been shown above, 
however, that in the later-formed regions of the rhizome, an essentially 
siphonostelic vascular system is present, and in the light of this evidence 
it is highly probable that the apparent anomaly existing in the young plant 
can be explained. The earliest transitional changes present no real 
difficulty ; the leaf-traces which do not involve the formation of leaf-gaps 
are given off directly from a cauline strand, in which, for some reason, the 
xylem has maintained the plate-like character it possessed in the root. 
This was found to be by no means without a somewhat similar parallel, 
even among the relatively small number of ferns examined by the writer. 
For instance, in some of the seedlings of Pteris Winsetti and Doodia aspera , 
the xylem at the level of the first trace had a distinctly oval outline in 
transverse section, and this occurred occasionally also in other plants. 
The splitting of the cauline strand is perhaps a point more difficult of 
explanation. The writer, however, regards the splitting merely as an 
anticipation of the characteristic double nature of the later leaf-traces. 
Such a splitting, previous to the actual nipping off of the petiolar bundles, 
seems to be of very general occurrence in connexion with such double traces 
(cf. Doodia aspera , &c.), and is indeed the most obvious preparation for 
their formation. If this view of the transitional changes be the correct one, 
the essentially siphonostelic character of the young vascular system is 
