704 Gwynne- Vaughan. — Observations on the 
Miss Wigglesworth. For not only are there two concentric 
vascular cylinders lying within the ordinary solenostele, but 
a small central strand is present in addition, which may be 
regarded as an indication of a third. 
It is evident that in all these cases the ordinary typical 
vascular cylinder is represented by the outermost vascular 
system. The internal vascular system is an accessory de- 
velopment, and from the consideration of the facts brought 
forward above it appears that, even in its most complex form, 
it is to be derived from the ordinary stelar cylinder by the 
progressive elaboration of a local thickening of the xylem- 
ring at the leaf-gap margin. The initial stage of such 
a development would be a simple marginal thickening some- 
thing like that in Dicksonia apiifolia. The first step in 
advance would be the further development of this thickening 
into an internal ridge resembling that in Dicksonia adiantoides. 
Very little is wanting to separate off this ridge so as to give 
rise to a free internal strand similar to those in Dicksonia 
rubiginosa. The internal strand might then become converted 
into a more or less closed cylinder, like that found in Pteris 
data , in two different ways : either by enlarging and at the 
same time curving round so that its two ends eventually meet, 
or as it enlarged it might also branch, and the branches 
eventually fuse up into a ring. It is difficult to decide which 
of these two methods is the more probable, indeed, it is 
possible that both may occur. If the same series of changes 
were to take place in the first internal ring a second would be 
produced, and thus again a third, one lying within the other, 
as exemplified by Pteris data and Matonia pectinata. 
It must at once be understood that the order in which 
these Ferns have been placed in order to illustrate this theory 
is not intended to represent a phylogenetic series. All that 
it is necessary to assume is that their relationship is sufficiently 
close for the various modifications of structure that they 
present to be taken in explanation of one another. According 
to the theory outlined above a strong distinction must be 
drawn between the internal vascular cylinders and the original 
