212 D. T. Gwynne-Vaughan. 
stated that this arrangement has been derived in the course of 
evolution from a single solid central cylinder with a central mass of 
xylem. It follows, therefore, that certain regions in the stem 
primitively occupied by vascular tissue are now occupied by ground- 
tissue. The exact manner in which this change took place is as 
yet undecided and is still under discussion. Nevertheless, as 
regards the Polypodiaceae at least, whatever the process itself may 
have been, there is a strong probability that it first of all came into 
action at certain points at the periphery of the protostele situated 
immediately above the departing leaf-traces. According to one 
view, at these points the vascular elements of the protostele have 
been directly transformed into ground tissue; according to another 
they were not formed at all, but their place has become occupied 
by the elements of the cortical ground-tissue lying immediately 
without them. In either case the recognizable outline of the 
central vascular cylinder would become changed at the points in 
question and the cortical tissue would appear to, or actually 
would, project inwards into the stele so as to form so many deep 
bays or pockets. These “ endodermal pockets,” as they have been 
called, at first end blindly below, later on, however, the pocket 
belonging to one leaf reaches down to meet that of the leaf next 
below, and thus the solid protostele becomes converted into a tube 
with perforated walls. A further increase in the relative size of the 
perforations will produce a structure that may be conveniently 
pictured as a tubular lattice-work. 
In this last condition of the vascular system the question as to 
what has become of the “ stele ” as a morphological unit leads 
naturally to the expression of two diverse opinions. In the first 
place it is obvious that the separate strands of the lattice-work 
(meristeles) correspond to parts only of the original protostele 
Now according to what may be called the theory of transformation 
the ground-tissue lying within and between the meristeles also 
corresponds to a part of the original protostele, and, therefore, the 
stele as a whole is still to be regarded as theoretically circular in 
outline. On the other hand, according to what may be referred to 
as the theory of substitution, the central ground tissue is cortical 
in origin, the protostele is broken up into several separate parts, 
and the “ stele ” as a whole cannot be said to exist at all. 
While admitting freely the great theoretical interest attached 
to this question, it seems to me that, whichever view of the point 
be taken, the real essential worth of the conception of the stele will 
