224 
Review. 
region having been displaced through the centre of the stele “ as 
one would pull the bottom of an inverted sack through the tubular 
part ” (6). 
Passing to the anatomical structure Mr. Holloway distinguishes 
two main types in the young stem. In the one the first vascular 
elements to develop in the stem are those of the leaf-traces; later 
the latter affix themselves irregularly to the vascular elements 
formed from the plerome cylinder; in this type, found in L. cernuum 
and L. laterale , xylem and phloem are irregularly intermingled. In 
the second type found in L. Billardieri, L. volubile and L. scariosum, 
the vascular elements of the plerome cylinder develop before those 
of the root and leaf ; here the structure of the stele is more compact 
and definite, and the leaves, fewer in number, are more regularly 
arranged. In the mature stem Mr. Holloway also recognises two 
main types, the “ mixed” and the banded. In the latter type he 
includes two very distinct forms : those in which the bands of xylem 
and phloem are arranged more or less radially and those in which 
these bands are more or less parallel to one another, except where 
the attachment of the roots causes some disturbance. Mr. Holloway 
considers that the type of stele showing parellel arrangement of 
the xylem and phloem is especially characteristic of plagiotropic 
stems and concludes that it arises as a modification of the type 
with radial bands of vascular tisssue, owing to the restriction of 
branching to one plane in these plagiotropic forms 1 In this he is 
in general agreement with Mr. Jones (4, p. 27) and Miss Wigglesworth 
(9), and the probability is that this view would be generally accepted. 
But Mr. Holloway’s further contention that the banded types thus 
grouped together are less primitive than the “ mixed ” type is open 
to many objections. In the first place the more regular arrange¬ 
ment of the type showing radial bands (of which Mr. Holloway 
himself considers the type with parallel bands merely a variety) is 
much more easily brought into line with the steles of the older 
fossil Lycopods and of the simpler species of Selaginella ; indeed 
the radial type of stele which is found in the roots of the Pteridophyta 
generally seems to be as old as the circular protostele. The “mixed ' 
type on the other hand seems to be peculiar to certain species of 
Lycopodium and if we regard it as primitive for the genus we must 
either assume that it originated from a protostele and gave rise to 
the type with radial bands of vascular tissue which it would seem 
more natural to derive directly from the protostele; or we must 
regard the stele of Lycopodium as having originated independently of 
the stele of other Lycopods, recent and fossil; this last solution of 
the difficulty seems very unnatural, for the stele of this genus 
agrees very closely in its histology with those of the other members 
of the phylum. Again we may note that in the “ mixed ” type the 
purely cauline part of the stele is less differentiated and in the 
young sporophyte the leaf-traces contain the first vascular elements, 
whereas in the banded types the purely cauline elements develop 
first. In a phylum of which the relatively great development of the 
stem compared with the leaf is one of the principal characteristics 
it is natural to regard the latter condition as primitive. Mr. 
Holloway seeks to support the primitiveness of the “mixed” stele 
by urging that the more definite arrangement found in L. Billardieri, 
