820 IITOK AND CASH : AFFINITIES OF LEPIDODENDRON. 
radial and at others collateral, and that the collateral bundles are 
capable of secondary growth. In all cases the xylem consists of a 
few scalariforni tracheides, with parenchymatous rays between them 
when they are collateral. 
11. The Stem. 
The best figure and description of the internal structure of the 
stem of a Lepidodendron are given by Williamson in the eleventh 
part of his memoirs. Taken from a specimen supplied to him by one of 
us, in which the whole structure from centre to circumference is pre- 
served, it affords a tolerably complete view of the structure and 
arrangement of the tissues met with in one of the best known types 
of Lepidodendron, viz., that described as Lepidodendron selaginoides, 
Carr. and Williamson, L. vasculare, Binney. It will be convenient 
therefore to summarise Williamson's account of this stem, and make 
it the starting point for the comparisons that are to follow. 
1. The centre of the stem is occupied by a string of tracheides 
and vessels, circular in transverse section, among which are a few 
parenchymatous elements. The tracheides are either isodiametric or 
elongated longitudinally, but in both cases are scalariform or reticu- 
lated. The vessels are restricted to the periphery of the string, and 
appear to be mostly spiral (pi. xvi . a) 
This string is spoken of by Williamson as the ' Vascular 
Medulla,' the ' Vascular Axis,' the 'Vascular Cylinder,' etc. It is 
obviously the xylem of either a single vascular bundle or of several 
vascular bundles. 
2. Surrounding this string of tracheides and vessels is a zone 
of tissue termed by Williamson the innermost cortex. Though narrow 
he finds it divisible into three layers : (i.) an internal one of ' very 
minute cells (b) ;' (ii.) a middle layer ' in which the cells are of larger 
size {h") ;' and (iii.) an outer layer ' in which the cells exhibit a ten- 
dency towards a cyclical arrangement (^^^)-' This ' innermost cortex' 
is the bast or phloem belonging to the xylem of the ' vascular axis.' 
8. At the outer periphery of the ' innermost parenchyma,' we 
come upon the middle cortex as Williamson terms it, {d) which con- 
sists of ' very delicate parenchyma, the cells of which at the inner 
margin exhibit a disposition towards a radial arrangement. 
