322 HICK AND CASH : AFFINITIES OF LEPIDODENDRON. 
7. Finally it should be mentioned that the vascular bundles 
that go to the leaves arise at the periphery of the ' vascular axis,' 
and therefore must pass through all the tissues external to that axis 
in their course outwards. 
Such then is a description, as given by Williamson, of the 
structure of the stem of Lepidodendron selaginoides. 
For purposes of comparison, it will be convenient to reduce this 
description to its lowest terms, and to express the essential facts in 
such a way as will enable us to detect points of resemblance or differ- 
ence when w^e pass from Lepidodendron selaginoides to other types, 
and from these again to other genera. 
Beginning at the centre of the stem, it is obvious that the 
" vascular axis' and the ' innermost cortex represent respectively 
the xylem and phloem of an Axile Vascular String or Cylinder, 
whicl) is made up of the prima^^y vascular bundles. Whether this 
axile string is a single concentric bundle or an aggregate of bundles, 
and whether it is wholly made up of leaf-trace bundles or is partly 
cauline, is not definitely known. The more probable view is that it 
is an aggregate of bundles. 
The ' external cortex'''' and the ' middle cortex' of the above 
description offer no difficulty. They are obviously the equivalents 
of those divisions of the cortex which De Bary1 terms hypoderma 
and inner parenchyma. 
Hence the primary and fundamental structure of the stem of 
Lepidodendron selaginoides may be said to exhibit the following 
structures : — 
(a) K central Axile Vascular Cylinder, representing one or 
more vascular bundles, and consisting of 
i. A central string of xylem, enclosed by 
ii. A zone of phloem. 
(h) An outer mass of tissue, forming the cortex, and divisible 
into 
i. The inner parenchyma, and 
ii. The hypoderma. 
* In the restricted sense explained above, 
t Compartive Anatomy of Phanerogams and Ferns. Eng. ed., p. 404. 
