ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-~MEASURES. 707 
the parenchyma. Beyond the sclerenchymatous zone, there are a few more layers of 
parenchyma more or less imperfectly preserved. The epidermis itself is never perfect 
and has generally been destroyed. 
One of the most important features remains to be mentioned, namely, the so-called 
cortical vascular bundles. These are in reality the leaf-traces on their way out to 
the petioles ; their true nature was pointed out in a previous memoir,* and additional 
evidence is brought forward in the present paper (see Plate 18, photographs 3 and 4, 
and Plate 23, fig. 10). In most specimens, including those shown in photograph 1 and 
fig. 1, five of these leaf-traces are present in the transverse section ; the majority of 
these traces are double, forming the “ twin-bundles” so characteristic of the plant. 
All these bundles are imbedded in the pericycle, a position which they maintain 
until they turn out into the leaf-bases. The innermost bundles constantly possess a 
fan-shaped mass of secondary xylem and phloém on their outer side. 
The above may suffice for a summary of the more obvious points of structure in a 
typical stem of Lyginodendron. In order to gain a more complete conception of the 
anatomy, the first thing necessary is to obtain a clear knowledge of the course and 
structure of the primary vascular bundles. To this subject we have devoted special 
attention, and we will take it first in our more detailed survey. 
2. Course of the Vascular Bundles. 
In considering the distribution of the primary vascular tissues of the stem we have 
to determine the course of the cortical bundles,t and of those which lie at the 
periphery of the pith, and to trace the relation between the two systems. The former 
are complete bundles with both xylem and phloém (see Plate 22, figs. 5 and 6); the 
latter are xylem-strands only, for their phloém has been removed towards the outside 
by the intercalation of the secondary tissues. That: the cortical strands are leaf- 
traces has already been proved ; we shall endeavour to demonstrate below, that the 
perimedullary strands are simply the lower internal portions of the same leaf-trace 
system, thus confirming a conjecture of Count Sotms-LauBacu’s.{ First of all, 
however, it is necessary to determine with greater exactness the course of the 
bundles in the cortical region, and their relation to the leaves. These points are best 
studied by the comparison of transverse sections. Longitudinal sections are obviously 
necessary as auxiliaries, but the leaf-traces pass out so very gradually, that it is never 
possible to follow their whole course in any one preparation. 
In any geod transverse section through an internode, we see that the external 
bundles lie at somewhat different distances from the centre of the stem. Thus in 
* Wiiamson, “ Organization,” Part XVIL., p. 90, 1890. 
+ The external bundles are conveniently spoken of as cortical, though we more usually find them 
within the limits of the pericycle. 
t “Fossil Botany,” English edition, p. 360. 
4yv2 
