ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 711 
strands, namely that on the kathodic side. Unfortunately the preservation of this 
part is not good enough for the course of this bundle to be followed with absolute 
certainty. 
One point, however, throws considerable light on this question. If we determine 
the outermost leaf-trace in any transverse section, we know that its place will be 
taken a little higher up the stem by a bundle passing out from the pith. Conse- 
quently it is from the medullary bundles adjacent to the gap corresponding to this 
leaf-trace, that the next cortical strand will be supplied. Now, we often find that 
this gap has a double bundle on its kathodic and a single one on its anodic side (see 
Plate 18, photograph 1, 1.t.5; the kathodic medullary bundle is shown in Plate 21, 
fig. 3, the anodic in fig. 2; see also fig. 1, L.t.5). 
If, however, we examine a bundle, which is already passing out through the wood, 
we find that the medullary strands on either side of it are both single bundles. All 
this points to the conclusion that a leaf-trace, when followed downwards into the 
pith, turns aside and joins the next medullary strand on its kathodic side. 
If this were all, it would involve the fact that each medullary strand is a 
sympodial bundle, built up of the lower parts of all the leaf-traces of one orthostichy. 
It is, however, more probable that connections also take place in the opposite 
direction, for it is not likely that the bundle system of each orthostichy was quite 
isolated from the rest. The frequent occurrence of double medullary bundles in 
other positions than that already determined (see Plate 21, fig. 1) perhaps points 
to the existence of these additional fusions. 
In any case, we may safely draw the following general conclusion as to the course 
of the vascular bundles : the bundle-system in the stem of Lyginodendron is entirely 
a leaf-trace system. The longitudinal course of each leaf-trace extends through at 
least ten internodes, about five of which are passed through in the cortex and 
pericycle, and the same number at the outside of the pith. 
The medullary strands are thus sympodial bundles formed of the united lower 
portions of the adjacent leaf-traces. 
The general similarity to the bundle-system of Osmundacee is evident.* 
3. Structure of the Vascular Bundles. 
There can be no question that the vascular bundles in the stem were of collateral 
structure. This is most certainly shown by the medullary strands, which consist 
solely of xylem elements, abutting directly on the cells of the pith. There is no 
trace of phloém on their inner side, and the preservation is often so perfect as to 
make it certain that no elements have been lost (see Plate 21, figs. 2 and 3, transverse, 
and Plate 22, fig. 4, longitudinal). The phloém of these bundles has been entirely 
displaced towards the exterior owing to the interposition of secondary wood by the 
* See pz Bary, “Comparative Anatomy of Phanerogams and Ferns,” English translation, p. 280. The 
similarity is not diminished by the more recent observations of ZEnzrtl, ‘ Bot, Zeitung,’ 1895, p. 53. 
