ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 875 
nodes, and in connection with this subject to examine, more fully than we have yet 
done, the longitudinal course of the vascular bundles. 
b. The Course of the Vascular Bundles and the Structure of the Nodes. 
In considering the question of the longitudinal course of the vascular bundles, it is 
obviously necessary to start from specimens in which we know for certain which is 
the upper and which the lower end. A specimen for which the cabinet is indebted to 
Mr. Witp affords this evidence in a very convincing manner. The branch in ques- 
tion has a pith which tapers rapdily at one end ; a series of eight transverse sections 
were cut from this part, three of which are shown in the photographs 7, 8, and 9, on 
Plate 73. We shall return to these sections later; here we need only point out that 
the base of the branch is the end at which the pith has its minimum diameter. 
That this is so has long been proved: it is sufficient to refer to our figures, Plate 79, 
fig. 20, and Plate 80, fig. 22; to the figures 27 and 30 in Part IX. of Witiiamson’s 
series of Memoirs in the ‘ Philosophical Transactions,’ 1878, Part II.; and to Plates 2, 
3, 4, &., in WEIss’s ‘Steinkohlen-Calamarien,’ Part II. From the upper part of 
Mr. WILp’s specimen a longitudinal section has been cut (C.N. 1937), on which the 
upper and lower ends are marked. A part of the section is approximately tangential, 
and shows the course of the bundles with perfect clearness, so that here we have the 
desired evidence in an unmistakeable form. The same section shows further that the 
lateral branches were inserted immediately above the node. We can, therefore, use 
this fact as a means of orientation in other cases; in all sections which show branches 
we know that the adjoining node is below the branch, and thus the top and bottom of 
the specimen are determined. Another useful clue is afforded by the fact that the 
outgoing foliar bundle is generally situated in the median line of the bundle running 
up to it from below, while this is seldom the case with the bundles above the node.* 
Another indication which can sometimes be made use of is the fact that the xylem 
of the stem-bundles is directly continuous with that of the outgoing leaf-trace, 
from below only. On the upper side the trachez overarch the leaf-trace bundle, 
but are generally separated from its xylem-elements by some parenchymatous tissue. 
(See Plate 78, fig. 8.) These latter indications (to which others might be added) are 
of service where there are no branches to afford a more certain guide. When we 
have once become familiar with the structure of specimens in which the direction is 
known, there is seldom any difficulty in determining the position of apex and base in 
any case where we have a tangential section through the wood. 
In Mr. Witp’s specimen, and in some others, the course of the bundles is essentially 
that of Hquisetum. If we trace any bundle from below upwards, we find that at the 
* This indication, however, can only be trusted where the tangential section passes very near to the 
pith. The width of each woody wedge increases towards the exterior, and this increase is not always 
symmetrical with reference to the leaf-trace bundles. 
