ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURKES. 709 
Plate 18, photographs 3 and 5, and Plate 23, fig. 10; of. Part VI, Plate 52, fig. 6 ; 
Part XVII., Plate 13, fig. 2), 
Another point of great interest is shown by the same series; the innermost of the 
leaf-traces shown is, at the bottom of the series, deeply imbedded in the secondary 
wood. In the next section it has passed a little way out, and its position and 
structure are almost exactly similar to those of the bundle (from another stem) 
shown in Plate 22, fig. 5. As we trace it further up, we find that its secondary wood 
gradually disappears. At the top of the series the strand, which now consists of 
primary tissues only, has become a double bundle, similar to that shown in fig. 6. 
Hence we may infer that, roughly speaking, the change of structure shown by the 
comparison of figs. 5 and 6 is gone through in about one internode. This is confirmed 
by the fact that the bundles shown in those two figures, both of which belong to the 
same transverse section,* are separated by a divergence of 2. 
In the series C.N. 1198-1191 only four cortical traces are present in each transverse 
section of the internode, because one trace has passed out at the bottom of the series 
and is not replaced until the next node is reached. More usually five such traces 
are seen in any transverse section of an internode, as shown in photograph 1 and 
in fig. 1. 
The following conclusions may be drawn from the facts brought forward, which are 
supported by confirmatory evidence from numerous specimens : 
1. The phyllotaxis is spiral, and the divergence (except in very small stems) 2. 
2. Asa rule each trace passes through five internodes between entering the pericycle 
and bending out into a leaf. 
3. In its course through the lowest of the five internodes the trace usually loses its 
secondary arc of tissue, and begins to divide into two bundles. 
4, The now double trace continues its course with but little change through the 
remaining four internodes, but passes very gradually outwards, the pericycle bulging 
somewhat to make room for it. 
5. Ultimately it becomes free from the pericycle, and passes out through the cortex 
into a petiole, where its two bundles often become partially reunited. 
As each internode must have been, as our specimens show, at least an inch long, 
it is evident that the outward passage of the leaf-traces must have been exceedingly 
gradual. The trace turned out rather sharply, however, where it entered the leaf, as 
shown in Plate 18, photograph 4. 
The nature of the “cortical bundles” and their relation to the leaves is thus made 
clear ; we have now to consider their relation to the internal xylem-strands. 
The number of these strands, as seen in transverse section, is more variable than 
that of the external bundles. We usually find five of the latter, never more. The 
internal bundles also often number five (see Plate 18, photograph 1), but are sometimes 
* As a matter of fact they were drawn from two adjacent sections, between which there is no appre- 
ciable change of structure. 
