710 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
more numerous, the maximum number observed being eight, as shown in Plate 21, 
fig. 1. Where their number is equal to that of the cortical bundles, they alternate 
quite regularly with the latter, as shown in photograph 1.* Where they are more 
numerous, we find the reason is, that certain of the single medullary bundles are 
replaced by a pair (see Plate 21, fig. 1). The alternation, however, is always preserved. 
That the medullary bundles are continuous with the cortical leaf-traces is proved 
by the fact that we often find bundles in the intermediate position, in the act of passing 
out from pith to pericycle. If, as is almost always the case, the stem possesses 
secondary tissues, the bundle is here enclosed by the secondary wood, which has 
formed on its outer side only, while behind it is a parenchymatous gap (see Plate 23, 
fig. 9). We find examples of these transitional bundles in all positions between pith 
and pericycle. In the case figured the bundle is scarcely half-way out. On one side its 
own cambium is continuous with that of the rest of the cylinder. On the other side the 
main cambium bends inward. In Plate 21, fig. 1 a corresponding bundle, L.t.1, is shown 
a little further out. Here the general cambium has already formed behind it. In 
the series C.N. 1198-1191 already referred to, a bundle can be traced all the way 
from near the pith into the pericycle. At the top of the series another bundle is 
just beginning to pass out from the pith.t 
The identity of the medullary and cortical bundles is further proved by the study 
of very young stems (which are rare), such as that of which a transverse section is 
shown in Plate 18, photograph 2. Here only three or four layers of secondary wood 
have been formed. Consequently, there is comparatively little separation between the 
medullary and cortical bundles. Four cortical traces are shown, the fifth appears to 
have passed out into a petiole. The innermost cortical trace, namely, that which 
has not yet split into two bundles, is only just free from the ring of wood. There 
are six medullary bundles, which alternate (either singly or in pairs) with those of 
the cortex. The study of this section at once shows that cortical and medullary 
strands are identical bundles cut through at different parts of their course, and thus 
confirms the evidence obtained from more advanced stems. 
We may, therefore, draw this further conclusion, as to the course of the vascular 
bundles in the stem: the medullary bundles form the downward continuation of the 
xylem-strands of the same leaf-traces which pass out through the cortex into the 
leaves. The question now arises, How do the bundles behave on entering the pith, 
and how is the alternation of the medullary and cortical strands to be accounted for ? 
The evidence on this point is imperfect, but certain indications are afforded by the 
comparison of transverse sections. In the series so often referred to (C.N. 1198- 
1191) a bundle is shown in the uppermost section just entering the pith. As we 
trace it downwards, it appears to attach itself to one of the adjacent medullary 
* This fact was already recorded in ‘‘ Organization,” Part IV., p. 383. 
+ C.N. 1191. See also C.N. 1140 and C.N. 1190, 1138, and 1885 E.; the three last form a series from 
below upwards. 
