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DR JOHN M'LEAN THOMPSON ON 
widens considerably, and the xylem assumes a more or less horse-shoe appearance in 
section (fig. 9). The phloem still completely invests the xylem, but the thinness of 
the former opposite the parenchymatous bay in the latter is maintained. At a 
slightly higher level an open horse-shoe form is established in both xylem and 
phloem, and the parenchymatous bay enclosed by the xylem is placed in open 
contact with the pericycle (fig. 10). This condition is likewise maintained through- 
out the dorsiventral portion of the branch. The petiolar nature of this branch is 
then structurally established, and the small conical protostelic branch is regarded 
as a displaced and arrested axis. Had the arrestment of this axis been more 
complete, merely a nodular swelling would have been developed at the base of the 
petiole, and Mettenitjs’s description of the external aspect of the leaf as nodular 
at its base, and a direct continuation of a branch of the axis, would have found 
its demonstration. 
It will be remembered that when the wide forking of the stem occurred, giving 
rise on the one hand to the branch bearing the arrested axis and the petiole, which 
have just been described, and on the other hand to the left branch, the stele which 
entered the latter was wide and parenchymatous (fig. 4). As this branch is ascended 
these characters of the stele are maintained, and as the distal swelling is approached 
the stele becomes very elliptical in section (fig. 11). This is immediately followed 
Jby a massing of the tracheides on the right of the ellipse, and a tendency on the 
part of this mass to draw off from the remainder of the xylem, leaving the latter in 
the form of a horse-shoe on the left of the stele (fig. 12). The division of the stele 
into two dissimilar parts thus prefigured is soon completed (fig. 13), forming on the 
right hand a small protostele, and on the left a large trace, with open horse-shoe 
xylem and phloem. The trace thus formed is continued directly into the left-hand 
member of the three branches into which the stem divides, and the petiolar structure 
of this branch is finally established. 
But while this foliar trace is passing upwards into its petiole, the small protostele 
on the right widens considerably, and quickly divides, giving rise to two dteles (figs. 
14, 15, and 16). The steps of this division are interesting, for the xylem and phloem 
first divide into two dissimilar portions, that on the left being protostelic, while that 
on the right is a crescentic mass of xylem with phloem on its convex face alone. It 
is, in fact, like a foliar trace. When the division of the stele is completed the proto- 
stelic condition is maintained in the left-hand portion, and assumed in the right. In 
the latter, in fact, is shown a state similar to that seen in the base of the first petiole 
considered. It only remains to state that this trace enters the remaining branch of 
petiolar appearance, and that, as the branch, becomes free, the protostele is trans- 
formed into a typical leaf-trace, with an open horse-shoe of xylem and phloem 
(fig. 17). 
There still remains for discussion the small columnar structure standing between 
the two petioles. It is supplied by the protostele which was the left-hand product of 
