712 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
cambium. In good preparations the group of primary phloém belonging to each 
medullary xylem strand can be clearly recognized at the corresponding point of the 
phloém-zone (see Plate 21, fig. 1 and Plate 22, fig. 7, ph.). 
The leaf-traces in the external part of their course demonstrate the same fact. 
Although there is small-celled tissue on the internal side of the bundle, yet the more 
delicate phloém (Plate 22, fig. 6, ph.’) is perfectly distinct and is evidently limited to 
the outer side of the xylem. The collateral structure is equally evident in bundles in 
the intermediate position, which have a cambial are of their own. Here there is 
secondary as well as primary phloém, both of which are found on the external side of 
the bundle only (see Plate 22, fig. 5, ph.”, also Plate 23, fig. 9). Longitudinal sections 
of the outgoing bundles afford confirmatory evidence.* 
In the petiole, as we shall see more fully below, the structure of the bundles 
becomes concentric. It is a question of considerable interest, at what point this 
important change of structure takes place. It is not easy to answer the question 
with absolute accuracy, for it is only in the best preserved sections that the position 
of the phloém, as distinguished from mere small-celled parenchyma, can be deter- 
mined with certainty. This much, however, is clear: so long as the outgoing 
bundles remain in the pericycle of the stem they maintain a collateral structure ; on 
the other hand, when they have definitely entered the leaf-base they are certainly 
concentric. Thus, in our Plate 21, fig. 1, the four traces, marked 1.¢.’-1.¢.4, consist of 
purely collateral bundles; the outermost trace, 1.5, which is already bulging the 
pericycle, shows some signs of an encroachment of the phloém on the inner edge of 
the xylem-groups. 
The double bundle in the base of the leaf, shown in Plate 18, photograph 3 and 
Plate 23, fig. 10, is distinctly concentric. The change then takes place in the region, 
where the bundle passes out through the cortex to enter a leaf-base.t 
We have next to consider in detail the structure of the xylem of the primary 
bundles. We will begin with the medullary strands, which, as we have seen, simply 
represent the leaf-traces in the lower part of their course. The inspection, even 
of transverse sections alone, at once reveals a characteristic feature; the smallest 
elements of the xylem lie neither at the outer nor inner edge, but are placed in 
an intermediate position, nearer the outer than the inner surface. This statement 
holds good without exception for all the very numerous transverse sections investigated. 
A few typical instances are shown in the illustrations (see Plate 18, photographs 
1 and 2, and more especially Plate 21, figs. 2 and 3, and Plate 28, fig. 8). The smallest 
xylem-elements are accompanied by a few parenchymatous cells; the surrounding 
xylem is entirely composed of tracheee. The similarity to the xylem of a bundle in 
* Hg., O.N. 1982, from which photograph 4 is taken. 
+ These statements are based on the comparison of many sections, among which the following may 
especially be mentioned, in addition to those figured for this purpose: C.N, 1144 D., 1190, 1191-1198, 
1640, 1885 G. 
