( Intraxylary) Phloem in the Stems of Dicotyledons. /. 577 
composing the cylinder, with here and there much smaller bundles in 
the pith closely adjoining them or on their sides ; these small bundles are 
inverted collateral or amphivasal in structure. The internal phloem of the 
large bundles of the ring has the form of a detached, large, rounded strand 
encircled by a cambium which, in nearly all cases, has formed some tissue 
which has the characters of the xylem parenchyma of the main bundle; it 
is usually most greatly developed on the inner (pith) side; in only one case 
was a small group of vessels seen attached to the inner side of the 
internal phloem ; but the soft-walled tissue formed by the cambium may be 
regarded as xylem. Not only are there distinct evidences of the scattered 
arrangement of the bundles of the cylinder, but there is also a distinct 
tendency in these towards amphivasal structure, for the bundle whose 
internal-phloem group possessed vessels was concentric, the phloem being 
completely encircled by the xylem; all the other bundles are very 
V-shaped. 
The above-described fusion of a small medullary vascular bundle with 
an internal-phloem group leads to the conclusion that the latter represents 
an independent vascular bundle which has lost its xylem, for the medullary 
bundle becomes one with it, and the internal-phloem group become?, at 
a lower level, a vascular bundle. 
No case is known in any plant of a vascular bundle fusing with the outer 
phloem of a bundle of the cylinder. Hence the internal phloem is not the 
equivalent of the outer phloem, in the sense of forming a constituent part of 
the bundle of the ring, but must be regarded as an independent bundle of 
the pith. This view is again strongly supported by the facts recorded in the 
second fruit-peduncle : the independent character of the rounded internal- 
phloem group, with its parenchymatous and, in some cases, woody xylem. 
It clearly represents an incompletely formed amphivasal vascular bundle, 
and is probably a vestigial structure. The bundles of the cylinder are also 
clearly more or less perfect or imperfect amphivasal bundles. The. whole 
thus constitutes, in the writer’s opinion, the vestige of a scattered system 
of bundles composed of several series or irregular rings ; the internal- 
phloem groups attached to each ring of bundles would represent always, on 
this view, a distinct series or ring of bundles (cf. Fig. 9). 
C. foetidissima (Cucumis perennis). 
Stem . 
In the extreme basal region is a single bundle-ring of quite cylindrical 
contour and composed of about twelve bundles of various sizes and rather 
closely approximated, the large internal-phloem masses occupying almost 
the entire pith. At a higher level the number of bundles is greatly 
increased and the ring is no longer of cylindrical contour, but exceedingly 
sinuous, consisting of five arms. In the outer or pericyclic zone of the 
