86 Hill .— The Structure and Development of 
type (excepting that here the xylem is interrupted by parenchy¬ 
matous passage-cells, see Fig. 2), the xylem surrounding the 
phloem, as ‘ occurs in the lower ends of the leaf-trace bundles 
of many, but not of all, rhizomes of Monocotyledons, where 
they lie at the periphery of the bundle-cylinder in the stem, 
e. g. Iris germanica , Cyperus aureus , Papyrus , Carex arenaria 
(but not, for example, C. disticha and C. hirta ), Acorus calamus , 
and A. gramineus ' 1 1 
A transverse section of a fully formed bundle is represented 
in Fig. 2, PI. VI. The xylem consists of tracheides, which 
are, comparatively speaking, of great length and frequently 
branched ; sliding growth has gone on to a great extent, so 
that these elements are much interwoven ; hence it is extremely 
difficult to isolate them. 
The protoxylem consists of a few elements thickened in the 
usual manner ; it is generally placed towards the centre of the 
axis, although cases have been observed in which the proto¬ 
xylem occupies a more lateral position. 
The ring of xylem in such a concentric bundle is not, as 
already stated, complete, but is separated into masses by 
parenchymatous passage-cells; five such passage-cells are 
frequently present. 
The phloem is exceedingly well marked and regular, the 
sieve-tubes (s. t.) and companion-cells (comp, c.) being very dis¬ 
tinct ; the former are apparently void of contents, and the 
latter filled with protoplasm, with a large and well-defined 
nucleus. 
The parenchyma surrounding a bundle may sometimes be 
thickened in various places ; such a case is shown in Fig. 2 
(th.pari). 
The ground-tissue in which the bundles are embedded is 
made up of parenchymatous cells, generally rounded in shape, 
but often elongated between two adjacent vascular bundles. 
As the rhizome increases in age the central parenchymatous 
cell-walls of the vascular cylinder thicken up and become 
1 De Bary, Comp. Anat. of the Phanerogams and Ferns, Eng. ed., 1884, 
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