128 Worsdell. — On the Comparative Anatomy of 
wide spaces in the ground-tissue. In different stems the 
bundles vary in number and in size ; some of them are of 
considerable proportions, being built up of a large number of 
elements. Most of them have the normal orientation ; some 
are frequently seen to have a peculiar concentric arrangement 
whereby the xylem comes to lie on the circumference and 
the phloem in the centre ; the bundle then becomes circular 
in outline. 
The xylem-elements are few and scattered, the small proto- 
xylem-elements lying on the innermost side of the bundle. 
The phloem is largely developed, the elements being much 
more numerous than those of the xylem. Medullary rays 
and parenchyma are very abundant throughout the bundle, 
and add greatly to its significance. A distinct cambium is 
present, giving rise to secondary thickening. 
In some places a small group of stone-cells lies on the outer 
edge of the phloem. 
Stomata occur here and there on the epidermis. They are 
characterized, as seen in this section, by the guard-cells being 
raised above the surface of the stem, the subsidiary cells 
also being bent upward ; the former have usually dark 
contents. 
In longitudinal section the cells of the ground-tissue are 
seen to be compactly placed, and undergo sliding-growth, but 
to a very slight extent. 
The xylem-elements have oblique terminal walls, on which 
there is always present a round or oval perforation, occasionally 
two ; this shows them to be vessels (Fig. 23). They are reti- 
culately thickened and run irregularly down in more or less 
isolated chains. The protoxylem-elements are much elongated 
and narrow ; the innermost have a very loose reticulate or 
spiral thickening. 
The sieve-tubes are short elements, with transversely-placed 
terminal walls, on which are apparently several plates. 
The companion-cells are usually of the same length as the 
sieve-tubes; they are either extremely narrow or broader 
(Fig. 24). 
