ARRANGEMENT OF VASCULAR BUNDLES 295 
the centre of the stem is occupied by pith parenchyma. Each 
bundle is surrounded by parenchyma cells, and in iris, calamus, 
and veratrum, rhizomes, and endodermis, surrounds the bundles 
located in the centre of the stem, consisting of thin-walled 
(mechanical) cells. 
In sarsaparilla root, the pith is composed of thick-walled, 
porous pith parenchyma cells with starch. Outside the pith 
are arranged radial bands of oval vessels which decrease in size 
toward the periphery. Between the ends of these bands occur 
isolated groups of sieve cells. 
Surrounding the sieve cells and vessels are thick-walled, 
angled fibres. 
External to these cells is an endodermis composed of lignified 
brownish-colored cells one layer in thickness. 
CONCENTRIC VASCULAR BUNDLES 
There are two principal types of the concentric bundle, 
namely, xylem-centric, in which the xylem is centric and the 
phloem is peripheral, as in veratrum root; and phloem-centric 
(Plate 125), in which the phloem is centric and the xylem pe- 
ripheral, as in calamus rhizome. 
COLLATERAL VASCULAR BUNDLES 
There are three types of collateral vascular bundles—namely, 
closed collateral, bi-collateral, and open collateral. 
In the closed collateral bundle the phloem and xylem are 
not separated by a cambium layer, and in many cases the 
bundle is surrounded by thick, angled walled fibres, as in palm 
stem. The term closed bundle refers to the fact that there is 
no cambium between the xylem and phloem, therefore the 
bundle is ‘‘closed” to further growth, and not to the fact that 
it is frequently surrounded by fibres which prevent further 
growth. In podophyllum stem (Plate 126) the xylem portion 
of the bundle faces the centre of the stem and the phloem portion 
of the bundle faces the epidermis. The xylem and phloem are 
separated by a cambium layer, and both are surrounded by 
thick-walled angled fibres which are the chief mechanical cells 
of the stem. This bundle is, in fact, mechanically closed, but 
not physiologically because a cambium is present. 
