ARRANGEMENT OF VASCULAR BUNDLES 295 



the centre of the stem is occupied by pith parench^-nia. 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, xyl em-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. 



