There are three well-defined types of the fibro-vascular bundle, namely, the =radial=, the =concentric=, and the =collateral= types. RADIAL VASCULAR BUNDLES The radial type of bundle is met with most frequently in monocotyledonous roots. In this form (Plate 114) the xylem forms radial bands of tissue which alternate with isolated groups of phloem. The space between the phloem and xylem is filled in with either parenchyma or fibres, or both. In some cases the vessels of the xylem meet in the centre of the root, while in other cases 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. [Illustration: PLATE 124 CROSS-SECTION OF A RADIAL VASCULAR BUNDLE OF SKUNK CABBAGE ROOT (_Symplocarpus fœtidus_ [L.], Nutt.) 1. Vessels. 2. Bundle sheath. 3. Parenchyma. 4. Sieve cells.] [Illustration: PLATE 125 CROSS-SECTION OF A PHLOEM-CENTRIC BUNDLE OF CALAMUS RHIZOME (_Acorus calamus_, L.) 1. Vessels. 2. Sieve cells. 3. Phloem parenchyma. 4. Parenchyma surrounding the bundles.] 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 peripheral, as in calamus rhizome. COLLATERAL VASCULAR BUNDLES There are three types of collateral vascular bundles--namely, closed collateral, bi-collateral, and open collateral.