Having familiarized ourselves with the different types of mechanical and conducting cells, we shall now consider the different ways in which these cells are associated to form the vascular and fibro-vascular bundles.
The simplest form of the vascular bundle occurs in petals, floral bracts, and leaves. In these parts the vascular bundle is made up of conducting cells only.
In the great majority of cases, however, the conducting cells are associated with mechanical cells to form the fibro-vascular bundle.
The fibro-vascular bundle is made up of, first, the phloem, which consists of sieve tubes, companion cells, bast fibres, and parenchyma; secondly, of the xylem, composed of vessels and tracheids, wood fibres and wood parenchyma; thirdly, of medullary rays (restricted to certain types); and fourthly, of the bundle sheath (restricted to certain types).
TYPES OF FIBRO-VASCULAR BUNDLES
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.
