CHAPTER X 
ARRANGEMENT OF VASCULAR BUNDLES 
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- vacua 
bundle. 
‘The fibro-vascular bundle is made up of, first, the obit 
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 medul- 
lary 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 
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