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 fibre-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 fibre-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 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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