74 STRUCTURAL BOTANY 



Consequently we should have to look for the first- 

 formed spiral vessels at the extreme ends, as seen in 

 a transverse section, of the xylem-plate of the root. 

 The phloem develops from without inwards, just as in 

 the stem, so we see that, in the root, wood and bast 

 are both formed in the same direction, and not in 

 opposite directions, as in the stem. We find that in 

 this root there is no pith, for the wood extends quite 

 to the centre. This is characteristic of very many 

 roots, though not of all. There are a few parenchy- 

 matous cells between xylem and phloem ; these cells 

 form the conjunctive tissue of the central cylinder. 



A root with the structure just described is called 

 diarch t because there are, to begin with, two groups of 

 xylem and two of phloem, the two xylem groups soon 

 uniting in the middle to form the continuous plate. 

 When there are three groups of each, the root is 

 called triarch ; when four, tetrarch ; and so on. 



We see that the arrangement of the xylem and 

 phloem is quite different in the root and in the stem. 

 While in the latter the xylem always lies just inside 

 the phloem, the two together forming a collateral 

 bundle, in the former the bundles of xylem and phloem 

 are distinct, and alternate with each other all round 

 the vascular cylinder. 



The arrangement in the root may be called radial, 

 because the xylem and phloem groups are placed on 

 alternate radii of the central cylinder, which, as seen 

 in transverse section, is almost circular. The chief 

 points, then, in which the vascular system cf the root 

 differs from that of the stem are 



