33] 



CHAPTER II. THE TISSUES. 



125 



The relative 

 position of the 

 plilo em-bundles 

 and xylem-bundles 

 tchen they are dis- 

 tinct from each 

 other is such that 

 they alternate with 

 each other so that 

 a radius drawn 

 from the centre to 

 the surface of the 

 member cuts 

 through either a 

 phloem or a xylem- 

 bundle, but not 

 through both (Fig. 

 102). This ar- 

 rangement obtains 

 only in mono- 

 stelic mem- 

 bers ; it is 

 common to all 

 roots, and oc- 

 curs in the 

 stem of Lyco- 

 podium and 

 Psilotum 

 though in a 

 less regular 

 manner than 

 in roots. It 

 is commonly 

 termed the 

 radial a r- 

 rangement. 



The Differ- 

 entiation of 

 the Primary 

 Vascular Bun- 

 dle. The first 



Fia. 101. Transverse section of a concentric bundle, with 

 external wood, from the rhizome of Iris (x 350) : t trachea; 

 V protoxylem ; s sieve-tubes ; g companion-cells, of the in- 

 ternal bast.] 



vi 



FIG. 102. Part of a transverse section of the stele of the Sar- 

 saparilla-root (Smilax) : r cortex; ed endodermis with passage- 

 cells d ; the pericycle and the interfascicular conjunctive tissue o 

 are sclerenchymatous ; v' the pith ; * the protoxylem, and t a pitted 

 vessel of a wood-bundle: s a bast -bundle. The alternation, or 

 radial arrangement, of the wood and bast-bundle is shown. ( x 300.) 



