33.] 



CHAPTER II. THE TISSUES. 



175 



bast-bundle on the inner (medullary) side of the wood of the con- 

 joint bundle ; such a bundle is distinguished as bicollateral. 



In a concentric bundle, either the bast is surrounded by the 

 wood, or the wood by the bast, more or less completely : the 

 bicollateral bundle is, in fact, a structure intermediate between 

 the collateral and the concentric bundle. The former type of con- 

 centric bundle occurs in the rhizomes of various Monocotyledons 

 (Acorus, Iris, Cyperus, Carex, etc.), and in the medullary bundles 

 of the stem of some Dicotyledons (Rheum, Statice, Ricinus, Piper, 

 etc.). The latter type is rare in Phanerogams (e.g. the cortical 

 and medullary bundles of the MelastomacesD) ; but it prevails in 

 the Filicinse and in 

 Selaginella,, when 

 the gamodesmic 

 bundles (two or 

 more) of each stele 

 of the polystelic 

 stem, form a cen- 

 tral mass of wood 

 completely, or 

 nearly completely, 

 surrounded by a 

 ring of bast. 



The relative posi- 

 tion of the phloem- 

 bundles and xylem- 

 bundles when they 

 are distinct 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. 136). This 

 arrangement occurs only in monostelic members ; it is common to 

 all roots, and occurs in the stem of Lycopodium and Psilotum 

 though in a less regular manner than in roots. It is commonly 

 termed the radial arrangement. 



The Differentiation of the Primary Vascular Bundle. The first 

 indication of the development of vascular tissue in the plerome 

 is the differentiation of one or more strands of narrow elongated 



t' 



FIG. 131. Transverse section of a concentric bundle, 

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

 tracheae ; t' protoxylem ; s sieve-tubes ; g companion-cells, 

 of the internal bast. 



