12.6 THE STRUCTURE OF ECONOMIC PLANTS 



of bundles entering the stem which penetrate the nodal zone hori- 

 zontally for various distances before they incline downward. In 

 addition to this, although many of the bundles of the internode 

 above pass directly through the node, a certain proportion of them 

 terminate at each node by anastomosing with other bundles. 



Evans (6) investigated the vascular anatomy of the node using 

 a technique involving staining with methylene blue and retting. 

 He found that the vascular bundles "seldom pass through more 

 than two or three nodes without branching"; and points out that 

 the nodal complex results from numerous small branches that arise 

 from the vascular bundles as they enter the node, and from the 

 small peripheral bundles. The division, subdivision, and anas- 

 tomosing of these bundles account for the vascular meshwork 

 which constitutes the nodal plate. 



The Stem Bundle. — The collateral or amphivasal bundle is en- 

 closed by a sheath of mechanical tissue, and is generally oriented 

 with the phloem toward the periphery of the stem. (Fig. 51.) 

 There are usually two protoxylem vessels in each bundle, one with 

 annular and the other with spiral wall thickenings. The two large 

 laterally placed metaxylem vessels are either narrowly reticulate or 

 pitted, the pits being of the simple unbordered type. Between the 

 two large metaxylem vessels is a connecting band of smaller pitted 

 tracheids. As the bundle matures, the parenchyma surrounding 

 the two protoxylem elements becomes separated from them, form- 

 ing an air space or lacuna; and at this time the unthickened portion 

 of the wall of the annular vessel is frequently crushed or destroyed. 

 The phloem, which lies in a collateral position with respect to the 

 xylem, consists of a small number of protophloem cells and meta- 

 phloem in which the sieve tubes and companion cells are very 

 regularly arranged. As the bundle matures, the protophloem 

 becomes non-functional and the crushed cells form a narrow band 

 just outside of the metaphloem. The bundle is of the closed type 

 and all the cells of the provascular strand become mature primary 

 vascular elements without the formation of a cambium. 



Vascular Anatomy of Stem. — All the bundles of the stem are 

 common and their downward course is nearly vertical, although 

 there may be some lateral and tangential curvature. The course 

 of the bundles has been described by de Bary (x) following the 

 classic account of von Mohl. The bundles of the leaf trace enter 

 the nodal plate from the base of the leaf and pass downward 



