1 9 1 2 ] JONES— DI A N THERA 1 1 



At this stage, the oblique cross-arms are imbedded in the 



parenchymatous tissue, there b 



3f any sheath. After 

 fused bundles become 



more or less well marked 



the characteristic Casparian dots on the side walls of the cells. 

 Just above the node below, most of the vascular elements have 

 died out, there being left only three or four phloem elements. 

 These lose their surrounding sheath, and pass across to the periph- 

 eral vascular bundles at a level where the endodermis surround- 

 ing the forks has just broken. The medullary vascular tissue 

 becomes applied to the outer side of one of the forks, on the opposite 

 side of the stem, however, from which it branched off at the node 

 above. Soon after this the nodal ring becomes closed. If we 

 examine closely the upper node, we find that the vascular elements 

 which turn in to form the central bundle may be traced through 

 the node into the forks of the side bundles of the overlying internode. 

 At the next stage, the central bundle is differentiated in the 

 next internode above in the manner just described, the lower end 

 of this new bundle becoming inserted at the top of the older central 

 bundle between the incoming cross-arms. A cambium makes its 

 appearance in the central bundle and cross-arms about two stages 

 after the differentiation of these bundles. This cambium fre- 

 quently forms a more or less complete ring. Usually, however, in 

 the seedling stages, the primary xylem and phloem crowd over to 

 one side of the bundle, the cambium coming in as an arc, forming a 

 collateral bundle of exactly the same structure as the peripheral 

 bundle, in spite of its entirely different origin. 



The internal endodermis 



In the first five stages, the endodermis which surrounds the 

 bundles throughout the internodes disappears from the inner faces 

 of the bundles on reaching the nodes, leaving only an endodermal 

 sheath surrounding the vascular ring in the node. From the sixth 

 stage on, however, it is usual for the endodermis to be continuous 



the 



node as a type, we find (fig. 3), just abov 



sheath 



that every bundle is surrounded by a regular endodermal 



differentiated 



