THE STRUCTURE OP STEMS. 



229 



ence. You see it crossing successively all the bun- 

 dles situated below it, and older than itself, and 

 taking its place above them. 



Now, the differences between the interior and 

 exterior of the stem result chiefly from the changes 

 in composition that each bundle under- 

 goes in its course from its origin at the 

 leaf insertion to its ending in the cir- 



FIG. 402. 



FIG. 401. 



cumference far below. In the region 

 of the bundle which descends toward 

 the centre, its woody portion is- greatly 

 in excess over the cortical part, while 

 below this region, in the part of the 

 bundle which descends toward the periphery, the 

 cortical part of the bundle greatly predominates, and 

 finally forms almost the whole of it. On approach- 

 ing the outside, the bundle grows thin, and divides 

 into several filaments, resembling fibrous roots. These 

 filaments interlace with those of neighboring bundles, 



