MORPHOLOGY 



119 



independent existence in the stem. The bundles enclosed in the 

 partial central cylinder of the Pteridophytes are continued as cauline 

 bundles in the stem, and those from the partial cylinder (Schizostele) 

 of the leaves join on to the bundles of the stem. 



The stems of many Dicotyledons (Begonias, Aralias) in addition to leaf-traces 

 possess cauline bundles, which are situated in the pith within the ring of leaf- 



Fio. 133. — Diagram showing the 

 course of the vascular bundles 

 of Monocotyledons of the Palm 

 type, with alternating, two- 

 ranked amplexicaul leaves. The 

 numbers indicate the sequence 

 of the leaves ; m, median bundle. 

 (After DeBary.) 



CCD 



Fio. 134.— Diagram showing the 

 course of the vascular bundles 

 in a seedling of Tants 1)accata. 

 rr, Cotyledons ; | , course of the 

 bundles in the part of the stem 

 above the cotyledons; ';, xylem; 

 and :, phloem after their sepa- 

 ration. 



trace bundles of the internodes ; while the arrangement of the bundles at the 

 nodes is more complicated, as the cauline bundles then branch and are connected 

 ■with the leaf-traces. 



Within the central cylinder of roots, the xylem and phloem strands 

 pursue their vertical direction without deviation. If the changes 

 occurring in the arrangement of the vascular bundles, during their 

 passage from the hypocotyl (p. 45) into the root, be followed in a 

 seedling, it will be found that the xylem and phloem portions of 

 collateral bundles separate from one another, and at the same time 

 the xylem portions twist through an angle of 180°, so that their 



