DICOTYLEDONES 



407 



usually pass downward through several internodes before they unite 

 with the older ones. The arrangement of the bundles is often very 

 complicated, and is dependent upon the number of bundles in each 

 leaf-trace, and on the arrangement of the leaves upon the stem. 



The bundles are somewhat wedge-shaped in sections (Fig. 384), 

 the xylem consisting of rows of tracheae, with more or less paren- 

 chyma and fibrous tissue between. The tracheary elements are 

 largely made up of true vessels, which exhibit various forms of 

 thickenings upon their walls. 



A .1 



FIG. 386. Tilia Americana. A, cross-section of the cortex of a young twig, cutting 

 through a lenticel, 1; pd, the periderm. fi, outer cortex of an older branch; 

 e, epidermis; pd, periderm. C, cross-section of the phloem, showing medullary 

 rays, m, sieve-tubes, s, and bast-fibres, b (X 200). 



The phloem is composed of sieve-tubes, "companion cells," or 

 cambiform elements, and often groups of fibrous cells (bast-fibres). 

 Outside the ring of bundles is the common endodermis, marking 

 the inner boundary of the secondary growth. Even in herbaceous 

 Dicotyledons there is developed in the vascular bundles of the stem 

 a zone of cambium, which permits a greater or less amount of sec- 

 ondary growth, but it is in perennial woody stems that this is 

 best developed. The cambium, as in the bundles of the Conifers, 

 is composed of several layers of radially compressed cells, lying 

 between the xylem and phloem. The central cells of the cambium 

 zone divide actively by periclinal walls, and the cells thus formed 



