STEMS 



51 



working region of the stem: since it is green it is able to 

 manufacture carbohydrates as do the leaves ( 14); and it 

 is also concerned in other work connected with nutri- 

 tion. The vascular cylinder, on the other hand, is the 

 great conducting region, as well as one that gives rigidity 

 to the stem. This work of conduction will be considered 

 later. 



If the vascular cylinder be examined closely, it will be 

 seen that it is broken up into segments by plates of cells 

 that traverse it from the pith to the cortex, these radiating 

 plates of cells being the pith rays (Fig. 51). The cylinder is 

 thus made up of a number of segments which are called 

 vascular bundles. The peculiarity of the structure of the 

 stem in Gymnosperms and Dicotyledons, therefore, can be 

 described as the arrangement of the vascular bundles so as 



FIG. 52. Cross-section of vascular bundle from pine stem, showing xylem (i), 

 cambium (c), and phloem (p) ; on each side of the single row of cambium 

 cells there are young xylem and phloem cells that pass gradually into the 

 mature condition. 



to form a hollow cylinder. In woody stems the bundles 

 are very close together in the cylinder, forming a compact 

 cylinder with narrow pith rays; but in the stems of herbs 

 the bundles are well separated, leaving broad pith rays. 



If the cross-section of an individual vascular bundle be 

 examined under the microscope, two regions will be 

 recognized (Fig. 52) : the inner one, toward the pith, being 

 called wood (xylem), and the outer one being called bast 



