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 
AY 
p C x 
Fie. 52.—Cross-section of vascular bundle from pine stem, showing xylem (x), 
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 
