1 68 STEMS 



vascular bundle. That cylinder of wood that you saw in 

 the one-year-old twig is made up of vascular bundles, but 

 they lie so very close together that the individual bundles 

 are not distinct. They are separated only by very thin 

 plates of pith-Kke cells which run between them out to the 

 cortex. In some stems, especially the stems of pine, these 

 pith-rays are quite prominent; they form radiating lines 

 which run through the wood from the center to the cortex ; 

 you can see them easily. (See Figure 58.) In other stems 

 the pith-rays disappear as the stem grows older. Pith- 

 rays are also called medullary rays {med'U-la-ri). 

 The structure of an individual bundle may be best un- 

 derstood by study of a stem whose 

 bundles are distinct, being separated 

 by wide pith-rays. (Study Figure jg.) 

 You note that the xylem part of the 

 bundles (the inner part) is larger than 

 the phloem. Do you see any relation 

 between this difference in the extent 

 of these two tissues and the difference 

 in the work which they do? The 

 phloem or bast lies outside the xylem 

 and is separated from it by a layer'of 

 cells which are narrowly oblong in 

 cross section. This layer of narrowly 

 ^ . , oblong cells is cambium (seepage 120). 



Fig. 59- — Section of a stem ° \ f 6< y/ 



whose vascular bundles This cambium has a very important 

 are distinctly separate, part to play in the Subsequent growth 



From bundle to bundle r .1 



runs a thin belt of tissue, *-'^ "-"^ Stem. 



thecambium. Within the As the young, bundles lie side by 



bundles the cambium -j . 1 ■ 1 ^ 1 • ii 



passes between the xylem ^ide, touchmg or nearly touchmg, the 

 and the phloem. layer of cambium in one bundle soon 



