STEMS 237 



is the active working region of the stem ; since it is green 

 it is able to manufacture carbohydrates as do the leaves, 

 and it is also concerned in other work connected with nu- 

 trition. The vascular cylinder, on the other hand, js the 

 great conducting region, as well as one that gives rigidity 

 to the stem. 



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 (Fig. 204), these 

 radiating plates of cells being the pith rays. 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 

 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 (Fig. 204) ; but in the stems 

 of herbs the bundles are well separated, leaving broad pith 

 rays (Fig. 205). 



If the cross-section of an individual vascular bundle be 

 examined under the microscope, two regions will be recognized 

 (Fig. 206), the inner om k , toward the pith, being the wood 

 (xylem), and the outer one being the bast (phloem). A vas- 

 cular bundle, therefore, is made up of xylem and phloem, 

 which differ from one another in the work of conduction, 

 the xylem chiefly conducting the water that enters the plants 

 by the roots and is passing to the leaves, and the phloem 

 chiefly conducting prepared food. 



The cells of the xylem that conduct water are called 

 tracheary vessels. They are more or less elongated, and have 

 very thick walls, upon which there appear markings of 

 various kinds (Fig. 207). These markings may be seen in 

 a longitudinal section through the wood. Some of the 

 vessels are marked by a spiral band that extends from end 

 to end, and are called spiral vessels (Fig. 207, A) ; others 



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