236 



ELEMENTARY STUDIES IN BOTANY 



If an active twig of an ordinary woody plant be cut across, 

 it will be seen that it is made up of three general regions 



(Fig. 204): (1) a zone of 

 spongy tissue, usually green, 

 the cortex, and covered by the 



FIG. 205. Cross-section of 

 the stem of a buttercup 

 (an herb) , showing the well- 

 separated bundles that form 

 the vascular cylinder, leav- 

 ing broad pith rays ; observe 

 also the very large pith, the 

 cortex, the hairs arising from 

 the epidermis, and espe- 

 cially the cambium cells be- 

 tween the xylem and phloem 

 and continuing across the 

 pith rays. 



FIG. 204. Cross-section of a branch of 

 box-elder one year old : c, cortex ; w, 

 vascular cylinder ; p, pith. ' 



epidermis ; (2) a relatively broad zone of firm wood, the 

 vascular cylinder; and (3) in the center the pith. The 

 special feature of this arrangement is that the wood oc- 



FIG. 206. 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. 



curs as a hollow cylinder, inclosing the pith and sur- 

 rounded by the cortex. In the older parts of stems the 

 pith often disappears, leaving a hollow stem. The cortex 



