110 



PRACTICAL COURSE EST BOTANY 



have any difficulty in distinguishing the parts. In a year-old 

 shoot (Fig. 125), the structural regions correspond closely to 

 those shown in Fig. 119, except that the ring of fibrovascular 

 bundles is here compact and woody, and crossed by the 

 radiating lines of the medullary rays. In a three-year-old 

 shoot (Fig. 126), the main divisions are the same, but the 

 soft parenchyma of the central cylinder is replaced by the 

 pith, and the vascular ring is composed of three layers corre- 

 sponding to the three years of growth. In general, mature 



125 126 



Figs. 125, 126. — Cross sections of twigs : 125, section across a young twig of box 

 elder, showing the four stem regions : e, epidermis, represented by the heavy bounding 

 line ; c, cortex ; w, vascular cylinder ; p, pith ; 126, section across a twig of box elder 

 three years old, showing three annual growth rings, in the vascular cylinder. The 

 radiating lines (m), which cross the vascular region (w), represent the pith rays, the 

 principal ones extending from the pith to the cortex (c). (From Coulter's " Plant 

 Relations.") 



dicotyl stems may be said to include four well-defined re- 

 gions: (1) the epidermis, or the bark; (2) the cortex, made 

 up of bast and certain other tis.sues; (3) the cambium; 

 (4) the woody vascular cylinder, made up of concentric 

 rings, each representing a year's growth. The pith, or me- 

 dulla, constitutes a fifth region, but is obvious only in young 

 stems. Notice the little pores or cavities that dot the woody 

 part in the cross section ; where are they largest and most 

 abundant ? How are the rings marked off from one another ? 



