110 PRACTICAL COURSE IN 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 
Fies. 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; e¢, 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 CouLTErR’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 tissues; (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? 
Pa 
