36 



THE VEGETATIVE ORGANS 



line (Fig. 16, 3, c), but as growth proceeds it becomes a 

 uniform ring (Fig. 15, 1, c), to the inside of which is the wood 

 and to the outside the bast. The root has now lost its 

 radial structure and grows in thickness by additions from 

 the cambium in the same manner as in the stem. Finally, 

 a layer of cork is formed from a ring of cambium, the cork 

 cambium (Fig. 15, r, c.c), which arises in the tissues imme- 

 diately to the outside of the bast. The cork layer thus 

 produced cuts off communication between the vascular 



Fig. 16. Diagrams illustrating the Development of a Root. 

 1, very young root, showing alternating groups of primary wood 

 (P.w) and primary bast (P.b) ; 2, older root, showing cambium (c), 

 arising on the outside of the wood and on the inside of the bast ; 

 3, old root, showing a complete cambium (c) ; co, cortex; e, epidermis; 

 s.w, secondary wood, formed on the inside of the cambium ; s.b, 

 secondary bast, formed on the outside of the cambium. 



bundles and the cortex, and the latter dies and crumbles 

 away, as shown in Fig. 15, 1, co. 



The loss of the cortex greatly reduces, for a time, the 

 diameter of the root, but growth goes on steadily and the 

 root continues to increase in thickness throughout life. 



Structure of a monocotyledonous root. The root of a Mono- 

 cotyledon differs from the above in an important respect : 

 no cambium is formed between the wood and the bast, 

 and when these tissues are once developed, no further 

 increase in thickness can take place. The bundles of alter- 

 nating wood and bast are also more numerous (often ten 



