The Inner Structure of the Plantlet. 



55 



72. The Vital Part of Woody Stems in plants having 

 more than om k cotyledon (40) is limited to a rather thin 



layer of bark and wood, of 

 which the cambium (69) forms 

 the center. The cells of the 

 so-called heart- wood and those 

 of the dry and furrowed outer 

 bark, have lost their proto- 

 plasm, and hence are no longer 

 alive, though they serve a 

 useful purpose in adding 

 strength and protection to the 

 vital layer. The heart- wood 

 of a tree may largely decay 

 without materially interfering 

 with the vital processes (Fig. 

 23). 



73. The Healing of Wounds. 

 Cambium cells exposed to the 

 air by partial or complete re- 

 moval of the bark, soon per- 

 ish, as a rule, hence growth 

 ceases in a part of the stem 

 thus injured. The uninjured 

 cambium cells on the borders 

 of the wound may, however, 

 FIG. ^Tluve~pop)iar tree with by division (15), form a cush- 

 hoiiow trunk, showing to what j on o f new material that grad- 



extent the heart-wood may decay 



without destroying the life of a ually extends over the injured 

 tree - part. A new cambium layer 



may thus be formed over the wound if it be not too 

 large, so that growth of the stem may be resumed at this 

 place. The same process occurs when a branch is cut off 



