48 



PRINCIPLES OF PLANT CULTURE 



cause, and is renewed the same season, an extra ring 

 is formed. 



71. The living part of 

 woody stems in plants hav- 

 ing more than one cotyledon, 

 e.g., apple or maple (45), is 

 usually limited to a rather 

 thin layer of bark and wood, 

 of which the cambium (68) 

 forms the center. The cells 

 of the so-called heartwood 

 and those of the dry and 

 furrowed outer bark have lost 

 their protoplasm, and hence 

 are no longer alive, though 

 they serve a useful purpose in 

 adding strength and protec- 

 tion to the vital layer. The 

 heartwood of a tree may 

 largely decay without mate- 

 rially interfering with the 

 vital processes (Fig. 23). 



72. The healing of 

 wounds. Cambium cells 

 exposed to the air by par- 

 tial or complete removal of 

 the bark soon perish, as a 

 rule, hence growth ceases in 



FIG. 23 Live poplar tree with 

 hollow trunk, showing to what 

 extent the heartwood may de- 

 cay without destroying the 

 life of a tree. 



a part of the stem thus in- 

 jured. The uninjured cam- 

 bium cells on the borders of 

 the wound may, however, by division (15), form a cushion 

 of new material (callus) that gradually extends over the 



