56 Principles of Plant Culture. 



wood of a tree may largely decay without materially 

 interfering with the vital processes (Fig. 23). 

 72. The Healing of Wounds. Cambium cells ex- 

 posed to the air by partial 

 or complete removal of tlie 

 bark, soon perish, as a rule, 

 hence growth ceases in 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 that 

 gradually extends over the 

 injured part. A new cam- 

 bium 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 near its 

 union with the stem. The 

 wound, if not too large, is 

 "healed" by new growth 

 Fig. 23. uve poplar tree from the adjacent, uninjured 



with hollow trunk, showing to 



what extent the heartwood may cambium Cclls (Fig. 24). 



decay without destroying the 



life of a tree. The youngcr the uninjured 



tissues are the more rapid is the healing. Ip planted 

 cuttings, the uninjured cambium cells at the base form 

 the callus (cal'-lus) by continued division (Fig. 25), 



