166 Our Field and Forest Trees 



Though the loss of heartwood is not a fatal 

 misfortune, the loss of sapwood Is disaster indeed. 

 An oak, or other broad-leaved tree, deeply girdled, 

 is marked for slow but sure death. 



The trunk of any growing tree is a tube of sap- 

 wood around a cylinder of dead heartwood. Every 

 year a little of the sapwood on the inside of the 

 tube turns to heartwood, and retires from active 

 life. 



Meantime, on the outside of the girdled tree, 

 where all the inner bark has been cut away, the 

 tubes and vessels of the sapwood are being dried 

 and shriveled by sun and wind, so that the fluids 

 can no longer move through them. When the 

 death, by old age, traveling outward meets the 

 death by parching, traveling inward, there is a 

 cylinder of dead wood in the trunk. Fluids 

 cannot pass this dead cylinder at all either way, 

 and so the whole tree dies of hunger and thirst. 



Though heartwood is dead, it will not molder 

 away unless it is attacked by insect or fungus 

 enemies. The sapwood covering it everywhere 

 is its armor of defense. Wounds made in this 

 live part of the tree are soon covered with gum 

 or resin — a sort of surgical plaster, which pro- 

 tects the injured place till nature heals it. But 

 any cut or wound, however small, if it goes deep, 

 may open a way for some fungus enemy to enter 

 in and devour. Heartwood has no vitality, and 

 cannot defend itself against its foes. 



