24 



A PRIMER OF FORESTRY. 



is easy to kill trees with heart wood by girdling them, 

 provided all the sap wood is cut through. But in those 

 which have no heartwood the tubes of the older layers 

 of wood can still convey water to the crown, and when 

 such trees are girdled it is often several years before 

 they die. 



A great many theories have been j)roposed to account 

 for the rise of water into the tops of tall trees, some of 

 which, as in the big trees of California, may be over 

 300 feet from the ground. But none of these theories 

 are quite satisfactory, and it must be admitted that we 

 do not yet know how the trees supply their lofty 

 crowns with the water which keeps them alive. 



