STEMS 103 



136. Path of sap in large trees. — Each year a 

 new layer of wood is added to the trunk of a tree, 

 thus increasing its thickness by the small part of an 

 inch. If the previous experiment might be repeated 

 with a large tree, it would be found that only the 

 outer layers of wood in the trunk would be colored, 

 showing that it is through these only that the sap 

 ascends. 



137. Girdling. — An interesting fact in this con- 

 nection is the behavior of a tree when girdled. 

 ■Girdling is usually done by removing the bark, 

 the living layer, and some of the outer wood of the 

 trunk, and is done for the purpose of killing the 

 tree. In most species the removal of the outer 

 layer of wood cuts ofE the conduits which carry the 

 water to the leaves, and if it is done iu the spring, 

 the tree generally dies the following summer. Some 

 are able to send water up through the inner layers, 

 and not only live that year but the following year 

 also. A few instances have been found in which the 

 tree has survived the operation many years. 



138. Downward path of material from the leaf. — 

 Trees which live through one summer but die at the 

 beginning of the next, are injured by starvation of 



