342 GENERAL SCIENCE 



cut end of a tree limb or the end of a piece of board, we 

 shall find it full of very small holes. The soil water 

 taken in by the root hairs passes up through these holes. 

 These holes or tubes are not usually more than a few 

 inches long. The ends are separated from one another 

 by a very thin partition or membrane through which the 

 sap passes by osmosis. By osmosis the sap continues 

 to pass from tube to tube until it reaches the leaves at 

 the ends of the limbs or the top of the tree. By examin- 

 ing the cross-section of a large tree like the oak, we find 

 a white ring an inch or more in thickness just inside the 

 bark. This white wood is living and is called sap wood; 

 most of the sap flows up through this living part. The 

 brown wood inside the white ring is dead and is useful for 

 food storage and to support the tree. This brown part 

 is most valuable for lumber because it does not decay so 

 quickly as the sap wood. All first-class lumber has the 

 sap wood removed. 



The bark of a tree is dead on the outside and is con- 

 tinually falling off; much of it may be pulled or cut 

 from many kinds of trees without injury to the tree. 

 The dead part of the bark on a young tree or twig is 

 very thin and should not be removed. This outer bark 

 serves to protect the tree from insects and diseases. 

 The inner part of the bark next to the sap wood is living 

 and is called cambium, or the cambium layer. This cam- 

 bium layer forms bark on its outer side and new wood 

 on the inner side next to the sap wood. It makes this 

 wood and bark out of the digested food which flows down 

 from the leaves through the cambium and the bark next 

 to it. 



The outer bark of some trees is very useful to man. 

 The bark of oaks and of hemlock is used for tanning 



