Woods and Their Properties 



45 



the summerwood denser than the wood formed in the 

 spring. 



Name kinds of wood in which the annual rings are very 

 noticeable ; other kinds in which they are not easily seen. 



Heartwood and sapwood. In a young tree the water 

 is conducted up to the leaves through the tracheae and 

 tracheids of the wood. As the tree grows older and 

 larger, the newer branches and leaves form their con- 

 nections with the outside layers of the wood and gradually 

 the vessels in the heart of the tree cease to carry water. 

 Often they become filled with resin, gum, coloring ma- 

 terials, or even mineral matter. In a large tree, there- 

 fore, only a circle of 

 wood around the out- 

 side is carrying water; 

 by far the greater part 

 of the wood is of no use 

 to the tree except to 

 support it. The pith 

 and the cells of the 

 medullary rays also die 

 in the old wood, so that 

 even in a sound tree of 

 large size it is only the 

 outer part that contains 

 cells that are alive. 



In a tree the wood 

 that is no longer con- 

 ducting water is called 

 heartwood. That which _ . 



FIG. 28. Wood cut from white birch trees, 

 Carries Water is Called showing heartwood and sapwood. 



