OUR COMMON WOODS 



219 



Very young trees are all sapwood, but, as they get older, 

 part of the wood is no longer needed to carry sap and it 

 becomes heartwood. Heartwood is darker than the sap- 

 wood, sometimes only slightly, but in other instances it 

 may vary from a light-brown color to jet black. It tends to 



cambium layer 

 sapwood 



medullary rays 

 Fig. 145. — Cross-section of Oak. 



fill with gums, resins, pigments and other substances, but 

 otherwise its structure is the same as that of the sapwood. 

 The wood of all our common trees is produced by a thin 

 layer of cells just beneath the bark, the cambium. The 

 cambium adds new wood on the outside of that previously 

 formed and new bark on the inside of the old bark. A 

 tree grows most rapidly in the spring, and the wood formed 



