Trees, Stars, and Birds 



or break them. Oak wood has in it dense bundles of 

 these fibers, and other hard and heavy woods also are 

 richly supplied with them. 



Annual rings 



Tracheae 



Pith 



Medullary rays 



FIG. 9. The structure of wood (diagrammatic). 



It should be understood also that in all woods many 

 of the cells do not change into vessels or fibers, but 

 retain their thin walls, and except in the older wood 

 remain alive. They are called parenchyma cells. The 



