Paleo Slide Set: Tree Rings: Ancient Chronicles of Environmental Change
Green ash (Fraxinus pennsylvanica) ring structure, including clearly visible vessels.

The main element in the rings of angiosperms is the vessel. Vessels are similar 
in function to tracheids in gymnosperms. Annual rings can be discerned in angiosperms 
through the decrease in vessel size in ring-porous species (such as oak) or through 
the change in fiber characteristics in diffuse-porous species (such as beech).

Photo Credits:
Rex Adams
Laboratory of Tree-Ring Research, The University of Arizona,Tucson, AZ