Ring-porous domestic hardwoodsClick on any image for a larger view.
White oakThe pores of the white oak wood, other than the sap wood, end up having these membranes in them. This seals up the wood a little. It's also a sure-fire way of identifying white oak over red oak.
Red oakThis piece of red oak was very porous. But the distance between the rows or pores varies quite a lot. The rows of pores are the growth rings. The up-down lighter lines are "rays". All the hardwoods have rays, but they are most strongly evident in Oak.