Medullary Rays

Medullary rays are bands of cells that run radially from a tree's pith (center) outward toward the bark, perpendicular to the growth rings, carrying nutrients horizontally across the trunk. On a plain-sawn board they're barely visible, if at all -- but on a quarter-sawn board, where the cut runs roughly parallel to the rays, they're sliced open face-on and show up as the distinctive silvery fleck pattern associated with quarter-sawn white oak in particular.

Ray size and prominence vary a lot by species -- oak and sycamore have famously large, showy rays; maple and cherry have much finer ones that barely register even when quarter-sawn.