Medullary Rays
Medullary rays are ribbons of cells that radiate horizontally from the pith to the bark, transporting and storing nutrients across the tree trunk. When exposed by quarter-sawing, they create the distinctive fleck or ray fleck pattern prized in species like white oak, sycamore, and beech. Their visibility depends on species and sawing angle.
In-Depth Explanation
Medullary rays (also called wood rays) serve as the tree's horizontal transport and storage system, moving sugars and minerals between the bark and the inner wood. In most species the rays are too small to see with the naked eye, but in some — notably white oak, sycamore (lacewood), beech, and London plane — the rays are large enough to create dramatic visual features. When these species are quarter-sawn (cutting the log so the growth rings are roughly perpendicular to the board face), the rays are sliced along their length, exposing their full width as shimmering, reflective flecks that can be several inches long. This ray fleck is one of the defining characteristics of quarter-sawn white oak and is essential to the authentic look of Mission, Craftsman, and Arts and Crafts furniture. The rays also influence wood movement — quarter-sawn boards are more dimensionally stable partly because the rays help restrain tangential movement.
Usage Example
The craftsman chose quarter-sawn white oak specifically for the prominent medullary ray fleck that would give the Craftsman-style bookcase its authentic character.