In botany, Medullary rays refer to a characteristic found in woods

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

. In this context the term refers to radial sheets or ribbons extending vertically through the tree across and perpendicular to the growth rings. Also called pith rays or wood rays, these formations of primarily parenchyma cells allow the radial transmission of sap and are essential in the process of tylosis

Tylosis (botany)

Tylosis is the physiological process and the result of occlusion in the xylem of woody plants as response to injury or as protection from decay in heartwood....

. In quartersawn

Quartersawn

thumb|250px|quarter sawn surface, showing ray fleck In speaking of wood, the term quartermortersawn is properly used for wood that is cut exactly radially , or to put it differently, at right angles to the growth rings...

 material, where the wood is cut into boards with the growth rings roughly perpendicular to the face of the board, the medullary rays often produce beautiful figure

Figure (wood)

In wood, figure refers to the appearance of wood, as seen on a longitudinal surface : a "figured wood" is not plain.The figure of a particular piece of wood is, in part, due to its grain and, in part, due to the cut, or to innate properties of the wood...

such as silver grain, medullary spots, pith flecks, etc.