In botany, medullary rays (pith rays or wood rays) are sheets or ribbons of cells running from the inside of the plant to the outside. That is, they run at right angles to the xylem and phloem, which run vertically. While the plant is alive, these medullary cells are alive. They carry nutrients (sap) towards the surface of the tree.
In wood products the rays can be seen running perpendicular to the growth rings. If wood is cut into boards with the growth rings roughly perpendicular to the face of the board, the medullary rays often produce beautiful "figure" such as silver grain, medullary spots, pith flecks, etc.
The function of medullary rays is to carry chemicals which fight invasion by insects and fungi, and block any damaged areas. The cells deposit tannins and resins which block up the damaged area. This process is called tylosis.