Medullary Rays of Maple
“Longitudinal radial section through the wood of a branch of maple one year old: P, pith; B, bark.” -Century, 1889 In botany, Medullary rays refer to a characteristic found in woods. 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.
The Century Dictionary and Cyclopedia (New York, NY: The Century Co., 1889)