Medullary rays, also known as wood rays or simply rays, are cellular structures found in the wood of certain trees. They are radial structures that extend from the center of the tree toward the bark, perpendicular to the growth rings. These rays primarily serve to transport water, nutrients, and other substances horizontally across the tree trunk.
Medullary rays consist of cells that are elongated radially. They originate from the vascular cambium, which is a layer of meristematic tissue responsible for secondary growth in woody plants. As the tree grows, the cells of the medullary rays differentiate and mature, forming a network that connects the innermost parts of the tree with the outer layers.