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medullary ray (ray) Any of the vertical plates of parenchyma cells running radially through the cylinder of vascular tissue in the stems and roots of plants. Each may be one to many cells in width. Primary medullary rays occur in young plants and in those not showing secondary thickening; they pass from the cortex through to the pith. Secondary medullary rays are produced by the vascular cambium and terminate in xylem and phloem tissues. Medullary rays store and transport food materials.