Tree growth rings. Polarised light micrograph of a cross-section through a woody stem, showing a portion of an annual ring of xylem tissue with periods of seasonal growth. The wood that formed in the spring has large xylem vessels (purple circular areas) followed (above) by summer and autumn wood made up of smaller vessels and bands of supporting fibres (yellow-brown). The large radiating columns of cells are the medullary rays (red-brown), which transport water to the outer tissues of this woody stem.