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Beech tree stem. Polarised light micrograph of a section through the woody stem of a common beech tree (Fagus sylvatica), showing diffuse-porous xylem vessels (dark blue) scattered throughout the tissue. Some of the vessels show the vessel walls, which have a network of bordered pits, or scalliform, which let water sideways if there is a blockage in the vessel. The multicoloured cells are fibres, and fibre tracheids, which carry water. The horizontal files are the medullary rays, which carry water, minerals and food to the outer and inner tissues. Magnification: x7, when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)