DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Birch stem. Light micrograph of a transverse section through the woody stem of a birch (Betula alba) tree, showing a portion of the xylem including some of the pith (bottom left). The xylem vessels (large circular holes) in the wood are in clusters but uniformly scattered (diffuse porous wood). The rest of the wood is made up of tracheid cells. The diagonal lines are medullary rays, which transport water and minerals to the outside of the stem as well as storing food. The solid pith is composed of parenchyma cells. Magnification: x103 when printed 10 centimetres wide.
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