DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Pine stem. Polarised light micrograph of a section through a growth ring in the stem of a Scot's pine (Pinus sylvestris) tree, showing thick-walled tracheid cells (yellow, with purple-red tubes) of older autumn wood. There are vertical files of parenchyma and tracheid cells (brown) of the subsidiary medullary rays, which pass food substances radially. Many of the tracheid cell walls have bordered pits (circles, yellow) that pass water and minerals side-ways. Magnification: x32 when printed 10 centimetres wide.
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