DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Pine stem. Polarised light micrograph of a section through part of the stem of a pine (Pinus sp.) tree, showing the growth rings. From outer to inwards: outer layer of cork (brown); cortex - made up of parenchyma cells (red) with resin canals; ring of secondary phloem composed of sieve cells (blue), parenchyma and albuminous cells (red); vascular cambium cells (blue); xylem - with ten growth rings each made up of large thin-walled tracheid cells (light brown) of spring wood, and smaller thick-walled tracheid cells (dark brown) of autumn wood; thin files of parenchyma and tracheid cells (brown). The xylem includes large circular resin canals (lacunae, green) Magnification: x13 when printed 10 centimetres wide.
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