DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Dicotyledon plant stem. Light micrograph of a section through the young stem of a water oak (Quercus nigra) tree, showing one-year secondary thickening. The epidermis has a thick cuticle (red) and is being shed. The outer cortex (dark brown) and the parenchyma tissue contain large air spaces. Below the inner cortex is the vascular ring and below this is a ring of phloem (grey and dark brown) with sieve tubes; next is a ring of cambium (pink). The wood under the cambium has a single ring of small radial vessels (light brown), and medullary rays (dark brown). The rest of the wood is made up of smaller vessel - tracheids and wood parenchyma. At the centre of the stem is the pith, consisting of very large parenchyma cells (brown) that store water.
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