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Sycamore leaf vein. Light micrograph of a section through the midrib (vein) of a leaf from a sycamore (Acer pseudoplatanus) tree. In the centre of the midrib is a ring of seventeen vascular bundles: xylem (red) and phloem (deep blue) vessels. Around the vascular ring is supporting sclerenchyma tissue (deep red) and surrounding that is parenchyma (light blue). Under the two epidermis's (outer layers) is a flexible supporting layer of collenchyma (red). The leaf lamina is the typical structure of a mesophytic dicotyledon with one layer of palisade mesophyll (deep red) on top of the lower layer of spongy mesophyll (red-blue). Magnification: x37 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)