DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Rhubarb leaf. Polarised light micrograph of a longitudinal section through a rhubarb (Rheum sp.) leaf. The dark blue structure is a vascular bundle, which contains cells that transport water and carbohydrates around the plant. Within the parenchyma tissue (light blue) are clusters of crystals of calcium oxalate, which is poisonous if consumed in large quantities. Magnification: x100 when printed at 10 centimetres wide.
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