Oxalic acid and xylose crystals. Cross-polarised light micrograph of grown crystals of oxalic acid with xylose. Oxalic acid occurs naturally and is present in larger quantities in parsley, sorrel and rhubarb leaf. It is used in rust removal compounds, for bleaching wood and mixed with sugar syrup for treating Varroa mite in honey bees. Xylose is a sugar first found in wood. It is the main building block of hemicellulose xylan, which forms around 30 percent of birch wood, though less in most other plants. Magnification: x36 when printed 10 centimetres wide.