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Mention of health at the end of the entry on rhubarb brings to mind purgative powers, plus questions about possible health risks if a lot of rhubarb is eaten. In fact the purgative principle in rhubarb is a group of substances allied to chrysophanic acid and is present mainly in the root. The stalks contain oxalic acid, which is harmful if eaten to excess, but the amounts are no greater than those in spinach and chard, for example. Rhubarb leaves contain much more, and are not edible.