 The leaves are believed to also contain an additional, unidentified toxin, which might be an anthraquinone glycoside (also known as senna glycosides).In the petioles (leaf stalks), the proportion of oxalic acid is much lower, only about a tenth of the total acidity of 2–2.5%, which consists mostly of malic acid.Serious cases of rhubarb poisoning are not well documented in modern medical publications.