Cumarin itself is widely distributed in plants. No glucoside containing cumarin as such has yet been isolated; but several glucosides of its oxy-derivatives are known. The following are common ones: =Skimmin=, C_{15}H_{16}O_{8} (glucose and skimmetin), is found in _Skimmia japonica_; =æsculin=, C_{15}H_{16}O_{9} (glucose and æsculetin), is found in the bark of the horse-chestnut, _Æsculus hippocastanum_, and its isomer, =daphnin= (glucose and daphnetin), in several species of _Daphne_; and fraxin, C_{16}H_{18}O_{10} (glucose and fraxetin), is found in the bark of several species of ash. The structural arrangement of the oxy-cumarin groups which are found in these glucosides is shown in the following formulas. It is not known to which OH group the sugar is attached, in each case. Skimmetin Æsculetin CH=CH·CO CH=CH·CO /\ | /\ | / \_O_| / \_O_| | | | | | | HO| | \ / \ / \/ \/ OH OH Daphnetin Fraxetin CH=CH·CO CH=CH·CO /\ | /\ | / \_O_| / \_O_| | | HO| | | |OH HO| | \ / \ / \/ \/ OH OCH_{3} =Scopolin=, C_{22}H_{28}O_{14}, found in _Scopolia japonica_, contains two glucose molecules united to a monomethyl ether of æsculin; while =limettin=, found in certain citrus trees, is the dimethyl ether of æsculin. THE PIGMENT GLUCOSIDES Many, if not all, of the red, yellow, violet, and blue pigments of plants either exist as, or are derived from, glucosides. These are of three types: the madder, or alizarin, reds are derivatives of various oxy-anthraquinones; most of the soluble yellow pigments are glucosides derived from flavones or xanthones; and the soluble red, blue, and violet pigments of the cell-sap of plants are mostly anthocyan derivatives. The four basic groups, or nuclei, which are present in these different types of compounds are complex groups consisting essentially of two benzene rings linked together through a third ring in which there are either two oxygen atoms in the ring, or one oxygen in the ring and a second attached to the opposite carbon in the (C=O) arrangement, as shown by the following diagrammatic formulas: Xanthone Anthraquinone