no AROMATIC COMPOUNDS [ch. 



The Flavone and Flavonol Pigments. 

 These yellow colouring matters are very widely distributed in the 

 higher plants (Shibata, Nagai and Kishida, 22). They are derived from 

 the mother substances, flavone and flavonol, the latter only differing 

 from the former in having the hydrogen in the central 7-pyrone ring 

 substituted by hydroxyl : 



A.^°^P / \ /\/°N 



L CH K , 



COH 



Flavone Flavonol 



The naturally occurring pigments, however, have additional hydro- 

 gen atoms replaced by hydroxyl groups, that is they are hydroxy-flavones 

 and flavonols, and the various members differ among each other in the 

 number and position of these hydroxyl groups. Some of the members 

 are widely distributed, others less so. Quite often more than one repre- 

 sentative is present in a plant. 



The flavone and flavonol pigments are yellow crystalline substances, 

 and as members of a class they have similar properties. They occur in 

 the plant most frequently as-glucosides, one or more of the hydroxyl 

 groups being replaced by glucose, or, sometimes, by some other hexose, 

 or pentose. In the condition of glucosides, they are much less coloured 

 than in the free state, and, being present in the cell-sap in very dilute 

 solution, they do not produce any colour effect, especially in tissues con- 

 taining chlorophyll. Occasionally they give a yellow colour to tissues, 

 as in the rather rare case of some yellow flowers {Antirrhinum) where 

 colour is due to sqluble yellow pigment. 



In the glucosidal state, the flavone and flavonol pigments are, as a 

 rule, readily soluble in water and alcohol, but not in ether. In the non- 

 glucosidal state they are, as a rule, readily soluble in alcohol, somewhat 

 soluble in ether, but soluble with difficulty in water. 



The flavone and flavonol pigments can be easily detected in any 

 tissue by the fact that they give an intense yellow colour with alkalies 

 (Wheldale, 24). If plant tissues be held over ammonia vapour, they 

 turn bright yellow, showing the presence of flavone or flavonol pigments: 

 the colour disappears again on neutralization with acids. (The reaction 

 is especially well seen in tissues free from chlorophyll, such as white 

 flowers.) This reaction will be found to be almost universal, showing 

 how wide is their distribution. With iron salts, solutions of the pigments 



