332 PIGMENTS 



generally having a deeper yellow colour than the glucoside. 

 In the plant their concentration may be insufficient to effect 

 the colour materially ; thus they are commonly found in white 

 petals and their presence is only revealed by exposure of the 

 petals to ammonia fumes whereby a yellow colour is developed. 

 This tendency to form yellow salts with ammonia or alkalis 

 also reveals itself in the formation of deeply coloured salts 

 with other metals ; for this reason many anthoxanthins were 

 in the past used extensively as cotton dyes in conjunction 

 with suitable mordants ; owing to the fact that the salt for- 

 mation is associated with the phenolic groupings, the sugar- 

 free compounds usually dye more deeply than the glucosides. 

 The anthoxanthins are widely distributed amongst the 

 higher plants ; they are most abundant in plants which grow 

 under conditions of high insolation, unless there be a protec- 

 tion in the form of hairs or thick cuticle. For this reason 

 they are looked upon as affording a protection against the 

 light rays of shorter length. There is sometimes an inter- 

 change between the anthoxanthins and anthocyanins, thus 

 young plants often contain red anthocyanin, which gives place 

 to a colourless flavone in the mature stage ; at leaf-fall the 

 anthocyanin may reappear.* 



YELLOW COLOURING MATTERS DERIVED FROM FLAVONE. 



The mother substances from which all these compounds 

 are derived and from which they derive their name are the 

 two compounds Flavone and Xanthone, both of which con- 

 tain the pyrone nucleus (see p. 336) — 



CH O CH— CH 



CH C C— C CH 



I II II \ / 



CH C CH CH=CH 



\ /\/ 



CH CO CH CO CH 



Flavone Xanthone 



There are a considerable number of yellow substances 

 occurring in plants derived from flavone, but only a few re- 



* Shibata and Nagai : " Bot. Mag. Tokio," 1916, 30, 149. 



