vii] THE FORMATION OF ANTHOCYANINS 111 



and according to Witt the colour is due to the chromophore 

 group : 



o 

 /\ 



-c c- 



c 



c 



II 



o 



in combination with the auxochrome (hydroxyl), -OH, and the intensity 

 of coloration is said to depend on the position of the hydroxyl groups. 



The flavones, as a class, are yellow crystalline substances with high 

 melting points (see accompanying table). They give either a deeper 

 yellow, or an orange-yellow, coloration with alkalies, correspondingly 

 coloured precipitates with lead acetate, and a green or brown coloration 

 with iron salts. That their distribution in plants is practically universal 

 can be readily demonstrated by the colour reaction with alkalies. 

 This reaction is best shown by colourless parts of plants, such as white 

 flowers. Placed in ammonia vapour, almost any white flower will turn 

 bright yellow (with the exception of certain albinos of Antirrhinum and 

 Phlox Drummondii, see pp. 158, 209). The same yellow reaction is given 

 by green organs, though it can only be detected microscopically on 

 account of the presence of chlorophyll. On reference to the accom- 

 panying table, it will be seen that some flavones occur in genera of many 

 natural orders, while others are limited to one or a few ; this apparent 

 limitation is probably only due to lack of knowledge of their occurrence 

 in a number of plants. Further investigation will no doubt show a 

 very much wider field of distribution for all the flavones. 



There is little doubt that the flavones, as well as many other aromatic 

 substances, are synthesised in the leaves from sugar. The actual 

 steps of the process would be very difficult to demonstrate. On fusion 

 with alkalies, the flavones split, as a rule, into phloroglucin and an 

 oxybenzoic acid. Conversely, they are probably synthesised from oxy- 

 benzoic acids, or their derivatives, and phloroglucin. As we know, 

 somewhere in the plant and at some stage of plant metabolism, some 

 aromatic nucleus must be synthesised, that is an aromatic substance such 

 as phloroglucin must be synthesised from an aliphatic substance such 

 as glucose. Putting together what physiological and chemical evidence 



