i] INTRODUCTORY 11 



information that these substances have a general distribution in nature, 

 and had attracted the notice of chemists on account of their value 

 as vegetable dyes. Some flavones have been found in many species, 

 others in but one, or a few, though this appearance of restriction may 

 be due to lack of information. All members of the group are yellow 

 substances differing slightly from each other in their chemical composi- 

 tion. In the plant they frequently occur as glucosides, in which 

 condition their colour is much paler, and their solubility greater, than 

 in the free state. All give intense yellow or orange-yellow coloration 

 with alkalies, and green or brown coloration with iron salts. This 

 information led the author to conclude that the widely distributed 

 substances, occurring in all plant tissues which give the yellow reaction 

 to ammonia vapour, are flavones, and that the pigment of the ivory 

 variety of Antirrhinum is of the same nature. The white variety of 

 Antirrhinum is without the flavone, and must contain some substance 

 capable of action on the flavone to form anthocyanin. 



At about this period a series of papers were published by Palladin 

 (203, 210) setting forth what is known as the theory of 'Atmungs- 

 pigmente.' Broadly speaking, Palladin supposes plants to contain 

 chromogens which are capable of being oxidised by enzymes to pigments 

 which in turn pass on oxygen to respirable substances in the plant. 

 Among the ' Atmungspigmente ' Palladin includes anthocyanin. 



Palladin's conception, together with the evidence provided by 

 Antirrhinum, led the author (212, 217, 226) to bring forward the hypo- 

 thesis that anthocyanin is formed from a flavone by the action of an 

 oxidising enzyme or oxidase. Thus the formation of magenta pigment 

 in Antirrhinum would be brought about by the action of an oxidase 

 produced by the white variety upon the flavone present in the ivory. 

 Likewise in every plant anthocyanin would be formed by the action 

 of an oxidase upon a flavone, and just as the flavones are a class of 

 substances having similar properties, though showing some variety in 

 structure, the anthocyanins also would have properties in common as 

 a class but would, many of them, differ somewhat in chemical structure. 



The oxidase hypothesis of anthocyanin formation has been taken up 

 by Keeble & Armstrong (230, 231). These authors invented a micro- 

 chemical method for testing for oxidases in plant tissues, and applied 

 the method chiefly to colour varieties of the Chinese Primrose (Primula 

 sinensis), a plant largely used for Mendelian experiments. By means 

 of their test they discovered that oxidases in Primula are chiefly 

 confined to the epidermis and the bundle sheaths, both regions where 



