10 INTRODUCTORY [CH. 



was the phenomenon of the production of a plant bearing purple flowers 

 (in the Sweet Pea the original type) by crossing two white-flowered 

 strains. From such results the hypothesis arose, that red colour 

 (anthocyanin) is due to the presence of two factors, C and R, in the 

 plant, the loss of either factor resulting in an albino. A third factor, 

 B, by its presence modifies the red colour to a blue or purple, but 

 unless C and R are also both present in the plant, B is without effect. 

 Hence white plants of known parentage can be selected which are 

 carrying C, R, or B only, or CB, or RB. 



Such a discovery is not only highly important in its connection 

 with heredity, but it also provides us with well-defined material for 

 the solution of the problem as to what chemical processes are involved 

 in anthocyanin formation. Conversely, these processes once discovered, 

 we should also be provided with a chemical interpretation of the 

 Mendelian factors for flower-colour. 



The problem was first attacked by the author in 1909. The plant 

 selected for investigation was Antirrhinum, majus, since the inheritance 

 of the flower-colour of this species had already been worked out (514, 

 535) during the years 1903-9, and confirmed independently by Baur 

 (536). Antirrhinum majus presents a case of singular interest. In 

 this species (as in Matthiola and Lathyrus) two white varieties, or more 

 strictly two varieties which are albinos as regards anthocyanin, when 

 crossed together, produce the magenta pigment (anthocyanin) character- 

 istic of the type; but, whereas in Lathyrus and Matthiola the two 

 whites producing a purple are identical in appearance, the two albinos 

 in Antirrhinum can be distinguished at sight. One, which is known 

 as ivory, is ivory-white in colour, the other, the true white, is dead 

 white. These two varieties must obviously, between them, contain 

 the chemical substances, or the power to produce the chemical sub- 

 stances, essential to the formation of anthocyanin. 



The first clue to the solution was provided by the difference in the 

 action of ammonia vapour on the two flowers. The ivory exposed to 

 ammonia turns bright yellow whereas the white is practically unaltered. 

 It has been mentioned previously that Filhol (125), and in fact several 

 of the earlier workers on plant pigments, had noted the occurrence of 

 colourless substances in the plant which gave a bright yellow reaction 

 with alkalies. Moreover, a tentative suggestion was made by Bidgood (18) 

 some years ago (1905) in the Journal of the Royal Horticultural Society, 

 that these substances were the colouring matters known as flavones. 

 Reference to the work of A. G. Perkin on the flavones provided the 



