M. Whbldale 135 



With regard to their chemical nature, I have previou8ly(19) given 

 reasons for the statement that the red, purple and blue pigments, 

 collectively termed anthocyanin, are oxidation products of chromogens 

 of an aromatic nature'. That, moreover, these chromogens, in the form 

 of glucosides, are present in solution in the cell-sap throughout the 

 living tissues and in the unoxidised state cause no colouration, but 

 under certain conditions through the agency of an oxidase the chromo- 

 gens may be oxidised to pigments, i.e. anthocyanin. This point of view 

 is in agieement with that held by other investigators: Wigand(21), 

 Pick(16), Mirande(9), Laborde(7), Overton(13) and Palladin(14), who 

 have considered the soluble pigments either to be themselves aromatic 

 compounds or to be intimately connected with tannins and allied 

 substances. 



That oxygen is necessary for pigment formation and that the 

 oxidation is probably brought about through the agency of an oxidase 

 has been suggested by Mirande(9), Palladin(14), and by Buscalioni and 

 Pollacci(l). 



Katic(5) and Molliard(ll) have also shown experimentally that 

 oxygen plays an important part in the appearance of pigment in 

 certain organs. 



So far, however, no hypothesis has been formulated as to the more 

 exact mechanism of pigment formation, the reasons for its appearance 

 only under certain conditions and for its localisation in definite organs 

 and parts of the plant. 



Wigand(21), it is true, has pointed out that the occurrence of antho- 

 cyanin in autumnal leaves, evergreen leaves in winter, injured or dying 

 leaves, flowers and ripening fruits, indicates a connection between 

 lessened assimilative activity and the production of pigment, though 

 the nature of the connection remains unexplained. 



Overton (13), on the other hand, basing his conclusions on results 

 obtained from feeding leaves and plants with sugar solution, maintains 

 that increase of sugar in the cell gives rise to formation of anthocyanin. 

 He considers the pigment itself to be a glucoside of which the non- 

 sugar part is a tannin-like compound. 



Again no indication is given as to the exact nature of the con- 

 nection between the excess of sugar and the appearance of pigment. 



Kati6(5), Molliard(ll), Mirande(9) and Palladin(14) also support 

 the statement that an accumulation of sugar increases the formation of 



1 In many cases, probably, members of the flavone and xanthone classes of com- 

 pounds. 



