146 On the Formation of Anthocyanin 



In the formation of anthocyauin the following reactions must be 

 taken into consideration : — 



Aromatic glucoside + water "^"^ aromatic chromogen + sugar. 



Sugar — »> aromatic chromogen. 



Aromatic chromogen + oxygen = anthocyanin. 



The following possibilities may therefore arise. The amount of 

 pigment is directly proportional to the amount of free chromogen. 

 Increase of sugar would naturally lead to decrease of free chromogen, 

 but if at the same time additional chromogen is formed from the sugar, 

 the ultimate concentration of the glucoside, if it is not removed by trans- 

 location, will be increased to such a degree that the final result is an 

 increase of free chromogen accompanied by formation of pigment. 



A decrease of sugar, on the other hand, will increase the free 

 chromogen, but at the same time it will lead to a decrease in the 

 concentration of the glucoside, so that the final result is a decreased 

 amount of free chromogen and less possibility of pigment formation. 



Or to state the case rather differently : so long as the concentration 

 of glucoside remains low either as a result of translocation or of decreased 

 formation, the amount of free chromogen is negligible, but if the 

 concentration of glucoside is raised beyond a certain point as a result 

 of diminished translocation or continual formation, the synthesis of free 

 chromogen and sugar can no longer take place and the former becomes 

 oxidised to anthocyanin. 



In the normal green leaf the absence of pigment from the mesophyll 

 is in all probability due to the rapid translocation of aromatic gluco- 

 sides away from the leaf. It is difficult to ascertain the precise reason 

 for the presence of pigment when it appears in the epidermis of the 

 lamina and in the epidermis and sub-epidermal layers of the veins and 

 petiole. It may be caused either by low concentration of sugar or by 

 increased concentration of glucosides due indirectly to excess of sugar. 

 These tissues are without chlorophyll and the power to assimilate, but 

 at the same time they are also apparently devoid of starch-forming 

 capacity, since starch does not as a rule appear in them, so that the 

 sugar concentration may or may not be greater than in the mesophyll 

 of the leaf. 



In general the chlorophyll-containing tissues are most free from 

 pigment, the non-chlorophyllous more frequently pigmented. Hence 

 the appearance of pigment is undoubtedly connected with the concen- 

 tration of sugar, but I am at present unable to give the exact sequence 

 of events which affects the reversibility of the reaction. 



