ANTHOCYANINS AND GENETICS 207 



THE CHEMICAL INTERPRETATION OF FACTORS FOR FLOWER-COLOUR. 



If we write out the factorial composition of the type of any species 

 which has been investigated on Mendelian lines, as for instance. 

 Antirrhinum (see p. 161): 



YYIILLTTDDBB 

 or Lathyrus (see p. 167) : 



CCRRBBDiDiDeDe 



we see that a number of definite factors go to build up the colour both 

 the amount and kind of colour of the type, and each of these factors 

 must represent a power in the plant to control a chemical reaction; 

 the sum-total of these reactions is the production of the pigment of 

 the plant. To know with what kind of process each of the factors 

 corresponds is of very great importance for the understanding of 

 genetical problems. It may be stated without hesitation that we can 

 only find out w r hat these processes are by means of exact chemical 

 analvses. 



ti 



Such chemical analyses have been attempted in the case of Antir- 

 rhinum, and we may first consider the kind of deductions which can 

 be drawn from the results. The factors which have been most investi- 

 gated are the yellow (Y) and ivory (I). The tissues (except those of 

 the palate) of the flowers of the ivory variety contain a very pale yellow 

 soluble pigment, the flavone, apigenin, which is present in all the cells, 

 both epidermal and mesophyll. The pigment is not recognisable in 

 the petal, for, as we have pointed out in earlier chapters, the flavones, 

 though practically ubiquitous, are too slightly coloured to affect the 

 colour of the cell-sap. The presence of apigenin in the ivory Antirrhinum 

 can be demonstrated by placing the corolla in ammonia vapour, when 

 the flower turns bright yellow owing to the formation of a more intensely 

 coloured salt of the pigment. If, on the other hand, we examine sections 

 of the corolla of the yellow variety, we find the epidermal cells filled 

 with a soluble yellow pigment, the flavone. luteolin, while the inner 

 tissues appear colourless. On treatment with ammonia vapour, the 

 yellow epidermis turns orange, and the inner tissues yellow, showing 

 that the latter contain apigenin. These observations are confirmed by 

 analyses of the extracted pigments ; apigenin, only, is found in the ivory 

 variety: both apigenin and luteolin can be extracted and identified 

 from the yellow. From the true white variety no apigenin could be 

 extracted, nor do the flowers turn vellow when treated with ammonia 



