SECTIONAL TRANSACTIONS.— D. 



431 



Plastid Pigments. — The plastid colouring matters comprise a nximber of 

 yellow or orange substances such as xanthophyll or carotin, which are 

 insoluble in the cell sap and are quite independent of the sap-soluble 

 colouring matters. In the absence of anthocyanins they are either solely 

 responsible for flower colour, or are supplementary to the yellow 

 anthoxanthins. In the presence of anthocyanins their effect is purely 

 that of a background ; thus in the tulip the introduction of yellow plastid 

 pigment changes the colour from pink, crimson or purple to orange, scarlet 

 or brown respectively. 



The first part of the exhibit includes examples of the various types of 

 pigments and shows the effect on anthocyanin colour of variation in the 

 number of hydroxyl groups, glycosidal type, methylation of hydroxyl 

 groups and copigmentation. Mixture and background effects of antho- 

 cyanins with anthoxanthins and plastids are also shown. In addition there 

 are three of the rarer flower colouring matters. Gesnerin from Gesnera 

 species is unusual in that it has no hydroxyl group in the 3 position. Celosia 

 and Iresine contain nitrogenous pigments similar to that found in beetroot. 

 Flowers of Papaver nudicaule also contain a introgenous colouring matter, 

 but of a different type. 



The Genetics of Flower Colour. 



Flower colour variation has been studied genetically for many years, 

 but until recently, as pointed out earlier, the only possible criterion of 

 colour types was the visual one. The result was a chaotic mass of informa- 

 tion about the inheritance of flower colour, which was only disentangled 

 when the means by which variation is brought about were recognised. 

 The position now is that nearly all of the factors listed in the table are known 

 to be controlled by single genes. 



Table. 



Variation in flower colour is brought about by one or more of the 

 following factors. Changes are shown in one direction only ; the reverse 

 may be inferred. 



>4 *•> 



vii. Interaction of anthocyanins and antho- 

 xanthins . . . . . 



^ ^ viii. Alteration in nature of plastid pigment 



05 C 



ca C 

 P-i • « 



Alteration of back- 

 ground. 



Change in copigment 

 effect. 



Partial suppression of 

 one or both types. 



Yellow becomes 

 orange. 



Alteration of back- 

 ground. 



