F. Keeble and E. F. Armstrong 299 



words the genetical hypothesis of the nature of dominant whites holds 

 that the latter are due to the inhibition of pigment formation. 



This Mendelian interpretation is so convincing that it scarcely needs 

 the collateral support to be derived from chemical investigation ; but, 

 although this be the case, it is at once evident that the hypothesis 

 suggests a promising line of inquiry into the physiology of pigmentation. 

 Investigations on this subject made by Miss Wheldale (1910) supply 

 confirmation of the hypothesis by indicating that an inhibitor of pigment 

 formation exists in flowers the pale shades of which are dominant to 

 deeper shades. 



A further advance has been made by Gortner (loc. dt. 1911) who has 

 produced experimental evidence confirming the conclusion reached by 

 previous investigators, that the black pigment (melanin) of various 

 insects and other animals is produced by the interaction of the oxydase, 

 tyrosinase with a chromogen, tyrosin, and has shown that, when these 

 substances are allowed to act in vitro, the addition of certain dihydroxy 

 phenols such as phloroglucinol, orcinol and resorcinol prevents the 

 reaction from taking place. These substances which exercise an in- 

 hibitory action on pigment formation Gortner terms antioxydases. He 

 suggests further that the inhibition of melanin formation in animals 

 may be due to a chemical change in the chromogen (tyrosin). Gortner's 

 views may be expressed schematically thus : 



Let T = Tyrosin, 



and T"** = Tyrosinase. 



Then T+T«««= Melanin, 



and T+ derivative of T4- T*"* = Inhibition (Dominant White). 



We proceed now to consider dominant white flowers of P. sinensis with 

 respect to their behaviour with oxydase reagents. The addition of 

 a-naphthol or benzidine and the subsequent addition of hydrogen- 

 peroxide to intact corollas of dominant white plants results in no 

 oxydase reaction whatever. Even after prolonged action of the reagents 

 both epidermis and veins fail to give the colouration characteristic of 

 oxydase. It follows therefore that either oxydase is absent from the 

 flower or it is inhibited from oxidizing the reagents. It so happens 

 that among the Primulas which one of us has been breeding at Reading 

 is a strain derived from a cross between a pure blue-flowered variety 

 and the white, magenta-flaked Snow King to which reference has been 

 made already. Among the descendants of this cross are certain plants 

 illustrated in Plate XIX, Figs. 10 and 13, the flowers of which are 



