F. Keeble and E. F. Armstrong 303 



the subsequent addition of the oxydase reagent brings about a uniform 

 colouration of the petals. The originally white areas are now as deeply 

 stained as the blue regions, and the veins in the former, which as we 

 have seen yielded at most a faint oxydase reaction give after this treat- 

 ment as marked a reaction as those in the blue areas of the flower. 



The results confirm the Memlelian hypothesis that dominant white 

 flowers owe their lack of pigment to the j)''csence of an inhibitor of 

 pigment formation. They show moreover that the inhibition is exercised, 

 not on the process which results in the liberation of chromogen, but on 

 the oxydase and that the inhibitor acts not b}' destroying oxydase but 

 by effective interference with its action. 



Attention may be drawn to the fact that, as shown by the results 

 obtained with dominant white flowers and contrary to the general 

 opinion, hydrogen cyanide does not destroy oxydase. 



We may summarise our observations on the oxydases of the flower 

 (sections B — F) thus : 



In all coloured and recessive white varieties of P. sinensis oxydase 

 is present in the petals. It occurs in two situations, namely in the 

 epidermis and in the bundle sheath of the veins. The epidermal and 

 bundle oxydases react differently with our reagents (a-naphthol and 

 benzidine). Dominant white varieties of P. sinensis contain an inhibitor 

 of oxydase. On the removal of the inhibitor from the petals of dominant 

 white flowers a strong oxydase reaction is obtained. The white areas in 

 the petals of certain races of blue-flowered Primulas also contain an 

 inhibitor which prevents the oxydase contained in those areas from 

 reacting with oxydase reagents. When this inhibitor is removed the 

 white areas give well marked oxydase reactions. Ever-sporting varieties 

 of P. sinensis and of Dianthus barbatus show most epidermal oxydase in 

 the most deeply pigmented flowers, less in the less pigmented and none 

 in the white flowers. 



The albino forms of P. sinensis, Pisum sativum, Lathyrus odoratus 

 (in the forms as yet examined) all contain oxydase and their floral 

 albinism is attributable to lack of chromogen. The white-flowered 

 Geranium sanguineum lacks oxydase and we are of opinion that it 

 owes its albinism to lack of oxydase. 



Albino or approximately albino forms of Dianthus barbatus are of 

 two forms ; in one form oxydase is present and from the other it is 

 absent. 



