THE PURE LINE HYPOTHESIS. 555 



5. The Red Colouration. 



The character of the red colouration may be dealt with in a 

 more detailed manner owin^ to the fact that very considerable 

 variation in its intensity could be observed. 



The most intense red of all the flowers was called 100. 

 Between this colour and zero twenty grades or steps were made, 

 and the red colour of any flower was referred to one of these 

 grades. This estimation of grade was repeated several times on 

 different days with various flowers of the same plant, and the 

 mean of the results was taken as the grade of the plant. 



It has been pointed out by Weiss* that the intensity of the red 

 colouration of the flowers of nasturtiums increases with the age 

 of the plant, and with the advance of the season. This was 

 found to be also the case in Natal. Young plants might have 

 flowers only faintly red, but as the plants grew older the colour 

 might increase very greatly. It is therefore very essential that 

 the estimation of the red colouration should be made only on 

 quite young plants of approximately the same age, and subjected 

 to a like environment. 



The pale yellow colouration of the cell-sap never appeared 

 to be entirely absent from the flower. 



It is presumed that this pale yellow substance is the chro- 

 mogen which can be converted into the red colouring matter, 

 anthocyanin, by an oxidative enzyme when no inhibitor is pre- 

 sent. That such an oxidative enzyme is sometimes present in 

 pale yellow nasturtium flowers, without a trace of red colouration 

 except for the " honey-guides," was demonstrated by the follow- 

 ing familiar test.f A pale yellow flower was soaked in alcoholic 

 solution of a-naphthol for two hours, and subsequently it was 

 transferred to a solution of hydrogen peroxide. In about five 

 minutes the fibro-vascular bundles began to stain blue, and the 

 colouration gradually increased and extended to the finest 

 branches in the petals. Afterwards the general tissue between 

 the bundles became strongly tinged, and especially such tissue as 

 was in direct contact with the bundles. 



This reaction indicates the presence of an oxidase. From 

 analogy with similar cases it is to be supposed that the flowers 

 remained pale yellow on account of the presence of an anti- 

 oxidase or a reductase. 



It was noticed that the oxidase was present in the tissues 

 of the petiole. It was further observed that in the young flower 

 which is in the act of opening the amount of oxidase present is 

 very much less than in the old flower wdiich is on the point of 

 fading ; and this perhaps explains the 'fact t hat th e older flowers 



* I am indebted to Prof. F. E. Weiss for his kindness in sending to 

 me a reprint of his paper, " Note on the Variability in the Colour of the 

 Flowers of a Trop^eolum Hybrid." Mem. and Pvoc. Manch. Lit. and 

 P/m7. 5-oc.. 54 [3I, 1-6 (TQio). , ^, . r 



t Haas. Paul, and Hill, T. G.— " An Introduction to the Chemistry of 

 Plant Products," 244. London (1913)- 



