Atkins — Oxtjdases and their Inhihitors in Plant Tissues. 163 



yellow plastid pigments produce an inhibitor of colour-formation. Accordingly 

 yellow is surrounded by white. This interpretation seems to be the correct 

 one rather than the alternative of absence of chromogen ; for the addition of 

 a chromogen does not give any very decided reaction. Indeed it frequently 

 appears that conditions suitable for the production of the natural pigment 

 are suitable for the benzidine reaction, but not for the a-naphthol. Where no 

 chromogen is present the epidermis usually gives an intense reaction with the 

 latter reagent. The causes of the differences in the shades of colour — blue, 

 purple, red, etc. — have been recently dealt with by Keeble, Armstrong, and 

 Jones (8). 



Table II shows the influence of treatment with hydrogen cyanide as 

 described in removing the inhibitor. All the benzidine reactions are now 

 very dark, while those witli a-naphthol are much intensified. Here again 

 it is noticeable that the presence of deep purple chromogen seems to inhibit 

 the latter reaction in the epidermis ; and this inhibitor is not removed by the 

 cyanide, or it may be that twenty-four hours was not sufficient for its 

 complete removal, or even that this body was regenerated by enzyme action 

 while the last traces of the cyanide were being washed out. However, if it is 

 correct that the white in No. 5 is due to the presence of the general inhibitor, 

 and since the conditions of cyanide treatment have sufficed for its complete 

 removal, it does appear as if the presence of the natural chromogen acted as 

 an inhibitor on the production of the a-naphthol reaction. 



Table III. contains the results afforded by the varieties of /. Xiphmm. 



Takle III. 



Xijjhion Group {continued). 



