316 Prof. Keeble, Dr. Armstrong, and Mr. Jones. [Feb. 3, 



the benzidine reaction ; but the colour of the latter is not suitable for tinto- 

 metric estimation. 



The results obtained in test-tubes are opposed to the view that the recovery 

 of colour in petals immersed in strong alcohol is due to the activity of 

 oxydase. General considerations, however, led us to suspect that although 

 alcohol of 70-80 per cent, may prevent the action of the oxydase in solutions 

 extracted from plant-tissues, it might prove less potent to retard the action of 

 oxydases in the tissues themselves. We have confirmed the truth of this 

 suspicion in the following way : — 



Petals of purple stocks were incubated with 99-per-cent. alcohol and when 

 decolorised they were placed, some in 70 per cent., and others in 80, 90, and 

 95-per-cent. alcohol. Equal quantities of a solution of benzidine in water- 

 free alcohol were added to each tube containing the petals, one drop of 

 hydrogen peroxide was introduced into each tube, and the preparations were 

 placed in the incubator at 37° C. Examination of the petals after half-an- 

 hour showed that the petal treated with 70-per-cent. alcohol gave a well 

 marked brown benzidine reaction for oxydase, the petal in 80 per cent, 

 showed a very distinct reaction, that in 90 per cent, an equally good or 

 even better reaction, and that in 95 per cent, a slight but distinct reaction 

 in the veins of the claw. 



Whence it follows that the peroxydase of stock petals is capable — if 

 peroxide be present — of bringing about the oxidation of benzidine even in 

 a medium containing 95 per cent, alcohol ; and we infer that what is true 

 of this artificial chromogen is true of the natural anthocyanic chromogen, 

 namely, that the latter may undergo oxidation even in the presence of 

 95-per-cent. alcohol. 



Thus the conclusion is reached that, although the experiments with oxydase 

 extracted from plant tissues are adverse to the view that recovery of colour 

 is due to oxidation, the more apposite and crucial experiments with the 

 oxydases contained within the petals lend powerful support to that view. 



The series of observations described in the foregoing pages lead us to the 

 following conclusions : — 



In concentrated alcohol the anthocyan pigments are reduced to the state of 

 colourless chromogens. The reduction is brought about by reducing agents, 

 the nature of which is unknown. The reducing agents may be specific 

 chemical substances : they may perhaps be of the nature of catalysts ; they 

 are probably not enzymes (reductases). It is interesting to observe that 

 an effect similar to that exercised by the reducing agent contained in 

 stocks is brought about by hydroquinone, though not by formaldehyde. 



When the concentrated alcohol is replaced by water, the oxydases, which 



