306 Oxydases and Pigments of Plants 



specimens some method must be devised whereby the oxydase is 

 destroyed. It is probable that the method of preserving the colours of 

 flowers by drying them in sawdust or sand at moderately high tem- 

 perature owes its efficiency to a destruction of oxydase. 



(3) The Infliience of Light and Darkness on the Oxydase Content 

 of Plants. 



We now turn to the consideration of the influence of external 

 conditions on the oxydase content of plants. From our experiments 

 with P. sinensis it would appear that light and darkness play an all- 

 important part in determining the amount of oxydase present in plant 

 tissues. As demonstrated by the facts which we record below, light 

 exercises a destructive influence on the organic peroxide constituent of 

 oxydase. Thus under normal conditions of illumination a tissue may 

 give no reaction with a-naphthol or benzidine, though it yields a well 

 marked peroxydase reaction when hydrogen peroxide is added. When, 

 however, a plant is maintained in darkness for 24 hours or longer, the 

 tissue corresponding to that previously tested gives a pronounced and 

 direct reaction with the oxydase reagents. In illustration of this fact 

 we may cite the following examples : — Sections of the flower-peduncle 

 of a reddish -stemmed plant which had been maintained in normal con- 

 ditions of illumination gave no direct reaction with benzidine. Similar 

 sections from a sister plant, which had been kept in the dark for 

 48 hours, gave a distinct oxydase reaction in the epidermal hairs. Other 

 sections of the same two plants treated with a-naphthol gave the follow- 

 ing results : the illuminated plant — a direct reaction (faint lilac) only 

 in the phloem ; the dark kept plant — a much deeper reaction in the 

 phloem (rose-colour to almost black) and a fair reaction in the epidermis 

 and hairs. The same experiment shows further that the effect of dark- 

 ness is also to increase the peroxydase content of the tissues. Thus, 

 if flowers of sister plants, one exposed to normal illumination and the 

 other maintained in the dark, be tested for peroxydase the reaction 

 of the latter is found to be so definitely more considerable as to be 

 appreciated by macroscopic examination. Text-figure 4 is typical of 

 the results which were obtained in numerous experiments. 



Whatever be the interpretation of these facts the phenomena them- 

 selves are definite. Darkness leads to the formation of peroxide and to 

 an increase of peroxydase. We cannot say whether the latter result 

 is to be interpreted as being due to a destructive action of light on 

 oxydase or whether it is to be regarded as a consequence of the 



