236 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



water. The retardation of oxidase action occasioned by 

 strong alcohol is well known. 



A great number of substances have been used thus for 

 the detection of oxidases macroscopically and microscopic- 

 ally. For the latter purpose those which easily penetrate 

 tissues and give crystalline oxidation products, such as 

 benzidine, are as a rule to be preferred to compounds, such 

 as guaiacum resin, which do not diffuse readily. But as 

 the behaviour of tissues towards different reagents fre- 

 quently presents one with a series of reactions of varying 

 intensity, it is advisable to employ several of them in each 

 investigation. 



A list of about forty oxidase reagents is given by Kastle 

 in his monograph. Of these, many have been but little 

 used, on account of their too great susceptibility to oxida- 

 tion by atmospheric oxygen, which renders it hard to 

 decide whether the colour produced in any tissue is really 

 due to enzyme action. Thus, according as the reagent 

 employed by an investigator is relatively stable or un- 

 stable, so will the number of tissues remaining colourless 

 increase or decrease ; since it really is a question of whether 

 the reducing action of the cell contents is sufficiently feeble 

 to permit of the oxidation of the artificial chromogen which 

 has been added. [See note, p. 302.] 



For example, when examining the tissues of Rosa rugosa, 

 the writer (1913) found that the petals of white flowers 

 contained tannin, and fail to react with a-naphthol. A 

 very faint darkening of benzidine is, however, produced 

 in the veins, whilst ^-phenylenediamine blackens the 

 veins and causes a slight general discoloration. The above 

 changes only take place after the addition of a few drops 

 of very dilute hydrogen peroxide. Red petals, on the 

 contrary, are free from tannin, and when decolorized in 

 alcohol give a faint reaction in the veins with a-naphthol 



