308 BOTANICAL GAZETTE [ocToBER 
48 hours, satisfactory. duplicates may be obtained, so the method 
was considered sufficiently reliable for comparative purposes. A 
few drops of toluol were added to each tube during incubation, to 
insure against activity of bacteria. 
CaATALASE.—A detailed description of the method for the cata- 
lase determinations will be found in a recent paper by the author 
published in this journal (4). In all cases the water bath tempera- 
ture was 25° C. during the experiment. 
PEROXIDASE.—In order to compare the rate of peroxidase 
activity in the material under investigation, various methods were 
tried and variously modified. The methods differed mainly in the 
preparation and treatment of the extract, for in all cases guaiaconic 
acid was used as the oxidizable substance (10, 11). A definite 
quantity of extract was allowed to act upon a definite quantity of 
guaiaconic acid in the presence of hydrogen peroxide. The 
time required for the oxidation of the guaiaconic acid to reach a 
depth of blue equal to that of a standard tube, containing a solu- 
tion of indigo carmine, was considered the index of the peroxidase 
activity. The standard tube was placed against a white back- 
ground and screens arranged to eliminate shadows and unequal 
lighting. If the test tube is held against the standard tube, and 
the eye fixed upon the line separating the tubes, and the time noted 
when the color is the same on both sides of the line or when the 
line seems to disappear, quite accurate readings may be obtained 
with a little practice. The error of reading is greatly reduced if 
the test solution is of such dilution or the standard tube of such 
depth of blue that the time of reaction is not less than ten seconds 
nor greater than one minute. If the solution of indigo carmine is 
made up with pure distilled water, the fading of color at the end 
of several weeks is so slight that it falls well within the experi- 
mental error involved in comparing the shades of blue. 
The present methods for peroxidase determinations are unsat- 
isfactory, but under careful manipulation the above method will 
give results of value for determining comparative activity of the 
guaiaconic acid oxidizing peroxidase in potato tubers. 
The method described by Griiss (2) for obtaining a peroxidase 
solution from the potato tuber was first tried. According to this 
