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dye 2,3,5-triphenyltetrazolium chloride was only weakly reduced in the copper-deficient plants. Maxi To supplement biological methods of auxin assay, a colorimetric method was first described by Mitchell and Brunstetter (4). This procedure was later improved by Tang and Bonner (7), and by Gordon and Weber (2). Essentially, the method takes advantage of the red coloration formed through the mild oxidation of IAA. 4 This oxidation product will be described in another publication. stable colloidal suspensions. Since difficulties are encountered with extracts of plant materials, it seemed advisable to describe further refinements of the colorimetric methods developed in this laboratory and especially those procedures adapted for the quantitative determination of IAA and its esters in corn kernels during the various stages of development. EXPERIMENTAL COLORIMETRIC MEASUREMENTS:The procedure is modified from that of Gordon and Weber (2), in that 0.05 M ferric chloride in 10% (by volume) perchloric acid was used to develop the red oxidation product, maximum intensity of which was attained in 45 minutes when held at 30°C. The 10% perchloric acid was used in preference to 35% to facilitate extraction in the final separation. The red color was quantitatively extracted with isobutyl alcohol, and the isobutyl alcohol layer centrifuged to remove the suspended water droplets. Since the red color is unstable in this medium, the time after extraction to reading was standardized at five minutes; the absorbancy was determined at 530 m/i on a Beckman D.U. spectrophotometer. Under these conditions, Beer's law was obeyed over a concentration range of 0 to 0.8 mg/25 ml isobutyl alcohol and an absorbancy index of 2.7 ml (cm mg IAA)-l. APPLICATION OF THE METHOD TO PLANT MATERIAL:To ascertain the accuracy of the method, weighed amounts of IAA were added to 500 gm samples of immature corn kernels at the milk stage and macerated with ethyl acetate in a Waring blendor. The

Surgeon General, Department of the Army. Carried out under the Plasma Volume Ex- § andera Program of the Subcommittee on hock, National Research Council. Part of a thesis to be submitted by J. D. Moyer to the University of Maryland in partial fulfillment of the requirements for the degree of Ph.D.

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