Urease activity is expressed in urease units. A urease unit (U.U.) is a quantity of urease which will produce 1 mg. of ammonia per minute when acting upon urea under the experimental conditions employed (2). The indicator method is not as sensitive as the quantitative method, and tissues containing less than 0.02 U.U. per gram of fresh weight are reported as negative with this method. Tissues containing less than 0.0001 U.U. are reported as negative with the quantitative method. The hypocotyl is that part of the axis between the cotyledonary node and the external collet.