1. The changes in growth substance composition of buds and scales of Wedgwood iris bulbs during flower formation were determined. 2. The scales contain several thermostable flower forming substances which, in the course of a low temperature treatment, are transported to the bud. 3. The flower forming substances have physiological and chemical properties of gibberellins. 4. Compounds with Rp 0.3, 0.45-0.50 and 0.75 in 80 % isopropanol successively accumulate in the bud and attain maximum concentrations at successive stages of differentiation of the floral primordium. 5. The significance and possible mode of action of the gibberellins in flower formation is discussed.