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Two pairs of alleles, Cu/cu and g/5, are known to influence coumarin biosynthesis in Melilotus alba Desr. (Goplen e t al., 1957 ; Haskins and Gorz, 1957). Young leaves from CuCuBB plants characteristically contain substant i a l akounta of both free and bound co-in, CuCubb leaves are high in "coumarin" but virtually a l l the substance is in the bound form, and cucuBB and cucubb leaves are low in both forms. Lt has been suggested that the gene is involved in the formation of bound coumarin from coumarin precursors and that the gene is concerned with the conversion of the bound form to the free form (Gorz and Haskins, 1960). Evidence indicates that bound coumarin, so called because it responds in the usual coumarin assays only after a hydrolytic treatment, is the glucoside of cis-o-hydroxycinnamic acid (Rudorf and Schwarze, 1958), the lactone form of which is coumarin. It seemed reasonable, therefore, to investigate the possibility t h a t tissues of the BB genotype might differ from bb tissues with respect to β -glucosidase activity.