Flower buds of tobacco (Nicotiana tabacum), excised before organ primordia are present or, except for the sepals, determined, will continue normal development and produce all floral organs in a usual sequence when placed on a simple, defined medium lacking growth substances By surgically manipulating organ primordia, it was possible to infer the source of influences regulating the initiation of each type of organ. The carpels, e.g., would emerge rarely if the primordial stamens were removed from a flower bud prior to carpel determination However, removal of either or both of the sepal or petal primordia had no such effect The initiation of the carpels depends on the regulatory influences of the primordial stamens and the bud apical region. Experiments similar to that outlined above indicated that (1) the initiation of the stamens depends on the regulatory influence of the sepal and petal primordia, (2) the initiation of the petals depends on the regulatory influence of the sepal primordia, and (3) the ini...