The developmental and morphogenetic process of pistil formation was examined by analysing flowers of wild type and six flower mutants of Arabidopsis thaliana, a small crucifer. The wild type is suggested to originate from two 'pistil-forming units' (carpels) arranged laterally against the axis of the inflorescence at a pistil primordium. Aberrant structures of the pistils of mutants indicate that a set of genes regulate each step of pistil development and morphogenesis, namely arrangement of the units at the pistil primordia, fusion of the units, growth of primordia, formation of the septum in the ovary, and formation of the stigma.