Annonaceae are a remarkably diverse family of Magnoliales. Xu and Ronse De Craene investigate the floral development of 15 species using SEM to understand the basis for evolutionary change in flowers of Annonaceae. They find that the merism is variable with transitions between trimery and tetramery. Members of the inner perianth are often in double positions through splitting. Stamen and carpel number are variable, although outer stamens are always formed in pairs. An increase of stamen number is correlated with their smaller size at initiation. Limits between stamens and carpels are unclear with easy transitions of one organ type into another in some genera, or the complete replacement of carpels by stamens in unisexual flowers.