Abstract: The investigation of the differentiation and development of inflorescences and flowers in Ailanthus altissima (Mill .) Swingle (Simaroubaceae ) using light microscopy (LM) and scanning electron microscopy (SEM) suggests: 1) Flower bud differentiation of A. altissima occurs in early April on a panicle bearing many flowers; 2) The sequence of floral development proceeds from calyx primordia to corolla primordia to stamen primordia and finally to carpel primordia . Five sepal primordia are initiated spirally and asynchronously . Five petal primordia form nearly simultaneously and growmore slowly than the stamen primordia . The ten stamens are alternately arranged in two whorls; the two whorls develop simultaneously. The 5-carpellate gynoecium grows quickly . 3) In hermaphrodites, the five carpels adhere to each other to form the style and stigma; in staminate flowers, in late developmental stages, the five carpels are sterile and fertile stamens differentiate into anthers and filaments . In this paper , we focus on the morphological transition from bisexual to unisexual flower development in A. altissima . We observed that the primordia of staminate flowers are initiated in nearly the same way as in
hermaphrodite flowers ( although the time of initiation of each organ is different) . The stamen and carpel primordia initiatesimultaneously in both hermaphrodite and staminate flowers. The formation of reproductive structures is due to the differentiation in the course of development of the floral primordia that form the anthers and filaments and􊄯or styles , stigmas, and ovaries . The formation of staminate flowers is due to suppression of the development of the gynoecium in A. altissima . The mechanism of transition from bisexual to unisexual flowers will only be clarified by further study .

Key words: Ailanthus altissima