Study of floral ontogeny in Amorpha fruticosa L. (Fabaceae)

The flower structure in Amorpha fruticosa L. is unique among legumes because it possesses only one petal. In order to clarify the ontogenetic basis for missing petals, the initiation and development of floral organs were examined using epi-illumination light microscopy. First, the flower buds of different ages were collected and fixed in FAA. After a fixation period of 24 h, the samples were rinsed, dissected in 70% ethanol and further dehydrated in 95% ethanol prior to staining with 0.5% nigrosin black in ethanol. The most distinctive features of Amorpha flower ontogeny were the abaxial-adaxial unidirectional initiation of organ primordia within each floral whorl, high overlap in the time of initiation of whorls and early carpel primordium initiation. Based on our results, although five petals were first initiated in the flower buds, the abaxial and lateral ones failed to enlarge and thus disappeared completely by midstage.