Floral organogenesis of Ranalisma rostratum (Alismataceae)

The floral development of Ranalisma rostratum Stapf. has been observed under scanning electron microscope (SEM). Floral organogenesis of R. rostratum is generally typical in Alismataceae, but it is unique in having unidirectional development up to the stage of initiation of the first six stamen primordia. The initiation of the three sepal primordia and the three petal primordia is in proper order. The former is clockwise and the latter too. The sepal primordia and the petal primordia initiate first towards the side of the flower adjacent to the lateral branch complex of the inflorescence. The transition from stamen to carpel initiation occurs at six sites alternating with the first six stamens, some of them may be occupied by stamen primordia. Initiation of first carpel primordia alternate more or less with previous stamen primordia, but the initiation of later carpel primordia appear to be spiral. In view of specific condition occurring on the process of floral organogenesis, floral unidirectional development in R. rostratum may really represent reservation of successive initiation of original spiral tactics and continuation of bilateral symmetry tactics of the inflorescence. Antisepalous characteristic of the first six stamens , has even not been paid attention to, provides the evidence that the existence of petal-stamen complex primordia is impossible in Ranalisma. Basing on the difference of differentiation floral constructure, the relationship between Ranalisma and Limnocharitaceae is still discussed.