Studies in the Alismataceae. IX. Development of the flower in Ranalisma humile

The solitary flower of Ranalisma humile has three sepals, three petals, 8–12 stamens, and numerous carpels. The first six stamens appear to arise in pairs associated with the petal primordia. With respect to the perianth and the first six stamens, the flower conforms to the basic trimerous pattern detected in other Alismatalean flowers, but it differs in other aspects of development. Organogenesis is unidirectional during the period of sepal and petal initiation, since both sepal and petal initiation occur first on the same side of the flower bud. After the initiation of the first six stamens there is no evidence that subsequent organs are initiated in any semblance of a trimerous pattern. The transition from stamen to carpel initiation occurs at some point after the initiation of the first six stamens, but it does not occur at a fixed point. There are six organogenetic sites above and alternating with the first six stamens, and these may be occupied indifferently by stamen or carpel primorida. The sequence of initiation of further carpel primordia is hard to define, but ultimately carpels appear to be arranged spirally. The features of floral organogenesis are discussed in relation to the position of Ranalisma among the Alismatales. Key words: flower, organogenesis, symmetry, unidirectional development, Ranalisma, Alismataceae, Alismatales.