We have previously proposed that the flowers of helobial monocotyledons are derived from assemblages of primitive unisexual units. The main reason for the proposal lies in the different and partly independent developmental patterns of the periath/androecial and the gynoecial components of the flower. In many cases in the Zosterales, e.g. Triglochin, Lilaea, Potamogeton, and Scheuchzeria, there is stamen/tepal superposition: each stamen is formed above a perianth member in a way suggesting an axillary relationship, and the set of organs occur in whorls of different numbers according to the taxon. Similarly, in the Alismataceae and related families, the perianth members occur in whorls of three, and there is commonly an association between a petal and a superposed pair of stamens which develop in association with it, often from what appears to be a common primordium. Further stamens may be initiated above (some Alismataceae) or below (Hydrocleis) the firstformed stamens. In all these cases carpels are initiated in what seems initially to be a whorled pattern above the androecium, with the numbers in a whorl corresponding to those in the perianth. However, it has been shown in Potamogeton and Ruppia that carpel positioning is more flexible and appears to operate like an ordinary phyllotactic system. The flowers can be seen as being made up of two different types of module, one being the perianth/stamen complex, and the other the carpel. We now have additional data for floral development in two genera of Alismataceae, Luronium and Wiesneria, which complement older data and seem to reinforce the concept of independence of development in the two regions. Phylogenetic analysis indicates these genera are in interesting position within Alismataceae: Luronium is near other genera showing some range of gynoecial organisation, and Wiesneria is part of a complex including Sagittaria which shows extreme variation in meristic complexity.