Floral asymmetry is best known in enantiostylous flowers, which are basically monosymmetric but have the style deflected to the right or the left side (Jesson & Barrett, 2003; Jesson et al., 2003a, b).
Interestingly, most enantiostylous flowers are buzz-pollinated pollen flowers (Buchmann, 1983), such as Chamaecrista and Senna (Fabaceae; Gottsberger & Silberbauer-Gottsberger, 1988; Marazzi et al., 2007), Cyanella (Tecophilaeaceae; Dulberger & Ornduff, 1980), Heteranthera (Pontederiaceae), Philydrum (Philydraceae), Solanum (Solanaceae), and Paraboea (Gao et al.
Saintpaulia and species of Streptocarpus (Gesneriaceae) are enantiostylous (Harrison et al., 1999; Jesson & Barrett, 2003).
In Tecophilaeaceae, Cyanella has enantiostylous flowers (Dulberger & Ornduff, 1980; Simpson & Rudall, 1998).
In Haemodoraceae, some genera are enantiostylous, either based on otherwise polysymmetric or monosymmetric flowers (Simpson, 1990; Jesson & Barrett, 2002; Jesson et al., 2003b).
Floral morphology, phases, rewards, attractants, visitors, pollen, reproductive system, P/O ratio, OCI and ISI indexes of enantiostylous Senna corymbosa were analyzed for morphological androecial differentiation and possible related functional differences between stamens groups.
It was also seen in other self-compatible and enantiostylous species of the genus Cassia (Dulberger 1981) and Cyanella (Dulberger and Ornduff 1980).
In a recent study on Chamaecrista fasciculata, one of the genus of the subtribe Cassiinae, it was found that there were small differences in outcrossing rate between enantiostylous and non-enantiostylous plants, artificialy created (Fenster 1995).
Such stamen dimorphism is referred to as heteranthery where, as in Pontederiaceae and several other enantiostylous taxa, it reflects a functional division of labor into predominantly attractive, contrastingly colored "feeding" anthers and a cryptically-colored "pollinating" anther (Muller 1883; Iyengar 1923; Vogel 1978; Buchmann 1983; Lloyd 1992; Graham and Barrett 1995).
Among all four major genera of Pontederiaceae are species that lack either the tristylous or enantiostylous floral syndromes.
Several monocotyledonous families (e.g., Commelinaceae, Tecophilaeaceae, Haemodoraceae, Philydraceae), some of which are closely allied to Pontederiaceae (see Graham and Barrett 1995), also contain enantiostylous taxa and it is therefore possible that enantiostyly in Pontederiaceae is homologous to that found in related families.
A hypothetical outgroup was therefore placed at the base of trees for the root positions discussed above, and was coded either as monomorphic, enantiostylous, or tristylous to examine the effect of different outgroup codings on the reconstruction of shifts in floral condition within the family.