Lythrum salicaria
In tristylous species, incompatibility varies, with some species showing varying degrees of compatibility outside of the reciprocal herkogamy pattern of pollination.  Darwin noted weak incompatibility commonly occurring in the M-morph of "Lythrum salicaria." Some species have shown weak or absent incompatibility in their mating system, however self-compatibility in tristylous species is still poorly understood. Research on "Eichhornia paniculate", found difference in pollen tube growth between intra- and inter-morph pollen, indicating that the incompatibility system is a case of cryptic self-incompatibility.