TRISTYLY, SELF-COMPATABILITY AND FLORAL VARIATION IN DECODON-VERTICILLATUS (LYTHRACEAE)

Heterostyly typically involves reciprocal polymorphism in stamen and s tyle lengths, physiological self- and intramorph-incompatibility, and a set of associated polymorphisms of pollen and stigma characters. Thi s study examined floral morphology and compatibility relationships in the monotypic, herbaceous perennial Decodon verticillatus (Lythraceae) . There have been conflicting reports on the occurrence of tristyly in the species, probably because of frequent loss of style morphs from p opulations in parts of the species' range. Floral morphology was exami ned using material collected from natural populations throughout the r ange. Detailed floral measurements revealed discrete trimorphism in st yle length and anther positioning in three populations. Data from two dimorphic populations showed similar patterns of floral polymorphism, except that both were missing the mid-styled morph. In one dimorphic p opulation, there was evidence for modification in the length of mid-le vel stamens. Measurements in three populations indicated pronounced fl oral variability, including high frequencies of modified phenotypes wi th reduced stigma-anther separation. Pollen size was only weakly diffe rentiated among anther levels, and there were no differences in pollen production among anther levels or morphs. In contrast, stigma size an d papilla length showed a strong negative correlation with style lengt h; a pattern opposite to most heterostylous species. Experimental cros ses performed under glasshouse conditions on plants from two populatio ns showed a high degree of both self- and intramorph-compatibility. A comparative analysis of floral morphology showed that D. verticillalus is not unusual in terms of the precision and reciprocity of organ pos itioning compared with 13 other tristylous species.