Developmental aspects of tristyly in Lythrum salicaria

The developmental basis of floral polymorphism was investigated in the tristylous invasive species, Lythrum salicaria L. (Lythraceae). Ln tristylous species, the stigmas are positioned above tin the long morph), below tin the short morph), or between tin the mid morph) the outer and inner staminal whorls. Flower samples were collected at three different growth stages (early, pre-anthesis, and post-anthesis) from three genotypes from each of the three morphs to observe morph-specific differences in growth patterns of filaments and styles and their constituent epidermal cells. From each flower, I measured the length of styles and two types of stamens and their epidermal cells at the basal, intermediate, and apical regions of each organ. Differentiation of organ levels begins at a very early stage. Growth rate of the long pistil is higher than in the mid pistil followed by the short pistil. However, the growth rate of epidermal cells is higher in the short style followed by the mid and long styles. The number of cells does not increase during style development in the short morph but does increase in the long and mid morphs. Although the relative growth of the outer stamens is greater than in the inner stamens in all three morphs, the relative cell size is greater in the inner stamens than in the outer stamens. Cell size differs between outer and inner stamens in the long and mid morphs but not in the short morph. The intermediate cells are larger compared with the basal and apical cells of the stamens and styles. The number of epidermal cells increases in the outer stamens during development, whereas it remains constant in the inner stamens of the mid morph and increases only slightly in the inner stamens of long morph.