Heterostyly

Heterostyly, or Heterogony, the possession of stamens and styles of different relative length in different flowers of the same species. In primroses and their allies dimorphic heterostyly occurs, one form, the long-styled or pin-eyed, having the stigma at the top of the corolla-tube and the stamens half-way down it, whilst in the other, the short-styled or thrum-eyed, their positions are reversed. The common purple loosestrife, Lythrum Salicaria, is an example of trimorphic heterostyly, long-, medium-, and short-styled forms occurring, each with two rows of stamens of lengths other than that of its own style. Darwin first showed heterostyly to be an adaptation to cross-pollination by insect-agency, corresponding parts of an insect's body touching the anthers of one flower and the stigma of another, whilst the pollen from any anther was prepotent on the stigma at a similar level.