Heterostyly
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Heterostyly is a unique form of polymorphism and herkogamy in flowers. In a heterostylous species, two or three different morphological types of flowers, termed morphs, exist in the population. On each individual plant, all flowers share the same morph. The flower morphs differ from one another in the lengths of the pistil and stamens, and these traits are not continuous. The morph phenotype is genetically linked to genes responsible for a unique system of self-incompatibility, termed heteromorphic self-incompatibility.
Heterostylous plants having two different flower morphs are termed distylous. In one morph, termed pin, the stamens are short and the pistils are long; in the second morph, termed thrum, the stamens are long and the pistils are short; the length of the pistil in one morph, equals the length of the stamens in the second morph, and vice versa. An example for a distylous plant is the genus Primula.
Heterostylous plants having three different flower morphs are termed tristylous. Each morph has two types of stamens. In one morph, the pistil is short, and the stamens are long and intermediate; in the second morph, the pistil is intermediate, and the stamens are short and long; in the third morph, the pistil is long, and the stamens are short and intermediate.
The different lengths of stamens and pistils in heterostylous flowers, are adapted for pollination by different pollinators, or different body parts of the same pollinator. Thus, pollen originating in a long stamen, will reach primarily long rather than short pistils, and vice versa. In case of pollination between stamen and pistil of different types (e.g. long stamen and short pistil), no fertilization will take place, because of the self-incompatibility mechanism.
