Primula

Primula is a genus of 400–500 species of low-growing herbs in the family Primulaceae. They include primrose, cowslip and oxlip. 
Many species are grown for their ornamental flowers. They are native to the temperate Northern Hemisphere, south into tropical mountains in Ethiopia, Indonesia and New Guinea, and in temperate southern South America.
Perennial primulas bloom mostly during the spring; their flowers can be purple, yellow, red, pink, or white. Generally, they prefer filtered sunlight. Many species are adapted to alpine climates.
The word "primula" is the Latin feminine diminutive of "primus", meaning first (prime), applied to flowers that are among the first to open in spring. 
Primroses are used as food plants by the larvae (caterpillars) of some Lepidoptera species.
Some flowering forms of (cultivated) "Primula" are commonly known as polyanthus ("P. elatior" hybrids) as opposite to primrose ("P. vulgaris" hybrids).
Heterostyly.
The common primrose, "Primula vulgaris", shows heterostyly. This is a type of morphism with flower forms which differ in the lengths of the pistil and stamens.
The morph phenotype is genetically closely linked to genes responsible for a system of self-incompatibility. That means the pollen from a flower on one morph cannot fertilize another flower of the same morph. 
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. The system prevents self-fertilisation.
All sections of the genus "Primula" have heterostyle species, altogether 354 species out of 419. Since heterostyly is characteristic of nearly all races or species, the system is at least as old as the genus.