Primroses are perennial, herbaceous plants in the genus Primula, family Primulaceae. There are about 500 species of primroses. Most of these occur in arctic, boreal, and cool-temperate climates, including mountain-tops in tropical latitudes. The greatest species numbers occur in the mountains of central Asia, and, to a lesser degree, in northern Eurasia and North America. Only one species occurs in South America, in southern Patagonia.
The flowers of primroses are small but very attractive. Primrose flowers occur as solitary units, or in small groups (inflorescences). The flowers of primroses are radially symmetric, and have five partially fused petals and five sepals. Primroses have a rosette of leaves at the base of the plant and a taller structure that bears the flowers.
Some native primroses of North America include several species commonly known as the birds'-eye primrose. Primula mistassinica occurs relatively widely in boreal and cool-temperate, often stream-side habitats in the northeastern United States and much of Canada. Primula laurentiana occurs more locally in eastern Canada and the northeastern United States. The arctic primrose (P. stricta) occurs widely in moist places in the Arctic of North America and western Europe. Another arctic primrose (P. borealis) occurs in the northwestern tundra of Alaska and Canada as well as in eastern Siberia.
Many species and varieties of primroses are cultivated as ornamental plants. For example, the European cowslip (Primula veris) is commonly cultivated as a garden plant, as is P. denticulata. Primula auricula and other arctic-alpine primroses are often grown in rock gardens. Primula obconia is grown as a house plant.
Many horticultural hybrids of primroses have also been developed. One of the classic cases is the Kew primrose (P. kewensis), developed in the famous English botanical garden of that name, from a cross between a Himalayan primrose (P. floribunda) and an Arabian species (P. verticillata). In this case the original hybrids were sterile, that is, they could not reproduce sexually by the fertilizing of the pistils of one plant with pollen from another. Several of the hybrids subsequently became fertile as a consequence of a spontaneous doubling of their chromosome number, a characteristic that geneticists call polyploidy. This unprecedented discovery of sterile hybrids becoming fertile through polyploidy is a famous story in botany and plant breeding.