DNA studies swelled the primrose family to nearly 2,800 species
The primrose takes its name from Latin for little first of spring, one of the earliest blooms of the year. Its family, Primulaceae, once seemed tidy. Then molecular studies reshuffled it, absorbing whole neighbouring families, until it held around 53 genera and 2,790 species, from garden favourites to tropical shrubs.
Despite the shared name, the family has no link to the evening primroses. Most members are perennial herbs, though some, such as the scarlet pimpernel, are annuals, and a few form low cushions stiffened with lignin. Leaves are simple and lack the small stipules at the base found in most flowering plants, and many species gather them in a rosette at ground level.
Two traits unite the group. Each petal has a stamen directly in front of it, whereas in most related families stamens alternate with the petal lobes. And the seeds attach to a free-standing central column inside the ovary, with no walls dividing them, an arrangement botanists call free central placentation. When the seed capsule matures, it splits and can fling its seeds out.
Classifying the family has been a long saga. Linnaeus sorted Primula by its six stamens and single pistil in 1753. Pax and Knuth's 1905 treatment described nearly 1,000 species in 22 genera. Arthur Cronquist in 1988 grouped Primulaceae with two related families in an order called Primulales. Genetic data later revealed these plants belong inside the heath order, Ericales, and the Angiosperm Phylogeny Group admitted in 2009 that deciding how to treat them had been its biggest single problem. Its solution was to merge the related families into one broad Primulaceae with four subfamilies.
Fossils are scarce, but molecular dating puts the modern family at roughly 46 to 61 million years old, with the primrose line splitting from the ebony family about 80 million years ago. The group probably arose across tropical America and Indo-Malaysia; the primrose subfamily itself developed in the Palearctic and began spreading into North America in the last 16 million years.
Source: Primulaceae
