Primulaceae is one of 22 families in the order Ericales. Formerly, this order included some closely related families: Myrsinaceae, Theophrastaceae and Maesaceae. However, morphological and molecular data strongly support their membership in the primuloid clade, which corresponds to the Primulales order in the 1981 Cronquist system. The four families have been re-circumscribed into a more broadly defined family referred to as Primulaceae s. l. The two uniting features of this clade are a free central placenta and one stamenopposite each of the corollalobes. Below is a proposed cladogram for Primulaceae s.l. based on genetic analyses. The names on the branches correspond to the older family definitions. In 2016, estimates of the number of genera and species were 53 and 2790, but as of 2017 there were 55 accepted genera:
Primulaceae are mostly herbaceous, having no woody stem, except that some form cushions and their stems are stiffened by lignin. The stems can grow upright or spread out horizontally and then turn upright.
Leaves
are simple, being directly attached to the stem by a petiole, but unlike the leaves of most flowering plants they have no stipules. The petiole is short or the leaf tapers gradually towards the base. Leaf arrangement is typically alternate but some are opposite or whorled, and there is generally a rosette at the base of the stem. The edges are toothed or sawtoothed. New leaves in the bud are usually involute or conduplicate, but a few species roll downwards.
Flowers
Each flower is bisexual, having both stamens and carpels. They have radial symmetry; the petals can be separate or partially or fully fused together to form a tube-shaped corolla that opens up at the mouth to form a bell-like shape or a flat-faced flower. In most of the families of Ericales, stamens alternate with lobes, but in Primulaceae there is a stamen opposite each petal. The calyx has 4 to 9 lobes and persists after flowering. They are grouped in unbranched, indeterminate clusters such as racemes, spikes, corymbs or umbels.
Reproductive anatomy
The fruit of Primulaceae begins as an ovary and inside it are the future seeds. These are attached to a central axis without any partitions between them, and they are bitegmic. Unlike in most other families of Ericales, both layers form the opening at the top.
Seeds and fruit
As seeds develop, an endosperm grows around the embryo through free division of nuclei without forming walls. The embryo forms a pair of short, narrow cotyledons. Usually multiple seeds are in a capsule that is carried on a straight stalk. After it matures, it splits apart, releasing the seeds ballistically.
