This is a large, mostly tropical and subtropical family of about 13,000, species, of which only relatively few are herbaceous (including the cosmopolitan genus Galium). Plants typically have entire, opposite, stipulate leaves or whorled leaves (the added leaves in a whorl representing transformed stipules), with flowers characterized by an inferior ovary and radial symmetry.
1. Plant an erect woody shrub; flowers and fruits in dense, spherical, peduncled heads.
1. Plant herbaceous (or at most a prostrate trailing subshrub); flowers and fruits nearly sessile or pediceled, but not in heads.
2. Leaves whorled (i.e., stipules, even if fused or further divided, closely resembling the leaves).
3. Corolla tube distinctly shorter than the lobes; flowers not in heads, the corolla white, greenish, yellow, or maroon.
3. Corolla tube much longer than the lobes; flowers in heads, the corolla pink to blue.
2. Leaves opposite (with stipules united around the stem).
4. Stem prostrate, rooting at the nodes; leaves evergreen, scarcely (rarely as much as 50%) longer than wide and usually ± variegated with the midrib broadly marked in pale green or whitish; fruit indehiscent, red, fleshy, formed by fusion of ovaries from the 2 adjacent flowers and hence with two perianth scars.
4. Stem erect; leaves deciduous (or overwintering only in a basal rosette), the blades distinctly longer than broad, not variegated; fruit dehiscent, a dry capsule formed from 1 ovary.
5. Flowers axillary, sessile; stipules with fringe 2–7 times as long as body of the stipule, equaling or surpassing the flowers and fruit; fruit consisting of two 1-seeded nutlets; stem pubescent.
5. Flowers terminal (or on a scape), usually stalked; stipules with no fringe or a very short one; fruit a many-seeded capsule; stem glabrous or pubescent.
6. Calyx lobes shorter than the inferior portion of the ovary (at least as fruit develops); capsule longer than wide; stipules of cauline leaves ciliate or sparsely fringed or with a prolonged bristle; leaves narrowly linear.
6. Calyx lobes, even in fruit, equaling or exceeding the inferior portion of the ovary; capsule no longer than wide; stipules of cauline leaves not ciliate or fringed (though may be pubescent at base); leaves often broad.