2. Lippia L. (fog fruit, frog fruit)

Plants perennial herbs (shrubs elsewhere), sometimes with rhizomes. Stems few to several from the rootstock, prostrate, sometimes loosely ascending at the tip (ascending elsewhere), irregularly branched, usually rooting at the nodes, moderately 4-angled in young growth but often nearly circular in cross-section at maturity, pubescent with appressed hairs, these with 2 opposite branches, thus appearing attached medially along a straight line (hairs unbranched elsewhere). Leaves sessile or with an indistinct, short (to 5 mm), winged petiole. Leaf blades unlobed, variously shaped, rounded to sharply pointed at the tip, the margins toothed, mostly above the midpoint, pubescent with hairs similar to those of the stems. Inflorescences axillary, dense, spherical to more commonly oblong-ellipsoidal (knob-shaped), axillary heads, these solitary at the tips of slender, elongate, erect, naked stalks. Calyces flattened, ovate to nearly circular in outline, 1.5–2.0 mm long, 2-lobed, the lobes triangular, somewhat incurved, and keeled, glabrous or sparsely hairy, sometimes with a line of hairs along each angle. Corollas 3.5–4.5 mm long, moderately to strongly zygomorphic, somewhat 2-lipped, 4-lobed, white to pinkish- or lavender-tinged, usually with a yellow central spot at the base of the limb, the limb 2–4 mm in diameter, the lobes slightly irregular or shallowly and broadly notched. Stamens inserted at 2 levels toward the tip of the corolla tube, lacking glandular appendages. Ovary 2-locular (1 of the carpels aborting early in development), not appearing lobed, rounded at the tip. Style 0.2–0.4 mm long, the minute lobes spreading, but obscured by the confluent stigmatic regions, these appearing as a single capitate stigma. Fruits 1.0–1.2 mm long (larger elsewhere), appearing somewhat flattened longitudinally, circular to broadly obovate in outline, consisting of 2 nutlets, these more or less hemispheric, usually rounded at the tip and base, the surface finely pebbled to minutely pitted, olive green to yellowish brown. About 50 species, North America to South America, Caribbean Islands; introduced in the Old World.

The five to nine herbaceous species of Lippia with trailing stems that range into temperate North America sometimes have been segregated into the genus Phyla Lour. More recently, Sanders (2001) concluded that, in the context of the overall generic classification within the tribe Lantaneae Endl., the species sometimes segregated as Phyla are better treated merely as a specialized subgroup within Lippia. He noted, however, that the taxonomy of the tribe is in need of more detailed study. A recent, comprehensive, molecular study of phylogenetic relationships in the family similarly concluded that generic limits within the Lataneae require reevaluation following more detailed taxon sampling (Marx et al., 2010). The most recent taxonomic revision of the group accepted the segregation of Phyla from Lippia based on differences in habit and pubescence types. However, the situation requires more detailed study and the taxonomic limits of Lippia, in the broad sense accepted here, remain controversial.

The vernacular names fog fruit and frog fruit have been used relatively interchangeably for the North American members of this genus