Lamiaceae (Labiatae)

Growth Form and Habit

Mostly annual or perennial herbs or shrubs; less commonly trees or lianas. Stems are characteristically square in cross-section in most species (though some have rounded stems). Plants are often aromatic due to essential oils produced in glandular hairs covering leaves and stems.

Leaves

Leaves are almost always arranged oppositely, and typically decussate (each pair oriented 90 degrees to the pair below). Rarely whorled or alternate. Leaves are usually simple, though sometimes pinnately lobed or dissected. Margins are commonly toothed (serrate or crenate), sometimes entire. Stipules are absent.

Characteristic square stem and opposite, decussate leaves of many Lamiaceae.

Inflorescence

Inflorescence structure appears complex. The basic unit is typically a pair of contracted cymes located in the axils of opposite leaves or bracts. These axillary clusters often encircle the stem, forming an apparent whorl called a verticillaster. Verticillasters can be spaced apart along the stem or aggregated terminally into spike-like, raceme-like, or head-like structures. Less commonly, flowers are solitary in leaf axils.

Verticillaster: Dense flower clusters in the axils of opposite leaves, appearing as whorls around the square stem.

Flowers

Flowers are usually bisexual and strongly zygomorphic (bilaterally symmetric), adapted for insect or bird pollination. Sometimes nearly actinomorphic (e.g., Mentha). Typically 5-merous perianth.

• Calyx: Composed of 5 fused sepals (synsepalous), forming a persistent tube or bell shape. The calyx can be actinomorphic (with 5 equal lobes) or zygomorphic (2-lipped, often 2 lobes up / 3 lobes down, or other combinations). Often ribbed and sometimes hairy or colored.
• Corolla: Composed of 5 fused petals (sympetalous), characteristically forming a 2-lipped (bilabiate) structure. The upper lip typically consists of 2 fused petal lobes and may be flat or arching (galeate); the lower lip typically consists of 3 fused petal lobes, with the central lobe often larger and serving as a landing platform. Corolla tube length varies greatly.
• Androecium: Stamens typically number 4 and are didynamous (arranged as 2 long pairs + 2 short pairs), or sometimes reduced to only 2 fertile stamens (e.g., Salvia, Monarda). Stamens are attached to the corolla tube (epipetalous), alternating with the lower corolla lobes. Anthers typically open via longitudinal slits.
• Gynoecium: Ovary is superior, composed of 2 fused carpels. A key feature is that the ovary becomes deeply 4-lobed early in development due to intrusion of the ovary walls or formation of false septa. A single style arises from the deep depression between the four lobes (gynobasic style). Each lobe contains a single ovule attached near the base (basal placentation). The style usually ends in two small stigma lobes. A nectar disc is typically present around the base of the ovary lobes.

Left: Typical Lamiaceae bilabiate flower. Right: Superior ovary deeply 4-lobed with gynobasic style arising from the center.

Fruits and Seeds

The characteristic fruit is a schizocarp that develops from the 4-lobed ovary. At maturity, it splits into 4 distinct, single-seeded, dry segments called nutlets (or sometimes small drupelets if slightly fleshy). These nutlets are often smooth or slightly textured and are dispersed individually. Seeds contain little or no endosperm; the embryo is typically straight.

Chemical Characteristics

The family is defined chemically by the widespread production of volatile terpenoid compounds (monoterpenes, sesquiterpenes) that constitute essential oils. These oils are typically stored in glandular hairs on the plant surface and are responsible for the characteristic aromas of mint, sage, basil, lavender, etc. These compounds likely function in deterring herbivores and attracting specific pollinators. Iridoids, phenolic acids (like rosmarinic acid), flavonoids, and sometimes diterpenoids or alkaloids are also present.