Plantaginaceae

Growth Form and Habit

Mostly annual or perennial herbs, but also includes subshrubs, shrubs (e.g., Hebe, now in Veronica), and some aquatics. Plants are autotrophic (non-parasitic).

Leaves

Leaf arrangement is highly variable: alternate, opposite, or whorled along the stem, or sometimes all basal in a rosette (Plantago). Leaves are usually simple, though sometimes lobed or dissected. Margins can be entire or toothed. Stipules are absent. Venation is typically pinnate, but notably parallel in Plantago.

Inflorescence

Highly variable; commonly indeterminate racemes or spikes (Veronica, Penstemon, Digitalis, Plantago), but also panicles, thyrses, axillary clusters, or solitary axillary flowers.

Flowers

Flowers are usually bisexual and typically zygomorphic (bilaterally symmetric), often strongly two-lipped (bilabiate). However, some major groups exhibit secondarily actinomorphic (radially symmetric) flowers, notably Plantago (wind-pollinated) and many Veronica species. Flowers are typically 4- or 5-merous in the perianth.

• Calyx: Usually composed of 4 or 5 sepals, which are typically fused at the base (synsepalous) and often persistent in fruit.
• Corolla: Usually composed of 4 or 5 fused petals (sympetalous). Corolla shape is highly diverse: commonly bilabiate (2-lipped) with varying degrees of tube length; personate (2-lipped with a prominent palate closing the throat, e.g., Antirrhinum, Linaria); rotate (wheel-shaped with very short tube, e.g., Veronica); tubular (Digitalis, Penstemon); or small, dry, and papery (scarious) in wind-pollinated Plantago.
• Androecium: Stamens typically number 4 (often didynamous: 2 long + 2 short) or sometimes only 2 (e.g., many Veronica). They are attached to the corolla tube (epipetalous) and alternate with the corolla lobes. The fifth (posterior) stamen is often absent or represented by a sterile filament (staminode), which can be small or sometimes conspicuous and bearded (e.g., Penstemon, giving it the common name 'beardtongue'). In Plantago, there are 4 stamens with long, flexible filaments and large anthers adapted for wind pollination. Anthers typically open via longitudinal slits.
• Gynoecium: Ovary is superior, typically composed of 2 fused carpels forming 2 locules (chambers). Placentation is usually axile, with typically numerous ovules per locule (sometimes few or only one). A single terminal style is present, usually with a simple or 2-lobed stigma.

Fruits and Seeds

The most common fruit type is a capsule, which dehisces in various ways, often septicidally (splitting along partitions) or loculicidally (splitting into the locules). A distinctive circumscissile capsule (opening via a lid) is characteristic of Plantago. Rarely, the fruit is an achene or nutlet. Seeds are usually numerous and small, sometimes winged or flattened, containing endosperm.

Chemical Characteristics

Many members produce iridoid glycosides (e.g., aucubin, catalpol), which are common in the Lamiales order and contribute to defense and medicinal properties. Mannitol, saponins, and various phenolic compounds are also widespread. Digitalis species are notable for containing potent cardiac glycosides (e.g., digitoxin, digoxin). Unlike Orobanchaceae, Plantaginaceae members are non-parasitic and lack associated chemical/structural adaptations.