Isanthus brachiatus (L.) B.S.P. - False Pennyroyal

Isanthus brachiatus plant

Family - Lamiaceae

Stems - To 50cm tall, herbaceous, erect, from a taproot, single from the base, branching above middle, pubescent with retrorse viscid hairs.

Isanthus brachiatus stemStem.

Leaves - Opposite, descussate, entire, narrowly ovate to elliptic, 3-nerved, to +2.5cm long, +1cm broad, pubescent to (less commonly) glabrous, sessile or very short-petiolate, cuneate at the base, acute at the apex.

Isanthus brachiatus leaves

Inflorescence - One or two pedunculate flowers in leaf axils. Peduncles short, 2-4mm long.

Flowers - Corolla purple, to 5mm long, tubular, bilabiate. Upper lobes 4, spreading, shorter than corolla tube. Lower lobe slightly longer than upper lobes. Stamens 4, exserted beyond the corolla. Stigma 2-lobed. Ovary 4-parted. Calyx with a pair of stipule-like bracts at base. Calyx lobes acute to attenuate, one nerved, 4mm long, entire, pubescent, viscid.

Isanthus brachiatus calyxCalyx.

Isanthus brachiatus flowerFlower.

Flowering - July - October.

Habitat - Along streams on gravel bars, glades, rocky banks, outcroppings.

Origin - Native to U.S.

Other info. - This plant is common along the many streams and creeks of the state. It is easy to identify because of its three-nerved leaves and small purple flowers.
Another variety, variety linearis Fassett., has linear leaves. This latter variety is not found in Missouri.
A synonym is Trichostema brachiatum L.

Photographs taken along the Current River, Shannon County, MO., 8-2-03.