Microthlaspi perfoliatum (L.) F.K. Mey.

Penny Cress

© DETenaglia

Family - Brassicaceae

Habit - Taprooted annual forb, glabrous and somewhat glaucous throughout, not producing an odor when bruised or crushed.

Stems - Ascending to erect, to 35 cm, glabrous, glaucous.

Leaves - Alternate and with usually few basal leaves at flowering, glabrous, glaucous. Basal leaves with petioles to 2 cm long, glabrous. Stem leaves 1-4 cm long, reduced upward, sessile, clasping the stem with mostly rounded auricles, ovate to narrowly oblong, entire or with a few shallow teeth.

	Basal leaves.
© SRTurner

	Leaf adaxial. Leaf is not truly perfoliate but instead clasping, with prominent auricles.
© SRTurner

	Leaf abaxial.
© SRTurner

	Cauline and basal leaf respectively.
© DETenaglia

Inflorescence - Terminal raceme, compact in flower, elongating in fruit to 10 cm. Pedicels to 4 mm long in flower, expanding in fruit to 6 mm, glabrous.

	Inflorescence.
© DETenaglia

	Inflorescence top view
© SRTurner

Flowers - Sepals 4, distinct, 1.0-1.5 mm long, ovate, green with whitish margins. Petals 4, 1.5-3.0 mm long, white, glabrous, spatulate to oblanceolate. Stamens 6, didynamous. Filaments 1.2 mm long, glabrous, white. Style absent or very short, to 0.2 mm long. Ovary green, superior, compressed, ovoid to rotund, 1 mm long.

	Calyces.
© DETenaglia

	Flowers.
© SRTurner

Fruits - Silicles 3.0-5.5 mm long, obcordate, the margins broadly winged toward the tip, the apical notch shallow and wider than deep, glabrous. Seeds 2-4 per locule, 1.3-1.5 mm long, obovate in outline, the surface light orange to tan, smooth or nearly so.

	Fruits.
© SRTurner

Flowering - March - May.

Habitat - Fields, pastures, lawns, railroads, roadsides, and open, disturbed areas.

Origin - Native to Europe.

Lookalikes - Broadly, small species of Cardamine and Thlaspi; Capsella bursa-pastoris.

Other info. - This small springtime mustard is extremely common in east-central Missouri and probably occurs everywhere in the state south of the Missouri River. Beyond Missouri its range extends mostly eastward through Virigina and Pennsylvania. Its prevalence is extremely variable. Though ubiquitous in the St. Louis region, just across the river in Illinois it is considered rare.

The plant is easily recognized by its diminutive stature, glabrous and glaucous herbage, and sparse, clasping leaves. Although the species name "perfoliatum" suggests leaves which surround the stem on all sides, they are in fact only strongly clasping, with prominent auricles. "Thlaspi" comes from the Greek name of cresses - "thlaspis" - with "micro" denoting a diminutive form. The roundish fruits slightly resemble those of Capsella bursa-pastoris, but the latter has fruits which are more triangular in shape and smaller when mature. The leaves of the two plants also differ greatly from one another.

Photographs taken at Eagle Bluffs Conservation Area, Boone County, MO., 3-7-04 (DETenaglia); also near Labadie, Franklin County, MO, 3-30-2015 (SRTurner).