Cephalanthus occidentalis L.

Buttonbush

Cephalanthus occidentalis plant

Family - Rubiaceae

Habit - Shrub or small tree, to 5 m.

Stem - Somewhat angled to rounded, glabrous, the twigs sometimes finely short-hairy, becoming woody with age.

Cephalanthus_occidentalis_stem.jpgYoung stem.

Cephalanthus_occidentalis_stem2.jpgWoody stem.

Leaves - Opposite or in whorls of 3 per node, simple, entire, with petioles 10-25 mm long. Interpetiolar stipules 2-4 mm long, triangular, generally persisting with the leaves, angled to a sharply pointed tip, sometimes with minute fingerlike glands on the margins. Leaf blades 2.5-16.0 cm long, 2-6 cm wide, elliptic to elliptic-oblong or ovate, angled or slightly tapered to a sharply pointed tip, rounded or angled at the base, the margins flat, the upper surface glabrous, often glossy, the undersurface glabrous or with small tufts of hair in the vein axils, occasionally more uniformly short-hairy, the midvein and 6-8 pairs of secondary veins expressed and conspicuous.

Cephalanthus_occidentalis_leaves.jpgStem and leaves.

Cephalanthus_occidentalis_leaf2.jpgLeaf abaxial.

Inflorescences - Dense, globose heads 1-4 cm in diameter, in a loose terminal cluster and sometimes also solitary from the uppermost leaf axils, each with a stalk 1.5-6.0 cm long.

Cephalanthus_occidentalis_inflorescence.jpgInflorescences.

Flowers - Numerous, intermingled with short, linear to narrowly club-shaped bracts. Calyces 0.8-1.1 mm long, persistent at fruiting, 4-lobed, the lobes 0.4-0.5 mm long (becoming enlarged to 1.0-1.5 mm at fruiting), rounded at the tips. Corollas 9.0-12.5 mm long, narrowly funnelform to nearly cylindric, white, externally glabrous, internally pubescent with short wavy hairs in upper part of the tube and sometimes also the lobes, the tube 7-10 mm long, 4-lobed, the lobes 1.5-2.5 mm long, narrowly elliptic, bluntly pointed at the tips. Stamens 4. Ovary fully inferior, 2-locular, the ovules 1 per locule. Style 1, the stigma 1, cylindric, exserted.

Cephalanthus_occidentalis_flowers.jpgFlowers.

Cephalanthus_occidentalis_flowers2.jpg

Fruits - Schizocarps, 5.0-5.5 mm long, 1.5-2.0 mm wide, dry, narrowly obconic, at maturity splitting from the base into two mericarps.

Cephalanthus_occidentalis_fruits.jpgFruiting heads.

Cephalanthus_occidentalis_fruits2.jpgFruits.

Flowering - June - August.

Habitat - Swamps, sloughs, marshes, streambanks, bottomlands, pond margins, ditches.

Origin - Native to the U.S.

Lookalikes - None.

Other info. - This is a common shrub found in wet areas throughout Missouri. It occurs throughout much of the continental U.S., except for the northwestern quadrant. It is easily recognized in flower by its unique globose inflorescences. Even when not flowering it can be readily identified, being the only shrub in the state with entire, opposite leaves and interpetiolar stipules. The latter feature, common to all members of the Rubiaceae (coffee family), can be seen in the young stem image above as a small, triangular brown flap of tissue between the leaf petioles.

The plant is often cultivated as an ornamental, and for bank stabilization and to attract beneficial insects such as bees. Although it has been used medicinally, it contains saponins and is considered toxic. Tiny tufts of hairs in the undersurface axils of leaf veins harbor symbiotic mites, which harvest and ingest potential invaders, such as fungal spores, from the leaves. The species has been subdivided by some authors on the basis of leaf pubescence. Since this character shows continuous variability and lacks discernable geographic correlation, these varieties are not currently recognized in Missouri.

Photographs taken at McGee State Park, Atoka County, OK, 7-26-2016, and along the Katy Trail near Femme Osage Creek, St. Charles County, MO, 11-27-2017 (SRTurner).



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