Adenocaulon is a small genus of flowering plants in the family Asteraceae (the daisy family), comprising around five species commonly known as trailplants. The genus was first described by William Jackson Hooker in 1829, in Botanical Miscellany, under the name derived from the Greek aden (gland) and kaulos (stem) — a reference to the distinctive stipitate-glandular hairs that clothe the upper stems and fruit.
Plants are perennials, usually rhizomatous, growing (10–)30–60(–100+) cm tall. The lower stems are whitish-tomentose while the upper stems and fruits are covered in stalked glands. Leaves are basal and cauline, the lower ones petiolate, the upper sessile; leaf blades are ovate to triangular with truncate to cordate or hastate bases and coarsely toothed to entire margins — the undersides densely tomentose, the upper surfaces nearly smooth. Flower heads are disciform, arranged in open panicles; outer florets are pistillate with white to pale yellow corollas, inner florets are functionally staminate; fruits (cypselae) are club-shaped and covered in sticky glandular hairs; pappi are absent.
The genus ranges across a striking intercontinental disjunct distribution: one species (Adenocaulon bicolor) occurs in western North America and disjunctly in the Great Lakes region; one (A. chilense) in Chile and Argentina; one (A. himalaicum) across the Himalayas, China, Japan, Korea, Nepal, and Russia; and further species in Central America and Nepal. The sticky fruits are adapted for dispersal on animal fur and feathers, and this mechanism is thought to explain the wide intercontinental gaps in the genus's range, possibly aided by migratory birds.
Etymology
The name Adenocaulon is derived from the Greek aden (gland) and kaulos (stem), referring to the conspicuous stipitate-glandular hairs that cover the upper portions of the stem and the surfaces of the fruit. The genus was formally described by William Jackson Hooker in 1829.
Distribution
Adenocaulon has a striking intercontinental disjunct distribution spanning the Americas and Asia. Adenocaulon bicolor grows in western North America (Pacific Northwest, northern Rocky Mountains) and disjunctly in the Black Hills and northern Great Lakes region. Adenocaulon chilense is found in Chile and Argentina. Adenocaulon himalaicum ranges across the Himalayas, China, Japan, Korea, Nepal, and Russia. Additional species occur in Guatemala, Chiapas, and Nepal.
Ecology
Adenocaulon species grow in forest understories and moist, shaded habitats. Their fruits are club-shaped and covered in persistent sticky glands, making them highly effective at adhering to animal fur, feathers, and clothing — a classic example of epizoochory. This dispersal mechanism is thought to explain the genus's remarkable intercontinental disjunctions: the wide gaps between North American, South American, and Asian populations were likely bridged by long-distance dispersal on migratory birds. An alternative hypothesis for the North American disjunctions of A. bicolor is a former transcontinental periglacial distribution fragmented by Holocene warming.
