Castanospermum is a monotypic genus in the legume family Fabaceae (order Fabales), containing a single species: Castanospermum australe, commonly known as Moreton Bay chestnut or black bean. It is native to rainforests on the east coast of Australia, from Queensland to northeastern New South Wales, as well as the islands of Vanuatu and New Caledonia in the southwest Pacific, and has been introduced to India, Malaysia, Papua New Guinea, Sri Lanka, South Africa, and the United States.
The black bean is a large evergreen tree that can reach up to 40 m tall, though most specimens are considerably smaller. Its leaves are pinnate, 15 cm long and 6–7 cm broad, with 11 to 15 leaflets. The flowers are strikingly bicoloured red and yellow, 3–4 cm long, produced in 6 cm racemes. The fruit is a cylindrical pod 12–20 cm long and 4–6 cm in diameter, internally divided by spongy tissue into one to five cells, each containing a large, chestnut-like seed.
First described in 1829 by English botanist Allan Cunningham, the genus name means "chestnut seed," a reference to the seeds' resemblance to true chestnuts. The seeds are toxic when raw, containing saponins that cause vomiting and diarrhoea, but Aboriginal peoples of eastern Australia have prepared and eaten them for at least 2,500 years by roasting, slicing, leaching in running water for several days, and pounding into flour. The tree held deep cultural significance as a seasonal gathering point for ceremonies. In 1981, the iminosugar castanospermine was isolated from the seeds, adding scientific interest to the genus.
The trees are widely cultivated as ornamental potted plants in Asia, Europe, and the Americas, and as shade trees in parks in Australia and abroad. The timber is fine-grained, resembling walnut, and takes a good polish, with an above-ground durability rating of over 40 years.
Etymology
The genus name Castanospermum combines the Latin castanea (chestnut) with the Ancient Greek spérma (seed), referring to the seeds' resemblance to true chestnuts. The species epithet australe derives from Latin australis, meaning "southern," reflecting its Australian and southwest Pacific origin.
Distribution
Castanospermum australe is native to eastern Australia from Kutini-Payamu National Park in far north Queensland south to Coffs Harbour in New South Wales, as well as the islands of Vanuatu and New Caledonia. It grows in rainforest and gallery forest, particularly along watercourses, from sea level to about 800 m elevation. The species has been introduced to India, Malaysia, Papua New Guinea, Sri Lanka, South Africa, and the United States.
Ecology
The large seeds of C. australe are toxic to most animals, making animal-mediated dispersal unlikely. Instead, the seeds float and can be dispersed by water, retaining viability even after immersion in salt water, which explains their distribution along coastlines and waterways. Aboriginal peoples historically spread black bean trees by hand into mountain areas on Australia's east coast, and all trees in New South Wales are descended from a single seed.
Cultivation
Black bean trees have been widely cultivated as ornamental potted plants in Asia, Europe, and America. In both Australia and other countries they are also planted as shade trees in parks and gardens.
Cultural uses
The seeds of the Moreton Bay chestnut have been a food source for Aboriginal peoples for at least 2,500 years. Though toxic when raw, they become edible when roasted, cut into small pieces, leached in running water for several days, and pounded into flour for damper. The tree served as a seasonal gathering point and catalyst for ceremonies, and songlines featuring black bean seeds have been recorded. Beyond food, the bark fibre was used for fish and animal traps, nets, and baskets, while empty seed pods served as toy boats. The tree also acted as a seasonal signal for when to hunt jungle fowl.
Taxonomy
Castanospermum is a monotypic genus in the family Fabaceae. The sole species, Castanospermum australe, was first described in 1829 by English botanist Allan Cunningham in Robert Mudie's The Picture of Australia.
