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Idiospermum australiense, the sole species in the genus Idiospermum, is one of the most primitive flowering plants known, having lived in the Daintree Rainforest of Queensland, Australia for 120 million years. It is only found in very few locations of the Daintree Rainforest in the very wet lowland parts of the forest, where it grows in groups of 10-100 trees together (rather than scattered individuals). Common names include Ribbonwood and Idiot Fruit.
It is an evergreen tree, growing to 20-30 m tall. The leaves are produced singly, in pairs or in whorls of 3-4; the leaf is simple, 12-25 cm long and 5-9 cm broad. The flowers are 4-5 cm diameter, with spirally arranged red petals. The fruit is a brittle globular nut-like seed around 8 cm in diameter, which splits into three or four segments once fallen; it is very toxic, with symptoms (in cattle) similar to strychnine.
Most plants have both male and female sex organs, but half of the flowers of the Ribbonwood do not obtain any female sex organs, the species using the process of cross-pollination. Attracted by the colour and smell of the flower, tiny beetles and thrips crawl in and lay their eggs within the center of the flower, which contains the flower's pollen. Within the flower some of the sticky pollen gets trapped on the insect's bodies, and if the next flower they visit is a receptive one, it will pollinate and produce the seeds.
While other modern flowering plants produce seeds which have one cotyledon (monocotyledons) or two (dicotyledons), the seedlings of the Ribbonwood have between three or four cotyledons. Also the Ribbonwood can produce more than one shoot per seed, while the seeds in all other plant species will develop and send up a single shoot.
The seeds are currently mainly spread through gravity dispersal, the seeds rolling down the steep mountain slopes to find their new home. The seeds are so toxic that most animals cannot eat them; however it is known that the native Musky Rat-kangaroo does disperse and bury some of these seeds. It has been suggested that the seeds were formerly dispersed by the now-extinct Diprotodon, on the basis that many Australian marsupials are adapted to cope with the toxins in Australian plants.
The plants have adapted a unique poison, a chemical called Idiospermuline contained within the seed, to prevent animals eating them. Researchers discovered the poison affects transmission of messages between individual nerve cells, which may cause seizures. In small doses this chemical can be used to save lives.
First found by timber cutters south of Cairns in the late 1800s, then thought to be extinct. The Ribbonwood was then brought to the attention of the German botanist Ludwig Diels, who described the species in the genus Calycanthus as C. australiense in 1902, a remarkable disjunction for this otherwise North American genus. It was later believed to be extinct again, because when Diels finally returned to the location where this tree was found, the area had been cleared for a sugar cane farm.
The species was re-rediscovered in 1971, after the poisonous seeds of the plant were found in the stomachs of dying cattle in the region. In 1972, the Australian botanist T. S. Blake reassigned it to the new family Idiospermaceae and the genus Idiospermum (idio-, "unusual", and spermum, "seed"). In its 2003 revision, the Angiosperm Phylogeny Group retained it in the new genus, but restored the species to the family Calycanthaceae.
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- The families of flowering plants: Descriptions, illustrations, identification, information retrieval by L. Watson and M. J. Dallwitz
- The story of the rediscovery of Idiospermum by Prue Hewett
- An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II by the Angiosperm Phylogeny Group