Floral Giants From Humble Beginnings
One fascinating recent phylogenetic discovery concerns Rafflesia, a group of plants that live in the tropical forests of southeast Asia.
These bizarre organisms produce the world’s largest flowers. Leafless, stemless, rootless and nonphotosynthetic, these plants live as parasites inside a host plant, a woody vine related to grapes. Their bodies have been reduced to something that resembles the thread-like filaments of a fungus, which allows it to steal its nutrition as it grows through the host stem.
Lacking almost all recognizable structural features — other than their extraordinary flowers, which look and smell like a rotting animal carcass — Rafflesia’s closest relatives have remained a mystery for nearly 200 years. It has even been proposed that they are not flowering plants at all, but fungi.
Despite the use of DNA sequences, this riddle has proven difficult to solve. The problem is confounded by drastic reduction of the chloroplast DNA in Rafflesia and the transfer of some genes from the host plant directly into the genome of the parasite. It was not until 2007 that a definitive study in the journal Science reported the findings of botanist Charles Davis of Harvard University and his colleagues.
Rafflesia and its closest relatives, Rhizanthes and Sapria are nested squarely within a large flowering plant group (with over 6,000 species) known as the Euphorbiaceae, the euphorbs or spurges. This group contains some familiar plants, including the poinsettia, the rubber tree, the castor bean and the cassava, or yuca root.
What is striking about this discovery is that related plants in the Euphorbiaceae (such as Ditaxis) have small or even tiny flowers, which implies an astounding rate of increase in flower size along the line leading to Rafflesia. The remarkable size of these smelly giants may help lure pollinating flies to their blossoms by better mimicking decaying animals on the forest floor.
The World's Largest Flower
Rafflesia arnoldii produces the largest known individual flowers, nearly three feet (one meter) across and weighing up to 15 pounds (7 kilograms).
The individual flowers of Rafflesia arnoldii are either male (pollen bearing) or female (seed bearing), and may be pollinated by “bluebottle” carrion flies attracted by the color and the smell of “tainted beef.”
A fly enters the chamber of a male flower and is guided to the anthers, where a sticky mass of pollen is deposited on its back for transport to a female flower.
Successful pollination is rare, however, because Rafflesia populations are few and far between. The flowers open only rarely and then only for about five days.
The flowers offer no reward to the flies, who are fooled into looking for food or a place to lay eggs. The distinctive projections on top of the disk in the center of the flower may help to radiate heat and spread the carrion odor.
Rafflesia and its Relatives
There are 13 species of Rafflesia living in southeast Asia. These differ in size, coloration, and the number of various flower parts.
The closest relatives of Rafflesia are Rhizanthes and Sapria, each with two species. These are also Asian parasitic plants with smaller but equally bizarre flowers.
The magnificent flowers of Rafflesia arnoldii have become a symbol of Borneo. Tragically, Rafflesia and its relatives are now all threatened with extinction from destruction of their rainforest habitats. It has not been possible to cultivate them.