Why does this flower smell like a dead body?

Deep in the Sumatran rainforest, a carrion fly is guided by the scent of its favorite egg-laying site: dead and rotting animal carcasses. But when it lands, it is not on liquefied flesh, but on the largest and perhaps strangest flower— in the world - Rafflesia arnoldii.

Rafflesia is a genus of over 30 species found in the tropical forests of Southeast Asia. While its smallest representative has a flower just a few centimeters wide, its largest weighs seven kilograms and is over a meter wide. And it is not only their foul smell that distinguishes these plants — all Rafflesia are parasites. For most of its life, the Rafflesia plant lives as an endophyte, a single thin strand of almost uniform cells under the bark of its host. It exclusively infects Tetrastigma, a genus of large vines related to the grape.

Like typical leafy plants, the cells of the host Tetrastigma also contain chloroplasts. These organelles convert sunlight into energy and are each equipped with their own DNA. The plastids of Rafflesia, however, appear to have lost their DNA and thus the ability to photosynthesize. This kind of loss is incredibly rare. Without roots and the ability to produce its own food, Rafflesia is completely dependent on its host, siphoning off the Tetrastigma’s water and nutrients to fuel its own growth. And Rafflesia’s propensity for thievery doesn’t end there. Through a process known as horizontal gene transfer (HGT), Rafflesia has stolen quite a bit of genetic material from its host and other plants in its habitat. While HGT is well known in bacteria, it has only recently been documented in parasitic plants. And scientists are still trying to understand exactly how this DNA transfer takes place. Rafflesia appears to use several of these stolen sequences as if they were its own, transcribing the DNA into RNA and then translating it into proteins that are used in important cellular processes.

After living embedded in the host vine for some time, Rafflesia hatches in the form of a single bud, which then takes several months or even a year to reach its full size. When it opens, its fleshy, maroon petals release several foul-smelling, sulfurous compounds. The evolutionary reason for this odor is relatively simple: to attract pollinators. In most Rafflesia species, the individual flowers are either male or female. In order to produce a seed, pollen must therefore be transferred from one flower to the next. The foul odor is ideal for attracting carrion flies, and the enormous size of the flower can help spread the smell in the stale air of the rainforest.

A deceived fly explores the inside of the flower and lays thousands of unfortunate eggs. But during the fly’s visit, the male Rafflesia’s liquid pollen can land on the fly’s back, where it dries. When the fly encounters an open female Rafflesia flower, the pollen is rehydrated when it's rubbed against the flower’s moist stigma and cross-pollination is complete. A pollinated Rafflesia flower will gradually wither and turn black, but this doesn't mean that it's dead.

Over the course of several months, a fruit forms containing thousands of tiny seeds. What spreads these seeds is still controversial, with hypotheses ranging from elephants to rodents to ants. What is known is that the seeds have an oily appendage called an elaiosome, a structure that ants often feed to their larvae. And scientists have even observed ants carrying Rafflesia seeds. But what happens to the Rafflesia seeds once they're in the ant nest remains unclear.

In any case, no one has ever seen Rafflesia seeds germinate or attach to a host root and infect it. As this crucial step in their development is still not fully understood, the cultivation of Rafflesia is difficult. Despite many attempts, botanists from around the world haven't yet succeeded in cultivating Rafflesia from seed outside its natural habitat. With these tropical forests under threat, we're in danger of losing Rafflesia and our ability to unlock some of its remaining secrets.