The surprising scent of the world’s largest flower - Daniel Nickrent

Deep inside the Sumatran rainforest, a carrion fly descends,

guided by the scent of its favorite place to lay eggs:

dead and rotting animal carcasses.

But when it lands, it isn’t on liquefying flesh,

but instead on the world’s biggest, and perhaps strangest, flower—

Rafflesia arnoldii.

Rafflesia is a genus of over 30 species

found across the tropical forests of Southeast Asia.

While its smallest representative has a flower only a few centimeters wide,

its biggest weighs seven kilograms and spans over a meter wide.

And its putrid aroma isn’t the only thing that sets these plants apart—

all Rafflesia are parasites.

For most of its life, the Rafflesia plant exists as an endophyte,

a single thin strand of almost uniform cells beneath the bark of its host.

It strictly infects Tetrastigma,

a genus of large vines related to the grape.

Like typical leafy plants, the host Tetrastigma’s cells contain chloroplasts.

These organelles convert sunlight into energy

and are each equipped with their own DNA.

Rafflesia’s plastids, on the other hand, appear to have lost their DNA,

and with it the ability to photosynthesize.

This sort of loss is incredibly rare.

With no roots and no ability to produce its own food,

Rafflesia is completely dependent on its host,

siphoning the Tetrastigma’s water and nutrients to fuel its own growth.

And Rafflesia’s propensity for theft doesn’t end there.

Using a process known as horizontal gene transfer, or 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 happens.

Rafflesia appears to utilize several of these stolen sequences

as if they were their own,

transcribing the DNA into RNA and then translating it into proteins,

which are used in key cellular processes.

After living some time embedded in the host vine,

Rafflesia emerges as a single bud,

which then takes several months, or even a year, to reach full size.

When it opens, its fleshy maroon petals

emit several foul-smelling sulfur-containing compounds.

The evolutionary reason for this odor is relatively straightforward:

to attract pollinators.

For most species of Rafflesia, single flowers are either male or female.

So, to produce a seed,

pollen must be transferred from one flower to the next.

The rotten stench is ideal for attracting corpse-loving carrion flies,

and the massive size of the flower may help broadcast it

through the stagnant rainforest air.

A deceived fly will explore the flower’s interior,

laying thousands of ill-fated eggs.

But during the fly’s visit,

the male Rafflesia’s liquid pollen may end up on the fly’s back where it dries.

If the fly encounters an open female Rafflesia flower,

the pollen will rehydrate when rubbed against the flower’s damp stigma,

completing cross-pollination.

A pollinated Rafflesia flower gradually withers and turns black,

but this doesn't mean it's dead.

Over several months, a fruit forms which contains thousands of tiny seeds.

But what disperses these seeds is still debated,

with hypotheses ranging from elephants to rodents to ants.

We do know that the seeds have an oily appendage called an elaiosome,

a structure ants often feed to their larvae.

And scientists have even observed ants carrying Rafflesia seeds.

But what happens to the Rafflesia seeds once inside the ant nest remains unclear.

In any case, nobody has seen Rafflesia seeds germinate,

or attach to and infect a host root.

Because this crucial step of their development is still not fully understood,

cultivation of Rafflesia is difficult.

Despite many attempts, botanists from around the globe

have been largely unsuccessful at growing Rafflesia

from seeds outside its natural habitat.

As these tropical forests are under threat,

we’re at risk of losing Rafflesia,

and our ability to unravel some of its many remaining secrets.