The World's Largest Flower

Have you ever wondered which plants produce the largest single flower in the world?

Meet Rafflesia. Well, I should say Rafflesia arnoldii, which produces flowers that are over 3 feet (1 meter) in diameter and can weigh as much as 24 lbs. (11 kg)! Rafflesia is a genus that contains roughly 28 species that hail from the jungles of southeastern Asia. What is crazy about this genus is not just the fact that it produces the largest flower in the world, but also that they are all holoparasites. They do not produce stems, leaves, or true roots. They live out their entire lives inside of a group of vines related to North America's grapes. Except for flowering, Rafflesia exists entirely as a network of mycelium-like cells inside their vine hosts.

For a long time, the taxonomic status of this plant was highly debated but recent DNA evidence puts it in the order Malpighiales. From there, things get a little funny. One recent analysis suggested that Rafflesia belonged in the family Euphorbiaceae, however, it most likely warrants its own family - Rafflesiaceae.

A view from the inside of another species of Rafflesia - Rafflesia tuan-mudae. The strangely spiked "disk" in the center is the column, which houses the reproductive organs.  Photo by Ch'ien C. Lee

A view from the inside of another species of Rafflesia - Rafflesia tuan-mudae. The strangely spiked "disk" in the center is the column, which houses the reproductive organs.

Photo by Ch'ien C. Lee

So, why produce such large flowers? Well, existing solely within a vine makes it hard to establish a large population in any given area, a difficult situation for any plants that rely on pollinators for reproduction. By growing very large and thus being able to produce a lot of "stink" (this plant is also referred to as the corpse plant), Rafflesia make sure that pollinators will come from far and wide to investigate, thus increasing their chances of cross pollinating. How this plant goes about spreading its seeds is still a mystery.

Most interesting of all, it has been discovered that there is some amount of horizontal gene transfer going on between Rafflesia and its host. Basically, Rafflesia obtains strands of DNA from the vine it lives in and uses them in its own genetic code. It is believed this incurs some fitness benefit to Rafflesia but it is yet not fully understood.

Sadly, many species within this family may be lost before we ever get a chance to get to know them better. Forests in this region are disappearing rapidly to make room for expanding populations and agriculture. What makes matters worse for the genus is that their lifestyle makes them very hard to study. It is especially difficult to obtain accurate population estimates. As more and more forest is cleared, we could be losing countless populations of these wonderful and intriguing plants. As with large mammals, it would seem that the world's largest flower is falling victim to the unending tide of human development.

Photo Credit: Tamara van Molken

and

Ch'ien C. Lee www.wildborneo.com.my/photo.phpf=cld05120647.jpg

Further Reading:

http://journals.plos.org/plosgenetics/article?id=10.1371%2Fjournal.pgen.1003265

http://www.sciencedirect.com/science/article/pii/S0960982208013432

http://legacy.earlham.edu/~givenbe/Rafflesia/rafflesia/biodiv2.htm

Rhizanthes lowii

Imagine hiking through the forests of Borneo and coming across this strange object. It's hairy, it's fleshy, and it smells awful. With no vegetative bits lying around, you may jump to the conclusion that this was some sort of fungus. You would be wrong. What you are looking at is the flower of a strange parasitic plant known as Rhizanthes lowii.

R. lowii is a holoparasite. It produces no photosynthetic tissues whatsoever. In fact, aside from its bizarre flowers, its doesn't produce anything that would readily characterize it as a plant. In lieu of stems, leaves, and roots, this species lives as a network of mycelium-like cells inside the roots of their vine hosts. Only when it comes time to flower will you ever encounter this species (or any of its relatives for that matter).

The flowers are interesting structures. Their sole purpose, of course, is to attract their pollinators, which in this case are carrion flies. As one would imagine, the flowers add to their already meaty appearance a smell that has been likened to that of a rotting corpse. Even more peculiar, however, is the fact that these flowers produce their own heat. Using a unique metabolism, the flower temperature can rise as much as 7 degrees above ambient. Even more strange is the fact that the flowers seem to be able to regulate this temperature. Instead of a dramatic spike followed by a gradual decrease in temperature, flowers are able to maintain this temperature gradient throughout the flowering period.

There could be many reasons for doing this. It could enhance the rate of floral development. This is a likely possibility as temperature increases have been recorded during bud development. It could also be used as a way of enticing pollinators, which can use the flower to warm up. This seems unlikely given its tropical habitat. Another possibility is that it helps disperse its odor by volatilizing the smelly compounds. In a similar vein, it may improve the carrion mimicry. Certainly this may play a role, however, flies don't seem to have an issue finding carrion that has cooled to ambient temperature. Finally, it has also been suggested that the heat may improve fertilization rates. This also seems quite likely as thermoregulation has been shown to continue after the flowers have withered away.

Regardless of its true purpose, the combination of lifestyle, appearance, and heat producing properties of this species makes for a bizarrely spectacular floral encounter. To see this plant in the wild would be a truly special event.

Photo Credit: Ch'ien C. Lee - www.wildborneo.com.my/photo.php?f=cld1500900.jpg

Further Reading:

http://www.jstor.org/stable/4222678?seq=1#page_scan_tab_contents

http://www.people.fas.harvard.edu/~ccdavis/pdfs/Nikolov_et_al_AJB_2014.pdf