Also known as Bunga Bangkai, the Corpse Flower, or Amorphophallus Titanium
The titan arum is, by one definition, the world's largest flower. It is an Indonesian plant that blooms only rarely and unpredictably in its 40 year life, and emits an intense stench of rotting flesh for about eight hours of its 3 day bloom when it does so. There are only a few dozen of these endangered 6 foot plants in botanical centers, and when they bloom they attract quite a crowd both for the novelty and to experience the stench, which attracts the carrion-eating beetles which pollinate it.
Well, that depends on how you define "largest"
It's rarely simple to determine recordholders for such poorly defined concepts as biggest, tallest, and largest, although the human mind holds a fascination with these concepts which demands to be satisfied. The most famous example of this ambiguity being buildings, which switch recordholders depending on whether or not one defines tallest as including any antennas, or the highest structural point, or the highest point intended for human occupation... and even how you define building.
Likewise the largest flower in the world has three claimants to the title. The Rafflesia arnoldii is the largest true flower in the world at three feet across, but it is dwarfed by two other flowers which are not single but rather examples of inflorescence — which means that what appears to be a single flower is actually a cluster of individual flowers, like a daisy. The largest branching inflorescence is the Talipot palm, which blooms once in its lifetime in huge fuzzy stalks of flowers branching off from the top of the palm.
The titan arum is the world's largest unbranched inflorescence at up to 9 feet tall (12 feet or more in the wild). It gets its Latin nomenclature "amorphophallus titanium" (giant shapeless penis) from the large central spadix — that is, a fleshy spike upon which the individual flowers grow. The spadix is wrapped in a large, showy spathe — a bract (modified leaf) which wraps around a spadix. The spadix and spathe together are considered to be the flower of the plant, and in this way is similar to the more famous jack in the pulpit.
Life cycle
The plant begins its life as a large corm, a starchy root similar to a tuber, which can be more than 18 inches across and weigh over 40 pounds. The corm will grow and store energy for about six to ten years before it is ready to grow its flower. In the intervening years it will periodically grow a leaf stalk, that can be up to 20 feet tall, which breaks off every year and must regrow. After this period, a central spike grows up from the corm instead of the leaf stalk and can grow up to six inches per day to reach a maximum height of 9 feet. During this growing phase it looks somewhat like a giant ear of corn. When the growth slows down to less than an inch per day, it is getting ready to bloom.
When the flower is ready to bloom, the spathe will begin to unwrap itself over a period of several hours to reveal the spadix underneath. The spathe is quite dramatic, unfolding into a frilly, inverted bell shape up to four feet across and revealing a deep purple or maroon on the inside of the leaf. The spadix is covered with hundreds of both male and female flowers. To prevent self-pollination, the female flowers open first, then the male flowers open a day or two later.
For eight to ten hours after the spathe unfurls is when it emits its strongest odor, which seems to generate a wide variety of responses from the hundreds of visitors which always show up to see this rare event when it occurs in captivity. Some people can't stand being around it, others simply wrinkle their nose in disgust, but nobody enjoys it. The odor, said to be like rotten flesh, rotten vegetable matter, or rotten eggs (see a pattern?), attracts carrion beetles and dung beetles which pollinate it. This odor has earned the plant its nickname, "Bunga Bangkai" (Indonesian for "corpse flower").
One more in a long line of unusual features of this plant is that it can produce its own heat, unlike most plants which remain at the ambient temperature of its environment. During the unfurling of the spathe through the hours-long production of its noxious odor, a specimen at the University of California, Davis raised its temperature from an initial 68 degrees to 90 degrees Fahrenheit, which may play a role in releasing and transmitting its not so delicate perfume.
From the time the corm produces its first shoot to the time the three-day bloom is over takes less than a month, and due to the long six to ten year period the corm needs to store energy for this show the whole process can be very difficult to predict, and will only occur 4–6 times in the plant's 40 year lifespan. Wild specimens are thought to bloom more frequently, perhaps every 1 to 3 years.
Fruit
If it was not successfully pollinated, the spathe will droop to the floor and the spadix will begin to lean and dehydrate until about a week after it first opened, when it finally falls over. The stalk will break off, and six to ten years later the corm will begin to grow a new shoot.
If it was successfully pollinated, the spathe will drop off and the spadix will collapse, but after four months the base of the stalk will erupt into attractive bright orange fruits (which are poisonous to humans) containing the plant's seeds. Pollinating the flower is a difficult task, as self-pollination has only ever succeeded once (Huntington Botanical Gardens in August 1999) and the flower's unpredictable blooming cycle makes it difficult to obtain fresh pollen for cross-pollination. The almond-like seeds inside the fruit are thought to be distributed by birds and will grow into new corms to start the cycle over again.
Endangered species
The titan arum was discovered on the island of Sumatra in 1878. These massive and dramatic plants are endangered in the wild as their natural range is suffering from human development and encroachment. Their long lifespans and rare blooming cycles make them poorly adaptable to the quickly-changing environment once man moves in. Nobody knows how many currently exist in the wild.
They are also difficult to grow in greenhouses, requiring heat, humidity, and ample water, and some specimens in captivity have never bloomed at all. When they do bloom, successful pollination is difficult. Any display of its dramatic bloom is sure to attract a huge crowd, with people willing to line up for hours to see (and smell) the world's largest flower. Just blooming is cause for attention. Successfully bearing fruit is cause for celebration.
Most of the botanical centers in the US owe their titan arums to James R. Symon, who collected wild fruits and brought them back to the US in 1993. Most of the rest of the world got theirs from a specimen in Bonn, Germany.
Sources:
http://www.npr.org/templates/story/story.php?storyId=1357271
http://www.qbgardens.com/blooming/corpseflowerinfo.htm
http://www.huntington.org/BotanicalDiv/TitanIntro.htm
http://news.nationalgeographic.com/news/2003/07/0718_030718_stinkyflower.html