THE dead horse arum is a curious flower. On a gram-by-gram basis, the male parts produce more heat than any known species. More even than the flight muscles of a hummingbird. The heat helps dissipate the flower鈥檚 smell, which is said to resemble 鈥� you guessed it 鈥� a dead horse.
Hot or 鈥渢hermogenic鈥� flowers are those that, like us, can make their own body heat. If you find that hard to believe, try feeling a Philodendron bipinnatifidum, a garden plant from the Brazilian rainforest, the flowers of which are warm to the touch.
How hot do they get? The temperature increase can be dramatic, depending mainly on the size of the flower. Although the dead horse arum produces the most heat relative to its size, the heat-producing tissue is so small 鈥� only a gram 鈥� that the plant鈥檚 temperature does not rise very much. By contrast P. bipinnatifidum鈥榮 heat-producing tissue, which weighs 125 grams, can warm to over 40 掳C even when it鈥檚 near freezing outside. Some flowers can even regulate their temperature. The eastern skunk cabbage Symplocarpus foetidus, for example, remains at between 23 and 26 掳C at air temperatures from -10 to 27 掳C.
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Why do flowers create their own heat? In some species it appears to be a survival mechanism. The eastern skunk cabbage almost certainly produces heat to protect itself from the cold 鈥� it鈥檚 common to find it flowering in early spring in a melted pool of snow ( picture). But the majority of hot flowers are found in the tropics. Why do flowers that grow in hot places produce heat, you might ask.
The reason appears to be sex. It鈥檚 thought the heat helps disperse the smell of the flower to attract insects to pollinate it. Research by Roger Seymour at the University of Adelaide, Australia, supports the theory that in some thermogenic species the heat may be acting as a reward for pollinators.
How does that work? In thermogenic species, the male pollen-producing stamens mature around a day later than the female stigma. It鈥檚 in the flower鈥檚 interest to encourage pollinating insects to stay until the pollen is released. The insects, often beetles, rub off the pollen they are carrying from other flowers onto the ripe female parts and then pick up pollen from the male parts before leaving. Seymour has discovered in Philodendron solimoesense and the Amazon water lily that the reproductive chamber stays hot for at least a day after the pollinators arrive.
Far from being a prison, Seymour says the chamber is more like a nightclub where the beetles can hang out and even take advantage of the hot environment to mate. Everyone鈥檚 a winner: the plant gets pollinated, the beetles get lucky.
鈥淭he heat makes the flower like a nightclub where the beetles can hang out and mate鈥�
