Botany under the Mistletoe
Milius, Susan, Science News
Twisters, spitters, and other flowery thoughts for romantic moments
A holiday merrymaker loitering under the mistletoe may not be thinking much about parasitic plants. That's a loss, because the world's mistletologists are making wondrous findings about the more than 1,300 species they study.
Some of the plants have flowers with trick openings. Some shoot their seeds farther than most watermelon spitters can spout. Some mistletoes grow as parasites on other parasitic mistletoes. And some give North Americans and Australians yet another way to misunderstand each other.
All in all, when you bump into someone under a suspended sprig, there's a lot more to say than "Kiss me, you fool."
"I can get rhapsodic very quickly," says Job Kuijt of the University of Victoria in British Columbia. He ranks certain mistletoes among the world's most beautiful plants. True, he's devoted decades to working out their classification and might be suspected of a certain bias. Yet he's sure that most people would be wowed by the 11-inch-long tubular red-and-yellow flowers of a rare mistletoe in Ecuador and Peru or a more common species that ornaments the coast of Chile with great billows of pink and white flowers. And then, there's the Christmas mistletoe.
What's to marvel at there? Viscum album, the holiday mainstay in England, puts out flowers no bigger than a pea and sprouts white berries. In North America, most holiday mistletoe comes from the equally modest genus Phoradendron.
No, that's not what Kuijt means at all. For a spectacular holiday-season mistletoe, one has to go to southwestern Australia. There, what people call the Christmas bush grows into a tree up to 30 feet high and flares into orange-yellow blooms around December. Whereas most mistletoe plants are bits of shrubby fluff parasitizing trees, Australia's Nuytsia floribunda--itself a tree--grows an underground network that parasitizes smaller plants, such as grass and even the domesticated garden carrot.
The various Christmas bushes raise the question of just what botanists mean by mistletoe. It's not a strict taxonomic term in the sense that holly means one of the plants belonging to the genus Ilex.
Mistletoes represent some, but far from all, of the flowering plant order Santalales. The order is named for the renowned fragrance source, sandalwood, which is a parasite but not a mistletoe. Mistletoe generally refers to those shrubby, parasitic cousins of sandalwood that poke into their hosts aboveground instead of attacking roots. The Australian Christmas bush and several other oddballs therefore don't quite fit the pattern. However, they're such close kin to standard mistletoes that botanists lump them into the group, anyway.
With their varied qualifications, 1,306 species have made it into the mistletoe club, according to Dan Nickrent of Southern Illinois University in Carbondale. He would like to know how their odd lifestyle came about.
At this year's Western International Forest Disease Work Conference in Kona, Hawaii, he reported on his ongoing genetic analysis of mistletoes. Nickrent and his colleagues compared DNA sequences from representatives of all the big groups of mistletoes.
By clustering plants with similar sequences, he drew a most-probable pedigree. It indicates that the mistletoe lifestyle evolved independently at least five times, says Nickrent.
Members of Viscaceae, the family of holiday cheer in North America and Europe, seem to have arisen from just one of these origins. A different origin gave rise to a family called Misodendraceae, Nickrent suggests. This family's scaly sprigs jut out of the branches of South American beech trees, and in blooming season, the female plants of several species grow silky blonde hair--swinging whisks of yellow filaments several inches long.
In all of the plant kingdom, parasitism evolved on nine separate occasions, says Kuijt, who wrote the classic Biology of Parasitic Flowering Plants (1969, University of California Press). …