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). The mistletoe lineage arose from just one of these beginnings, he says.
Hitching a ride
Mistletoes’ attacks—as good a place as any to start tracing their life cycle—usually begin with a seed hitching a ride on or in an animal, usually a bird. For instance, the seeds of the European mistletoe spend about 30 minutes in transit as they pass through a bird called a mistletoe thrush. A trip through a more specialized mistletoe-eating bird, such as one of the Indonesian flowerpeckers, can take only 4 minutes.
A species of Chilean mockingbird provides most of the transport for a mistletoe called quintral or Tristerix aphyllus. This plant requires the rather large favor of being deposited on the spiny surface of a cactus. An oddity even among mistletoes, quintral lives almost entirely deep inside the sugar-transporting tissue of the cactus. The mistletoe shows itself only during the Chilean winter, when its brilliant red tubular flowers burst out of the cactus column.
When the quintral mistletoe berries ripen to a pale pink, birds dodge the spines to nab a feast. The diners often deposit seeds for the next generation on cacti that already host some quintrals. Even a near miss that just snags a seed on a spine works for the quintral. The seeds germinate within a day of arrival, and each sends out a red strand that can extend a millimeter or two a day for up to 8 weeks or until it reaches the main body of its host.
Could mistletoe mooching push the cactus to evolve ways to discourage birds from dropping seeds on it? In the June Ecology, Rodrigo Medel of Universidad de Chile in Santiago reported that the quintral might be driving at least one host species toward longer spines. Among the big columns of the cactus Echinopsis chilensis, the plants with the longest spines had the fewest and shortest visits from perching birds. From the cactus’ point of view, that must seem a trend worth encouraging, Medel notes, because mistletoe-ridden plants produced less than half the seeds of uninfected plants.
In Eulychnia acida, the other cactus host that Medel studied, mistletoe infection didn’t seem to sabotage reproductive success. The underlying difference isn’t clear, and Medel warns against oversimplifying notions about relationships among hosts, parasites, and their carriers.
A few mistletoes “have emancipated themselves from birds,” as Kuijt puts it. Australia’s Christmas bush needs only a breeze to deliver its winged seeds. In the Northern Hemisphere, dwarf mistletoes (Arceuthobium) depend on tiny, green, exploding fruits that can shoot a seed 24 meters per second. When the seeds mature, at only a few millimeters long, the flower reaches an explosive point where a slight jiggle prompts it to launch its seeds for distances that commonly extend 5 m and can reach 15 m. It’s a silent shot—people strolling in the woods don’t hear popping mistletoe. By developing a sharp eye for flowers, Kuijt has learned to identify blossoms about to pop and gives them a gentle prod.
For the next stage of a freeloader’s life, settling into a host, mistletoes have some advantages over many other parasitic plants. Mistletoes tend toward the generalist’s rather than the specialist’s style of attack, Kuijt says. The European Christmas mistletoe takes up residence on any of more than 200 plant species, and a distant relative, Dendrophthoe falcata, infiltrates at least 343 host species.
Some mistletoe species, however, live on a more restricted diet of hosts. The dwarfs specialize in conifers, sometimes only a single species.
Botanists have found three small mistletoes almost exclusively on other species in the group. A report from the 1920s even describes a parasite triple-decker: one mistletoe attacking a second, which grew on a third. And the third grew on a tree.
Kuijt argues that people often shift host relations for one of South America’s most entertaining mistletoes, Gaiadendron punctatum. This glossy-leafed beauty with long, slim, yellow trumpet flowers inhabits the rainforest’s roof garden atop giant, old trees. The mistletoe doesn’t sink its strands into the supporting tree but preys on other plants specialized to perch in the canopy.
As people clear these grand forests, the Gaiadendron isn’t dying out. Instead, it often grows as a tree itself, still parasitizing small plants, but on the ground. Kuijt has encountered one such mistletoe tree with a trunk more than 25 centimeters in diameter.
After arriving on a host, a typical mistletoe seed’s first exploratory root grows away from light, and once in shadow, grows upward. When it bumps into something—in the best case, a young branch—the mistletoe sends in special wedge tissue in search of the plumbing. Hitting the host’s network of water-carrying cells deep inside the plant, the mistletoe builds its own system of ducts to steal water and nutrients.
With their parasitic lifestyle, mistletoes kick up a mix of emotions in people. While some observers are struck by the plants’ beauty and ingenuity, others worry about the well-being of the hosts.
All the sapping of host resources disturbs pathologist Brian Geils of the U.S. Forest Service’s Rocky Mountain Research Station at Flagstaff, Ariz. He’s one of the editors of a guide to dwarf mistletoes, Agriculture Handbook 709. Geils, who describes his job as “public health for trees,” frets that these parasites are getting out of hand.
Mistletoes attack young trees more readily than they infiltrate thick-barked oldsters. The dwarf mistletoes prey on conifers primarily in western North America. “We probably have a lot more young trees than we used to,” Geils says. Logging, replanting, and other reshaping of the landscape have shifted the balance away from old-growth granddaddies toward arboreial teenagers.
Also, suppressing forest fires has reduced natural pruning of dwarf mistletoes.
People passing through a forest may dismiss mistletoes as relatively harmless frills on the trees, since these parasites don’t seem to do much damage. “The tree’s got to sit out there for 100, 200 years,” Geils protests. A human being just doesn’t have the attention span to understand the destruction under way, he says.
Dwarf mistletoes can weaken or kill trees by sapping their resources and making them easy targets for marauding insects such as bark beetles, Geils warns. Also, the mistletoe’s growth compounds can send a tree into a tizzy of sprouting weird, ultradense foliage called witches’ brooms. Neither beetles nor brooms please the timber industry. A survey by the Canadian Forestry Service blames dwarf mistletoes for the loss of wood worth roughly $1 billion (U.S.) each year in North America.
Discouraging the growth of dwarf mistletoes has not turned out to be an easy matter, Geils says. Plenty of herbicides could kill the parasite—and the tree to which they’re intimately connected. Cutting down infected trees would slow the spread, Geils says. However, such proposals have proved contentious and even provoked suits by citizens with a different view.
For one thing, the witches brooms improve bird habitat. Forests with abundant brooms have three times as many cavity-nesting birds as other forests do, according to studies in several states. And research published in 1995 found that the Mexican spotted owl, a species listed as threatened in the United States, prefers mistletoe-infected tree stands to uninfected ones.
Some people oppose cutting down trees because they don’t want to be left with stumps, and others see mistletoe as a natural force in the forest.
Mistletoe “is quite the political hot potato around here,” Geils sighs. He’s pinning his hopes on work by Simon Shamoun at the Canadian Forestry Centre in Victoria. Shamoun has found two fungi that don’t hurt trees but kill mistletoes. Colletotrichum gloeosporiodes strikes the leaves and stems, and Nectria neomacrospora attacks the parasite’s tissues inside the host. Shamoun plans to combine them in a mistletoe-control cocktail.
On the far side of the world, Dave Kelly and other New Zealand ecologists are working to save mistletoes. In the austral summer, Peraxilla tetrapetala bristles with clusters of narrow, hot-pink buds, which eventually open upside-down. The petals stay connected at their tips but split apart from each other at the stem end. Kuijt’s ended his 1969 account of the flower, “We cannot even guess at the meaning of this bizarre performance.”
Kelly and Jenny Ladley, both at the University of Canterbury, published the answer to the mystery in 1995, based on surveillance videos of the flowers. The blooms do open from the top but only if given just the right twist by a bird. Then, the pink petals rip apart, flinging pollen onto the bird’s head.
Bellbirds and tuis have the knack. It turns out that some small bees can wrestle the flowers partially open to get at pollen. This is the first instance in which scientists have found invertebrates that can work trick flowers apparently tailored for vertebrates (SN: 12/21&28/96, p. 390:
https://www.sciencenews.org/pages/sn_arch/12_21_96/fob2.htm). The bees prove as effective as the birds in pollinating blooms, according to recent observations.
Unfortunately, competent birds and bees have grown so scarce at two sites on the South Island of New Zealand that the twist-off mistletoe and a close relative, lacking sufficient pollination, have seen their seed output drop by 90 percent, reported Kelly, Ladley, and their colleagues in the June 1999 Conservation Biology.
Worldwide, more than 20 species of mistletoes have ended up in the endangered category of the Red List of the Switzerland-based World Conservation Union, or IUCN.
The London-based Plantlife and other British conservation organizations issued a more cheerful report a year ago on the supply of England’s traditional holiday mistletoe, V. album. As the first British mistletoe survey in 30 years, it found that the plant’s old haunts—in apple orchards—indeed were shrinking. However, other niches seemed to be providing ample habitat. At least for the time being, no one has to worry about kissing that mistletoe good-bye.