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Counterpoints in Science The Abominable Mystery Plant of the Namib
Namibia in southwest Africa, where I went on a three-week safari last July, is a vast parched land of mountains, canyons, and deserts. It offers many attractions, including the lowest human population density of any nation on Earth, vast animal parks like Etosha, and the world's largest meteorite. But for me, the high point of the trip was a visit to the Welwitschia Plains in the Namib Desert, a fascinating moonscape of weathered granite and marble featuring one of the world's strangest plants, Welwitschia mirabilis. What's a Welwitschia? That question was the apt title of an article by Edward S. Ross in the Fall 1994 issue of Pacific Discovery. During the past few years, botanists, paleontologists, and researchers in molecular evolution have been trying hard to find the answer. When Friedrich Welwitsch, an Austrian physician and naturalist, first saw the plant that bears his name on September 3, 1859, he was so astonished at this assemblage of large leaves sprawled out on the desert floor—a luxuriant growth that seemed to belong in a tropical forest rather than the driest of deserts—that he knelt on the hot sand and stared in bewilderment, wondering whether this weird apparition was a mirage. He had already described 550 new species but had never seen anything like this. Mirabilis is the Latin for wonderful or astonishing, and Charles Darwin's friend Joseph Hooker, who gave the plant its name, thought it the most interesting but ugliest plant he had ever seen. "Welwitschia resemble giant octopuses stranded on the desert floor," writes Chris Bornman, a South African botany professor who studied at the University of California at Davis. They possess two enormous olive green, leathery leaves that are the longest lived of any plant's. Other plants like the creosote bush can live longer than Welwitschia, but their leaves fall off and are replaced. Welwitschia leaves grow at the rate of about five inches per year throughout the life of the plant, arcing out from the thick woody stem onto the desert floor, where the tips are roasted. The leaves split into strips as they are whipped about by the wind. The huge leaves, which can reach several hundred square feet in size, seem paradoxical in a desert environment where most plants survive by reducing surface area to counter loss of water by evaporation. The Namib's other plants have tiny leaves or none at all. How can Welwitschia survive and thrive in an environment where the average annual rainfall is less than an inch? Welwitschia is found only in a fairly narrow strip of land extending from central Namibia to southern Angola. This stretch of Africa's southwest coast features heavy fogs, similar to those in San Francisco. The prevailing westerly wind carries the morning fog from the Atlantic coast out over the desert. The secret of Welwitschia's survival is that it absorbs fog droplets through millions of tiny pores on both surfaces of its leaves, and closes the pores at about 10 a.m., when the fog burns off. Welwitschia provides shade, shelter, water, and food for many desert organisms, including birds like Layard's chat, snakes like the horned adder, lizards, chameleons, scorpions, spiders, and distinctive red beetles and black beetles. Looking down on a Welwitschia, one sees a bustling habitat. The largest and oldest plant here, believed to be about 1,500 years old, is a veritable oasis for desert creatures. There are two groups of seed-bearing plants; the more ancient gymnosperms ("naked seeds") that include pines, redwoods, podocarps, and the primitive cycads and gingko, and the angiosperms, the flowering plants with seeds embedded in a "seed vessel," such as an apple or avocado. Presumably some gymnosperm was ancestral to the angiosperms, but the linkage between these groups has baffled botanists for more than a century. Darwin even tried to figure it out and was so frustrated that he called the origin of the angiosperms "the abominable mystery. Some botanists have thought Welwitschia might be the key to that mystery. Welwitschia has been a favorite candidate for the missing link between gymnosperms and angiosperms. It possesses many angiosperm-like attributes: large net-veined leaves, a flower-like male reproductive structure, double-skinned ovules on the female plants, and seeds and sap-conducting vessels like some angiosperms. Welwitschia has separate male and female plants, both of which have cones, like the conifers. The male cones bear pollen that under the microscope looks like rugby balls; the female cones bear distinctive winged seeds. Botanists have been debating Welwitschia's evolutionary relationships with conifers and angiosperms for nearly a century and a half, without coming to any consensus. The fossil record has not been very helpful either in solving Darwin's abominable mystery. Innumerable fossil species have been proposed by one paleo-botanist or another as being "the earliest angiosperm," only to be shot down by other experts. New candidates continue to be nominated, one of the latest being Archaefructus, of Jurassic age (about 150 million years old) from northeast China, discovered by Ge Sun and colleagues at Nanjing Institute of Geology and Paleontology (Science, November 27, 1998). They maintain that their fossil, unlike most other contenders, has the right kind of enclosed ovules to be an angiosperm. During the past two decades, many evolutionary puzzles of this kind have been approached by the methods of molecular evolution. Indeed, I first heard about Welwitschia while working on a molecular family tree of the conifers, in collaboration with two graduate students from Berkeley, Robert Price and Jeanine Olsen. We constructed our family tree of pines, redwoods, podocarps, and several other groups of conifers by comparing their seed proteins, using the technique of radioimmunoassay, and published our results in the journal Systematic Botany. Price was eager to include Welwitschia in the comparison, because of the plant's disputed evolutionary status, but we were unable to obtain seeds at that time. During the past year, two other groups of molecular researchers, from the Institute of Botany in Taiwan and the Institute for Genetics in Braunschweig, Germany, have reported comparisons of Welwitschia with angiosperms and conifers. One group studied ribosomal RNA of various plants, and the other looked at chloroplast DNA. Ribosomes are the protein-manufacturing centers; chloroplasts are the chlorophyll-bearing organelles that give plants their green color. Both sets of researchers found Welwitschia and its division, Gnetopsida, more closely related to the conifers than to the angiosperms. These results pretty much ruled out Welwitschia as the earliest angiosperm and left Darwin's abominable mystery unsolved. If Welwitschia is not the missing angiosperm ancestor, what is? The answer came much sooner than expected, in last year's August 13 issue of Science, and as in all good mystery stories, the identity of the perpetrator was something of a surprise. Fed up with all the confusion about plant phylogeny, Brent Mishler, a botanist at University of California at Berkeley, initiated an effort to build a family tree for all 500,000 known green plants. The project, called Deep Green, involves 200 scientists from twelve countries. Pamela and Douglas Soltis, a husband-and-wife team at Washington State University in Pullman compared the chloroplast DNA of 560 species. Of the living angiosperms, most botanists have considered magnolias or water lilies as the most "primitive," based on their floral structure. But Deep Green disagrees. The most basal angiosperm, lying closest to the root of the family tree, according to the Soltises’ DNA comparisons, is a virtually unknown shrub called Amborella, with bright red fruit and creamy yellow flowers, that is found only on the island of New Caledonia in the South Pacific. The Soltises’ results were confirmed by three other research groups. Preliminary reports from Deep Green do not mention Welwitschia. Though acknowledged as a primitive plant, Amborella had previously garnered little attention. But you may be certain that battalions of botanists are even now winging or sailing their way to New Caledonia to investigate. If Welwitschia is the platypus of the plant kingdom, as Darwin dubbed it, Amborella could become its coelacanth, a living fossil unexpectedly flushed out of hiding by the zeal of those wishing to study it. As I was writing this article, I opened my San Francisco Chronicle of August 28 to find a delightful piece about Amborella and Deep Green by science editor David Perlman. He reports that one of the few places Amborella can be found outside New Caledonia is the Arboretum of the University of California at Santa Cruz. Professor Ray Collett, who founded the Arboretum, asked his students to bring him plants from their foreign travels. Twenty-five years ago, Virginia and Todd Keeler-Wolf spent a year in the South Pacific and transported Amborella cuttings from New Caledonia to Santa Cruz, little suspecting that this unassuming shrub had royal sap in her stem. I’m looking forward to going over to the Arboretum to pay homage to the new princess of plants. Meanwhile, though, my heart still belongs to her ugly stepsister Welwitschia, the wonderful, paradoxical, abominable fog-drinker of the Namib. I can understand why Friedrich Welwitsch, though he discovered hundreds of species of plants and animals, chose to have an image of Welwitschia carved on his gravestone.
Jerold M. Lowenstein is professor of medicine at the University of California in San Francisco and chairman of the Department of Nuclear Medicine at California Pacific Medical Center in San Francisco. jlowen@itsa.ucsf.edu |
Winter 2000
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