Summer World: A Season of Bounty - Bernd Heinrich (2009)
Chapter 17. Moss, Lichens, and Tweedlaarkanniedood
THE ROBINS AND THE PHOEBES ON OR NEXT TO OUR house hopped out of their nests, adult-size, at two weeks of age at the most. They were aided first by the warmth of a brooding parent, and then they warmed themselves by their own metabolism. The plants’ growth, stimulated by the warm summer days, was as impressive. Rachel kept track of what grew in the garden, and I was more focused on what went on outside it. On my daily jogs past a beaver pond I was especially impressed by how fast the stump shoots grew where the beavers had chewed off trees. Some ash shoots went up nine feet in a single season, and red maple shoots grew as much as sixty-six inches. They had been growing at a steady rate of almost an inch per day for the whole summer. Surprising as the fast growth was to me, I was even more impressed by how quickly it could come to a halt. Most trees stopped lengthening their twigs entirely by mid-June, when there were still three months of summer to come, but vines and some tree stump shoots (those in direct sunshine) kept right on growing at the same furious pace. Warmth and sunshine may translate into growth, but only if everything else is equal. In deserts there is plenty of both, but growth tends to be very slow.
Deserts are a source of marvels of survival in extreme summer, and so an extreme desert—one with the least water and the most heat—should be a place to find the most marvels of biological ingenuity. The Namib Desert along the Skeleton Coast of southern Africa provides examples of the exotic and the bizarre—silver ants, head-standing beetles, small plants that mimic stones to reduce water loss and avoid being detected by thirsty and hungry grazers, and a fern that can dry up and revive. The ferns that I knew about from Maine and Vermont grow in wet places, and when they run out of water they run out of life. But in the Namib I saw a fern that can dry and curl its fronds into a tight ball, and when wetted it uncurls and there it is—instant live fern, the “resurrection fern.” It is probably the perfect house plant, for me. But I never thought much about that fern until unexpectedly last summer in Vermont, when I turned our garden hose onto our serviceberry tree.
Our serviceberry (Amelanchier) tree is much like many others growing wild in the surrounding woods. We sometimes hang suet on it for woodpeckers and chickadees in the winter. Otherwise we pay no attention to it—except in May, when, several days before the trees leaf out, it erupts for a week in a mass of white flowers. That is also when the ground thaws, the time people used to bury the dead and hold funeral services here in New England (hence the serviceberry name). By June this tree bears purple berries (hence its other common name, Juneberry). Even before they are ripe, these berries already attract cedar waxwings, and then in late June and early July they also attract robins, along with rose-breasted grosbeaks, purple finches, wood thrushes, catbirds, veeries, tufted titmice, and cardinals.
In 2007 our summer, like most summers, had long dry spells. I got out the garden hose to water our serviceberry tree, remembering all the birds who feed there. I did not want its roots to dry up, because as with most plants, even a temporary absence of water kills. As I was idly spraying the ground beneath this slender tree I noticed for the first time what I had undoubtedly seen hundreds of times before: yellowish green moss on the rocks under the tree. Surely this moss would be totally dry! I bent down and peeled off a patch of it—dry indeed, dryer than a bone. I realized then that this moss was already many years old, and it must have been dry on many occasions during past summers.
I peeled off a handful of moss and put it into a bowl of water in the sun, and—presto—it soaked up water like a sponge; in seconds its slender fronds expanded and became vibrant green. It was just like the resurrection fern in the Namib Desert, which I had thought to be a unique marvel. In an hour small silvery bubbles (oxygen) formed on the submerged moss—it was respiring; it was alive. I put a sample of the moss back on the rock, where it again dried, more quickly than the clothes we put out on our clothesline. I then collected samples of five other species of moss from our woods. I dried and then wetted them at intervals of months. They froze outside in the winter, and then I brought them in again to dry out. I let a portion of them stay almost totally dry for six months, and when I dunked them in water they again soaked up water in seconds, and then looked as fresh and green as when I had picked them. I did the same with three species of club mosses (Lycopodium digitatum, L. clavatum, and L. obscurum). They all died when they dried, and once they dried, it was difficult to get them to absorb water. For a further contrast with real moss, I picked eight kinds of green herbs and let them dry. After a week the dried leaves looked a dull green, but when wetted (with difficulty) they were black and dead.
In mid-November, while I was up in a balsam fir tree in Maine, sometimes for hours at a time, I had further opportunity to admire the miracle of mosses. Right next to me on the limb where I was perched, I counted at least three species each of mosses, growing in part intermingled with as many species of lichens. Every limb was loaded with both, as were those of the neighboring trees. The ground beneath me was covered with already browning fallen leaves, but the rocks that poked out among them were covered with vibrant, luminous green cushions of moss in moist areas, and with lichens in more dry and exposed areas.
The lichens from the branches dried as quickly as moss. They also absorbed water as quickly, and then they seemed as vibrant as any in spring and fall, when they are normally wet all the time. The water-absorbing property of moss is of course well known, and sphagnum moss, especially, is a traditional diaper material used by northern peoples. In their deathlike state lichens are protected by several antibiotic chemicals, should any microbes attempt to consume them. Lichens are a cooperative association of a fungus and an alga, in which the alga provides carbohydrate for the fungus, and the fungus provides minerals and shelter for the alga. The summer had revealed common marvels, which I had seen before but not noticed. They remind me of the resurrection fern of the Namib, but another unique plant from there, the two-leafed Welwitschia mirabilis, is in a category by itself.
WELWITSCHIA IS NAMED AFTER FREDERICK MARTIN JOSEPH Welwitsch, an Austrian medic, naturalist, and collector who first found it in Angola on 3 September 1859, the year that Charles Darwin published On the Origin of Species by Means of Natural Selection. This plant resembles no others, and its evolutionary origin is still an enigma. It is the sole representative of its genus and the sole species within its plant family. Its Latin species name, mirabilis, means unique or wonderful. The Afrikaners of South Africa also call it Tweedlaarkanniedood (literally, “two-leaved-cannot-die”). This plant is also radically different physiologically from all other desert plants; its two huge leaves stay green and hydrated continuously, and it may live more than 1,000 and possibly 2,000 years.
Fig. 34. Welwitschia plant, a unique denizen of the Namib Desert that does not shed its leaves as most other plants do, and that stays hydrated when others dry out. It has two lifelong leaves that may grow (as they fray) for more than 1,000 years.
Other plants adapt to extreme heat and drought by having no leaves, or small leaves that are shed when water becomes scarce. Welwitschia mirabilis has two straplike leaves that are more than a yard in diameter and several yards long, and they are never shed. Like hair, they just keep growing from the base, and gradually wear off or disintegrate at the end. The functional (living) part of the leaf may be up to seventy years old and earns the distinction of being the oldest known living leaf tissue.
Leaves lose water primarily through their stomata, the pores needed for gas exchange, and most desert plants minimize the number of these microscopic openings and locate them on the lower leaf surface. Welwitschia’s leaves have about 250 stomata per 0.0016 square inch, more than most temperate and tropical plants, and these are located on both the upper and the lower leaf surfaces. In short, this plant is both a botanical and a physiological paradox of desert adaptation that could not have been fully appreciated by Welwitsch when he first found and described it, even though he wrote, “I am convinced that what I have seen is the most beautiful and majestic that tropical South Africa has to offer.”
Leaf stomates typically stay open during the day to allow carbon dioxide to diffuse in so that it can become fixed into small carbon compounds during the process of photosynthesis, in which sugar is produced. Water then necessarily evaporates, leaving passively through the open stomates, especially when the leaves are heated by the sun and the air is dry. However, most desert plants (in this case including Welwitschia), have evolved the ability to conserve water by closing their stomata during the day when water loss would be high, and they have a special mechanism that still allows them to perform photosynthesis. They conserve water by opening their stomates at night when the air is cooler and more saturated with water. And the carbon dioxide then enters (it can’t be used for photosynthesis just then, because there is no sunshine) following the diffusion gradient (from high concentration outside to low concentration inside the leaf ). The gas concentration is lowered inside the leaf as gas that enters is removed (by being incorporated and thereby stored) in malic acid. Then, in the daytime, the reaction reverses itself—the malic acid breaks down and releases the carbon dioxide, which stays inside because the stomates are closed then. The stomates are closed to conserve water, and the carbon dioxide then held captive is available to be used in the normal way to make sugar by photosynthesis. Despite this water conservation—a trick that is also used by many other desert plants—Welwitschia still needs water, and it employs a mechanism like that of the “butt-head” tenebrionid beetles who inhabit the same environment: the capture of water condensed from the air. It has no mouth, though, to suck this water up.
Welwitschia’s stomates are arranged in troughs formed by parallel ridges on its leaves. These ridges are much like the ridges on the tenebrionid beetles’ backs, and function as they do in catching water vapor. Water that condenses on cold nights runs between the ridges, along the troughs, where it is absorbed by these stomates.
According to NASA’s definition, life is “a self-sustaining chemical system capable of undergoing Darwinian evolution.” Almost all signs of the chemistry of life point to one origin here on Earth. Despite the great diversity of forms, the internal machinery of life on our planet may be so conservative because all life here evolved from common ancestry: all organisms are constrained by their evolutionary history. However, life as we know it is also constrained by the physical properties of the elements that compose it, and that possibly further constrain it into its specific configurations by temperature and pressure. We assume that life is not likely ever to be radically different from ours, or in all the billions of solar systems where it is almost certain to exist, to have existed, or to exist in the future. When (in 2007) astronomers discovered Gliese 581c, another of 227 new planets discovered thus far, it generated excitement because its distance from its sun and its size suggested that its temperatures probably ranged from 32°to 104°F, conditions that are just right for the possibility of liquid water. Hence it is one of the first planets thought hospitable to life. We automatically make the assumption that life requires certain conditions regardless of where it is found and that it would be like our life. But who is to say that oceans of ammonia or methane might not evolve other, bizarre, life?
In his book The Fitness of the Environment, Lawrence L. Henderson argued (in 1912) that the properties of matter, especially water and carbon, are requisite for life to evolve, and that possible abodes of life not unlike Earth must be a frequent occurrence in space. George Wald (in the preface to a reissue of this book in 1958) also assumed that life must exist elsewhere and it would be “life as we know it, for no other kind, I believe, is possible.” Henderson agrees, but concludes, “The biologist may now rightly regard the universe in its very essence as bio-centric.” Others now extend this to homo-centric. But if Welwitschia could speak it would say, “God has been kind and thoughtful to me above all others. He has given me two leaves, no more, no less, just exactly the right number that I need, and he has made them to last me a lifetime, and he has put me here in this environment that is so hospitable for me that I don’t need to move from the spot and can exist here. He supplies all my needs so that I can live without worries for centuries. The temperature—extreme summer to anything else—is perfect. I never overheat, and food is provided from the ground and the air. Water and carbon dioxide come to me in the foggy air at night. I’m in paradise. He has foreseen every little thing to make my life complete. Therefore, when he created the world, he must have had me, specifically, in mind.” Or the serviceberry tree, the lichens, and the mosses that thrive exactly two feet from my back door.