by Colin Tudge
More cleverly yet, trees do not respond simply to the here and now. They anticipate what is to happen next. The deciduous types of the north, like oaks and birches and lindens, have to shed their leaves in autumn and send out their hopeful flowers in the spring—and both of these procedures, the shedding and the blossoming, must be prepared for weeks in advance: the shedding in the height of summer, the flowering at a time when all thought of tender growth seems ludicrous.
Finally, trees, like all of us, have to find mates and interact with them; and all trees of the same type must be sexually active at the same time, so each must know what the others are up to—or, at least, each must respond to the same clues of climate, or length of day, or whatever, so that all are coordinated. Many—particularly though not exclusively in the tropics—rely on insects, too, or birds or bats, to spread their pollen, and on yet more animals to disperse their seeds; and so they must attract their collaborators—and, again, must make sure that they do all that is necessary in due season. But trees too, like all of us, are besieged from conception to the grave by potential parasites and must find ways of warding them off.
Trees have no brains or nerves and instead run their entire lives with the aid of a remarkably short short list of chemical agents: just five basic hormones, plus a handful of pigments and a miscellany of other materials, through which they convey information to others of their own species or to other organisms, including those that would attack them. The hormones control their growth and hence their overall body form, the emergence of buds, and the shedding of leaves. Of the essential pigments, the interplay of just two in particular enables them to keep track of the seasons and to anticipate what is to come. The various agents by which they communicate with other trees and with animals seem diverse but, even so, belong to only three classes of chemical compounds. These are of the kind known as “secondary metabolites”—virtual by-products of metabolism. All green plants produce the basic five hormones and the principal pigments, but only some plants produce just some of the secondary metabolites that help plants communicate: they are a movable feast. The chemistry of animals, by which they coordinate their lives and communicate with others, is at least as complicated—yet they have nerves and brains as well. But then, a tree might ask, why bother with brains and all the expense and angst that go with them, when you can run your life just as well without?
HOW TREES SHAPE THEMSELVES
The five hormones by which plants run most of their lives are auxins, the gibberellins, abscisic acid (ABA), the cytokinins, and ethylene—a gas. Hormones in general (whether in plants or animals) affect body cells by interacting with receptors on the surface of the cell. The receptors in turn link up to secondary messengers within the cell, which transmit the information of the hormone to the parts of the cell that are supposed to respond. Immediately there is scope for further subtlety, because different cells have different receptors, linked to different secondary messengers. On cell A, a hormone may make contact with receptor X and have one effect; on cell B, the same hormone may be picked up by receptor Y and have a different effect. In short, each cell extracts from each particular hormone the information it wants to extract—just as any of us, reading a text, focuses on certain aspects of it. The message is partly in the words and partly in the particular interest of the reader.
In addition, the different hormones work together in pairs or groups. Sometimes two acting together will enhance each other. Sometimes a particular cell will not respond to a particular hormone unless some other hormone has first primed it to do so. Sometimes two hormones oppose each other’s action. And so on. Five hormones may indeed seem ridiculously few. But when each can have different effects (depending on the receptors), and when they act in permutations, the total amount of information that the simple few might convey becomes effectively infinite. Even so, the underlying simplicity of the system is wondrously elegant.
Among the first to study plant responses seriously were Charles Darwin and his son Francis. In particular, as they described in 1881 in The Power of Movement in Plants, they studied the way plants modify their growth according to the light: phototropism. The Darwins studied not trees in this context but oats, which are easier to work with—but what works for oats in the first few days of their life also works for oaks and redwoods and all the other forest giants through century after century.
Oats (like oaks and redwoods) grow up toward the light; and when the light shines from the side, they bend toward it. The Darwins showed that it was the region just below the growing tip that changes direction: bending occurs because when the light shines from the side, the tissue on the shady side grows faster than the tissue on the side that’s lit. They found, too, that if they put little opaque caps over the growing tips, the bending stopped.
About forty years later (in the 1920s), other biologists revealed the mechanism. The growing tip of the oat (or the leading apical bud in a twig) produces a chemical that flows down to the tissue below and prompts it to grow. But this chemical, it turns out, migrates away from light. So if the light shines from the side, the chemical ends up on the shady side of the stem—and so stimulates the growth on the shady side, and not on the illuminated side. Nothing could be simpler. An engineer who came up with such a scheme would warrant a Nobel Prize. The chemical involved is the first of the five major plant hormones: one of the auxins.
But auxins do not always act as growth promoters. An auxin flowing down from the apical bud of a twig or from the lead shoot of a tree suppresses the development of lateral buds lower down. If the apical bud is damaged, the flow of auxin ceases, and then the subsidiary buds burst into life. So in northern woods we may often see a conifer with a kink in the trunk: sometime in the past the growing tip must have been damaged, and the topmost lateral bud has taken over the job as lead shoot.
Often, too, the trunk of a tree seems simply to stop, and out of the top grows a mass of branches like a bush. Again, the terminal bud at some point in the past has been removed, and not one but in some cases dozens of lateral buds beneath have been liberated as the flow of suppressive auxin stopped. Sometimes this happens in nature—I remember a huge Terminalia tree at the Indian Forestry Research Institute that took this form; in the 1940s, so the FRI’s Dr. Sas Biswas told me, the young tree had been cut off by lightning. Disease, too (like the shoot-boring caterpillars that infest mahoganies), may produce this effect. But in addition, foresters and horticulturalists may cut the tops of trees deliberately to provide an instant source of sticks and staves from the lateral buds below. This is called “pollarding.” Pollarded willows, hornbeams, oaks, hazels, and chestnuts have for many centuries been major industries in Europe. Some of England’s best-loved woods are former pollard plantations. Topiary, too, makes use of this effect. Yew, privet, beech, and box all make respectable trees if left alone—but if they are clipped the surface is crammed with subsidiary twigs that otherwise would remain repressed. City trees, such as planes or lindens, are often lopped to produce neat tops like mops or lollipops: a straight stem, and then a crown of more or less equal branches.
Auxins also prompt cut stems to produce roots of the kind known as “adventitious,” which are those that grow directly from stems. Many trees produce adventitious roots in various circumstances—indeed, as we saw in Chapter 7, all the roots of a palm are adventitious. Growers make use of this propensity. They dip cuttings in an auxin commercially known as “rooting hormone” to help things along. In similar vein, a steadily growing catalog of valued trees—including coconuts and teak—are now raised by “micropropagation”: the production of whole plants from cells grown in culture. Auxins are essential in this (though they are not the only hormone involved).
Fruits won’t normally develop unless the flowers are first fertilized, and so produce seeds—and it’s an auxin produced by the seeds that makes the fruit grow fleshy. Pick off the seeds from a strawberry, and the succulence stops. But some plants will produce seedless fruits if treated artificially wit
h an auxin, and so we are now regaled with seedless tomatoes, cucumbers, eggplants, and grapes. Cultivated bananas produce seedless fruits as a matter of course. Presumably they contrive to produce this auxin even in the absence of seeds.
The role of auxins in prompting trees to drop their fruits and shed their leaves in autumn (“abscission”) is still uncertain. Levels of a certain auxin drop as the leaves fall, but correlation is not cause. Yet the addition of this auxin can prevent leaf fall; and so it is applied commercially to stop the leaves and berries falling from decorative holly, and to keep oranges firmly on their branches until the pickers are ready. On the other hand, large amounts of another auxin can promote fruit drop—and so it is sometimes deployed to thin crops of apples and olives, enabling the remaining fruit to grow bigger.
Finally, another auxin has been modified to make weed killers, basically by promoting overrapid growth. This has great commercial value in agriculture and is not all bad: chemical control can be relatively benign for wildlife, if used selectively and decorously. But an auxin has also been modified to make the infamous Agent Orange, which was used to defoliate trees in Vietnam and thus (or so it was intended) to reveal the Vietcong. The policy was horribly destructive of wildlife as well as of people and was of very limited military use, not least because the Vietnamese army dug themselves in and lived largely underground. Agent Orange also contained dioxin as a contaminant, which causes horrible blistering of the head and body, and is probably carcinogenic. Thus the findings of science may be corrupted. Manufacture of Agent Orange is now banned in the United States.
The gibberellins, also discovered in the 1920s, also promote growth as auxins do—but again, they do many other things as well. In particular, they are highly concentrated in immature seeds and help break their dormancy. Gibberellins too, like auxins, can cause fruits to flesh out even in the absence of seeds. So they are used to produce seedless apples, mandarins, almonds, and peaches, as well as currants, cucumbers, eggplants, and grapes.
Soon after the gibberellins were discovered, abscisic acid (ABA) came to light. In contrast to auxins and gibberellins, its prime role is to suppress cell division and expansion—in other words, to suppress tissue growth. It also serves in seeds to suppress premature germination—germination before conditions are favorable. But despite its name, abscisic acid seems to play very little part in the shedding (abscission) of leaves or fruit.
The fourth of the major plant hormones, the cytokinins, have the opposite effect of ABA: they prompt cells to divide. The cytokinins were first discovered in 1941. They turned up first in coconut milk, but they are now known to occur in all plants. In some contexts cytokinins act in opposition to auxins and override them; so horticulturalists may use cytokinins to make side buds sprout, even when the apical bud is still in place.
The fifth of the basic hormones is ethylene, which affects many different aspects of a plant’s life, from the ripening of fruit to the falling of leaves and a great deal more besides. Ethylene is a chemically simple gas, and it may seem an odd choice for a hormone, for gases are wayward clouds of molecules that seem too unruly for precision work. But then, one of the surprises of recent years has been that the physiology of animals—not least of human beings—is profoundly influenced by nitric oxide, which is even simpler than ethylene. Like ethylene, nitric oxide seems to be involved in just about every system that has been looked at. It is the key even to Viagra—which operates by prompting release of nitric oxide, which, in turn, in this context, acts as a muscle relaxant and so allows engorgement. Ethylene does not act simply as a hormone within any one plant. It may also travel from plant to plant and so acts as a pheromone: an airborne hormone that acts on creatures other than the one that produces it. Whether nitric oxide also behaves as a pheromone, as ethylene does, is a most intriguing question.
Ethylene’s role as a hormone was first revealed in the 1880s, when the trees that lined the streets in many a city began to lose their leaves. German scientists were the first to find the reason: the streetlights were run on gas, and some of it escaped unburned. In 1901 a scientist named D. Neljubov showed that none of the several components of the gas had any defoliating effect, except ethylene. Ethylene was active at astonishingly low concentrations: 0.06 parts per million (or 6 parts per 100 million). On the other hand, the light from streetlights can prompt city trees to retain their leaves, so that the same tree may lose its leaves on the shaded side, but keep them far longer on the side nearer the lamp. City trees in the days of gaslights must have been horribly confused.
Ethylene soon proved to have other effects too. It causes fruit to drop, as well as leaves: and, like auxins, ethylene is used to thin commercial crops of plums and peaches, and to loosen cherries, blackberries, grapes, and blueberries in preparation for mechanical harvesting. Ethylene also promotes ripening, and herein lies another bizarre tale. In the early 1900s growers ripened fruit and walnuts in sheds that they warmed with kerosene stoves. But the more go-ahead growers switched to electricity. This was altogether more satisfactory: cleaner, more reliable, more modern. The only trouble was that the fruit no longer ripened. It wasn’t the warmth that did the ripening. It was the ethylene, leaking from the smelly and despised old heaters.
So it is that plants control their form. Darwin wrote of the English wayside as a “tangled bank,” and in the jungle the tangle can be beyond unraveling. Yet each plant in the melee knows what it’s doing. Each contrives to position its leaves in the light, and send its roots to the ground (except for a few specialist epiphyte orchids, which send some of their roots upward), while the vines wrap around others for support. In short, all in the apparent havoc know exactly what they are doing. Each adjusts its shape to the conditions, and to the presence of the others; and all this achieved, it seems, through astonishingly simple chemistry. The workings of trees, like plants in general, are indeed wonderfully elegant.
But trees do not dwell only in the present. They remember the past, and they anticipate the future.
THE PAST AND THE FUTURE: MEMORY AND ANTICIPATION
How trees remember, I do not know: I have not been able to find out. But they do. At least, what they do now may depend very much on what happened to them in the past. Thus if you shake a tree, it will subsequently grow thicker and sturdier. They “remember” that they were shaken in the past. Wind is the natural shaker, and plants grown outdoors grow thicker than those in greenhouses, even in the same amount of light; and so it is too that parkland oaks grown in splendid but breezy isolation are much more sturdy than those of the forest. Similarly, a larch tree remembers attacks by caterpillars. In the year after an assault it produces leaves that are shorter and stouter than usual. (Larches are among the few deciduous conifers.) Short, stout leaves do not photosynthesize as efficiently as the thinner, longer kind, but they are better at fending off pests. In subsequent years, however, the once-infested larch can and does revert to normal foliage. By then, the population of predatory moths would have plummeted, since the offspring of the original plague of caterpillars (which became moths) would have found nowhere to feed.
Most trees, like most plants of all kinds, are also aware of the passing seasons: what time of year it is and—crucially—what is soon to follow. Deciduous trees lose their leaves as winter approaches (or, in the seasonal tropics, as the dry season approaches) and enter a state of dormancy. This is not a simple shutting down. Dormancy takes weeks of preparation. Before trees shed their leaves they withdraw much of the nutrient that’s within them, including the protein of the chlorophyll, leaving some of the other pigments behind to provide at least some of the glorious autumn colors; and they stop up the vessel ends that service the leaves with cork, to conserve water.
How do the temperate trees of the north know that winter is approaching? How can they tell, when it is still high summer? There are many clues to season, including temperature and rainfall. But shifts in temperature and rain are capricious; they are not the kind of reliable signal to run your
life by. Sometimes a winter may be warm—but frost is never far away. Some autumns and springs are freezing, some balmy. The one invariable, at any particular latitude on any particular date, is the length of the day. So at least in high latitudes, where day length varies enormously from season to season, plants in general take this as their principal guide to action—while allowing themselves to be fine-tuned by other cues, including temperature. So temperate trees invariably produce their leaves and/or flowers in the spring, marching to the rigid drum of solar astronomy; but they adjust their exact date of blossoming to the local weather. This phenomenon—judging time of year by length of day—is called “photoperiodism.” Most of the basic research on photoperiodism has been done on crop plants, which for the most part are herbs. But trees and herbs work in the same way. What applies to spinach and tobacco applies to trees too.
Knowledge of photoperiodism again dates from the 1920s, when agricultural scientists in America found that plants like tobacco, soybeans, spinach, and some wheat and potatoes would not flower if the days were shorter than a certain critical number of hours (often around twelve). But other plants would not flower if the days were too long: strawberries and chrysanthemums were among those that remained resolutely sterile if the days were longer than sixteen hours. There were some, though, that didn’t seem to mind the length of day. The three groups became known as “long-day,” “short-day,” and “day-neutral.” Long-day plants generally flower in high summer, and short-day plants in spring or autumn. As a further refinement, plants also seem “aware” that absolute day length has different significance at different latitudes. At very high latitudes, the longest days are twenty-four hours—the sun never sets—and a fourteen-hour day is of modest duration. But in the subtropics, fourteen hours is a long day—as long as any day gets. Sometimes the same species may grow both at high and at low latitudes, including, for example, the aspens of North America. Then the northern ones will treat a fourteen-hour day as short, and the more equatorial ones will treat a fourteen-hour day as long. Adaptation is all.
