The Tree (34 page)

Mangrove trees in general combat the airlessness of the soil by ingenious anatomy. Most have at least some aerial roots, directly exposed to the air. Their surface is perforated with lenticels, apertures that enable air to enter, and inside the root the tissue is spongy, with huge air spaces between the cells that may account for 40 percent of the total volume. Many have stilt roots, mostly out of the water. In
Rhizophora
these stilt roots arch away from the trunk, enter the mud, but then may reemerge and form another loop, snaking along half in, half out. In some species, including
Avicennia,
the stilt roots thicken to form buttresses. Many species from unrelated groups have independently evolved “pneumatophores,” which grow vertically into the air to act as snorkels. In
Avicennia
the pneumatophores are thin like pencils. In other species they may be secondarily thickened, and develop into tall, substantial cones. As a final refinement, it seems that the air that does get into the roots is not left simply to find its own way around. The rising tide pushes the old air out; and when it recedes, fresh air flows in again through the lenticels and pneumatophores. Thus the roots of the mangrove trees effectively
breathe.
They use no muscle power to do this, as an animal must. The sea is their diaphragm. The tide serves to aerate their roots; wind and fleets of obliging animals spread their pollen and seeds. Trees just don’t need the elaborations of muscle and blood and nerves on which animals expend so much.

All in all, the main problems for the trees of the mangroves are chemical: all that fierce salt; too little oxygen. Yet, chemically speaking, the seashore is by no means the most hostile environment that the earth provides.

In particular, land that has been polluted by volcanoes—naturally polluted, that is—may contain an array of metals in concentrations that would be lethal to most plants and, indeed, to most life. Some of them are simply innately toxic. Others are present in most soils and may be essential in very small amounts but are lethal in high concentration. Yet again, many plants, from many different families, have evolved tolerance. So it is that an array of plants from various families have been definitely shown to be highly tolerant to nickel, zinc, cadmium, or arsenic. Others have been reported (though not confirmed) to withstand high doses of cobalt, copper, lead, or manganese.

Most outstanding is a tree from New Caledonia, the island that is so extraordinary in so many ways.
Sebertia acuminata,
of the Sapotaceae family, grows in soils rich in nickel. It does not exclude the metal from its conducting vessels, as mangrove trees generally contrive to exclude salt. Instead
Sebertia
accumulates the metal. Indeed, it accumulates so much that if the trunk or branches are damaged, the rubbery sap within (the latex) runs bright blue. Analysis reveals that the latex contains 11 percent of nickel by weight—and it accounts for an extraordinary 25 percent of the dry weight.

Exactly why some trees accumulate such metals (although few match
Sebertia
) is unknown. Clearly it’s a way of coping with metal-rich soils. But some other plants may grow in the same soil without accumulating the metals—simply excluding them. Perhaps the metal-or arsenic-rich stems and leaves are natural pesticides. Some experiments suggest that this is so; others give less clear results. In any case, such accumulators seem to offer a means of replanting and, indeed, reforesting land that has been polluted unnaturally, as by mining. There are many examples worldwide of such reclamation. It has even been suggested that soils might be freed of toxic metals by growing hyperaccumulators and then harvesting and removing them. But this could be too slow to be worthwhile and also raises the problem of what to do with the harvested metal-rich plants. Perhaps the metals might be recovered from them and pressed back into industrial service. But calculations suggest that this would rarely be worthwhile economically: it might be for nickel and cadmium, but not for zinc. Meantime, the hyperaccumulation of metals remains a botanical oddity.

So trees make use of what the soil and the atmosphere have to offer them; and so too they have evolved to endure the extremes of both. But they do not merely endure. Trees are not passive players. They are much more subtle than that.

HOW TREES KNOW WHAT TO DO (AND WHAT TO DO NEXT)

Trees live simple lives—or so it may seem to us: nothing to do all day but stand in the sun with their feet in damp and nutritious earth. But there’s a lot more to their lives than meets the eye. Trees, like all of us, have to do many different things, and they have to do the right things at the right times. Taken in the round, their lives are as intricate as those of Hamlet or Cleopatra, albeit without such conspicuous drama. Living is innately complicated.

All living things must respond to their surroundings, and trees respond in many ways. Many trees, like all plants, can move bits of themselves as the world changes around them, opening and closing their flowers or the stomata of their leaves; these movements are known as “nastic.” More broadly, all plants—including all trees—shape themselves according to circumstance, their stems growing away from gravity and toward the light, their roots generally doing the same in reverse. Trees do not simply grow: they grow
directionally,
most economically to fill the space available, and this directed growth is called “tropism.” Growth toward the light (as in stems) is called “positive phototropism”; growth away from the light (as in roots) is “negative phototropism.”

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 with 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.