by Colin Tudge
To be sure, tropical seashores are in many ways favourable for growth (nutrients, water, sunshine), but conditions overall are as tricky in their way as in any desert, or on any frozen tundra. So you might expect that only a very few, extremely specialist plants could live there. Yet mangrove forests worldwide contain many species of trees and shrubs and there is a core group of thirty or forty that occur in most of them, and these come from several plant families. The core group includes various ‘white mangroves’ from the Combretaceae family; the red mangrove, Rhizophora, and others from the Rhizopho-raceae family; Xylocarpus from the mahogany family, the Meliaceae; Avicennia from the somewhat recondite Avicenniaceae family in the order Lamiales (the order of teak and mint); and palms of the genus Nypa.
Independently of each other, the different tree families of the mangroves have evolved a series of physiological tricks to cope with the otherwise disastrous conditions in which they find themselves. Thus, the tissues that form a sheath around the xylem of the roots provide ‘ultrafiltration’, preventing the salt from entering the conducting vessels and thus polluting the rest of the plant. All mangrove species can do this but some are particularly good at it, including the red mangrove. Some that are less efficient at filtering out the salt, like Avicennia, do absorb at least some salt: but then they excrete it actively (by processes that use energy) from special glands in their leaves, where it may dry in the sun to form palpable crystals. Many seabirds do the same thing through their beaks.
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 per cent 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 re-emerge 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 pneuma-tophores. 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 to aerate their roots; wind and fleets of obliging animals to 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 has 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 then the rubbery sap within (the latex) runs bright blue. Analysis reveals that the latex contains 11 per cent of nickel by weight – and it accounts for an extraordinary 25 per cent 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, as mangroves exclude salt. 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, for example by mining. There are many examples worldwide of such reclamation. It has even been mooted that soils might be freed of toxic metals by growing hyper-accumulators 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 hyper-accumulation of metals remains a botanical oddity.
So trees make use of what the soil and the atmosphere has 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 of 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, and 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 towards 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 towards the light (as in stems) is called ‘positive phototropism’; and 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 limes, 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 th
e aid of a remarkably short shortlist 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 to communicate: they are a moveable 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 goes 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 auxin, the gibberellins, abscicic 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; and 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 many indeed seem ridiculously few. But when each can have different effects (depending on the receptors), and when they act in permutations, then 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 towards the light; and when the light shines from the side, they bend towards 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, it finishes 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, and is known as auxin.
But auxin does not always act as a growth promoter. 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: some time 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 time 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 stops. 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 limes, are often lopped to produce neat tops like mops or lollipops: a straight stem, and then a crown of more or less equal branches.
Auxin also prompts 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 auxin to help things along. In similar vein, a steadily growing catalogue of valued trees – including coconuts and teak – are now raised by ‘micro-propagation’: the production of whole plants from cells grown in culture. Auxin is essential in this (though it is not the only hormone involved).
Fruits won’t normally develop unless the flowers are first fertilized, and so produce seeds – and it’s 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 auxin, and so we are now regaled with seedless tomatoes, cucumbers, aubergines and grapes. Cultivated bananas produce seedless fruits as a matter of course. Presumably they contrive to produce auxin even in the absence of seeds.
The role of auxin in prompting trees to drop their fruits and shed their leaves in autumn (‘abscission’) is still uncertain. Auxin levels drop as the leaves fall, but correlation is not cause. Yet the addition of 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 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, auxin has been modified to make weedkillers, basically by promoting over-rapid 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 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 US.
The gibberellins, also discovered in the 1920s, also promote growth as auxin does – but again, they do many other things as well. In particular, they are highly concentrated in immature seeds and help to 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, aubergines and grapes.
Soon after the gibberellins were discovered, abscisic acid or ABA came to light. In contrast to auxin and the 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 favourable. But despite its name, abscisic acid seems to pay very little part either in the shedding (abscission) of leaves or fruit.