by Max Adams
The beech tree teaches us that if something is missing from the historical record, it does not mean that it was never there.
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Engineers
Hormones—Machines—Hydraulics—Bodgers—How high can a tree be?—Standing up straight—
TREE TALE: THE HAWTHORN
When a child is cutting a tree in the forest, it is the elders who know in what direction it will fall.
YORUBA PROVERB
Hormones
TREES ARE engineering masterpieces. The key problem that they have to solve—to get their solar panels (leaves or needles) as high up as possible while maintaining their supply lines to and from the earth—requires subtle solutions. Many things can go wrong. The extraordinary thing is that trees manage to overcome the challenge using just five hormones.
Hormones are like the switches and diodes in an electrical circuit, controlling signals and the relay of information, turning mechanisms on and off. In trees auxin is the most important. First identified in 1926 by the Dutch scientist Frits Went, auxin controls the growth of plant cells. But it does much more too, including preventing senescence in leaves and delaying fruit from ripening, and playing a vital role in the way that plants respond to light. The auxin present in the growing tip of a tree (its apical bud) prevents hundreds of lateral buds along a tree’s trunk and branches from growing. Under normal circumstances, these buds would be wasted, producing leaves where there is the most shade. When a tree is felled, cut down or damaged by storm, the apical bud is often broken and at that point auxin’s suppressing effect on lateral buds stops. This apparently simple piece of chemistry is what allows deciduous trees to re-grow from their stumps when felled and allows humans to coppice them in regular cycles.
Auxin’s role in determining how a tree responds to light was studied by Charles Darwin long before Went isolated it. Darwin observed plants leaning towards the source of light, a feature perhaps most obvious in the landscape when walking along a tunnel of trees arching over a road, and being the obsessively curious man that he was, he began to experiment by removing parts of the plants he was watching. He realized that it was some sort of receptor in the tip that pushed or pulled the plant towards the light. We now know that auxin pushes rather than pulls: it concentrates on the dark side of a stem, where it elongates the cells to bend the stem away from the shade.
The other tree hormones—cytokinin, gibberellin, abscisic acid and ethylene—are all implicated in one way or another in growth promotion, suppression, fruit development and ripening and controlling the various other chemical functions of the tree. Some work with each other, some work to hold other hormones in check. It would be nice and convenient if each hormone did just one job or had simple relationships with the others, but it is not so. Scientists are still a good way from understanding all the complexities.
Machines
There are supposedly six simple machines, from which all complex mechanisms have evolved. Each one is a means of converting one form of energy into another. The bow-drill converts linear motion into rotary motion. It is a relatively unsophisticated ‘compound’ mechanism. But even the most basic tool probably gave humans what is known as ‘mechanical advantage’. Humans are comparatively weak for their size, as anyone who has tried fighting a six-foot crocodile or a similarly sized wolf would be able to confirm. So people have to make up for their deficiency by replacing force with effort.
Perhaps the simplest version of the principle is the person carrying a heavy burden uphill. Taking the shortest route straight up is too difficult: one can hardly move. Taking a longer zigzagging route condemns the bearer to walk further but renders the task possible. Look at ancient trails the world over and you’ll find the zigzag path. This mechanism is called an inclined plane and it has myriad applications, most obviously in the humble metal screw—though wooden screws have been around for a long time, since Archimedes figured out how to raise water with one. The other obvious and early application is the ramp, much beloved of modern engineers trying to figure out how the pyramids were built. The next time you drive round and round a multistorey car park it’s worth appreciating how hard it would be to lift a car that height, even with an engine.
The inclined plane might easily have emerged without planning, strategy or forethought: it’s an almost subconscious means of saving energy, and even having invented it humans probably didn’t realize how clever they were being; after all, animals use the inclined plane to get up and down hills too. The wedge, which is no more than a small ramp, was the tool that led to our understanding of the principle of mechanical advantage: insert a triangular wedge beneath a rolling log to stop it; place its point against the grain of a log and the log will split with a blow from a mallet. The wedge shape itself is a natural derivative of this process, the result of splitting logs into segments.
From the wedge to the lever is a short intellectual step, one which demonstrates with mathematical precision how advantage works: lengthen the lever either side of its pivot and you get a visual grasp of the effect that the tool has on the object and the mover. The lever is merely another of the many adaptations of the stick, allowing humans to move or raise objects that they cannot otherwise lift. One only has to think of the shadoof, the elegant lever-operated water-sweep that seems to have been invented in Egypt before 2000 BC, to see how embedded the principle is in our cultural history.
And then there is the spring. Nature is full of inspiration for our ideas. A bendy tree is a spring; a coiled seed pod, like that of Indian balsam, is a spring. A catapult and a bow are simple, if not at first obvious, adaptations. Snares and traps, bows and the mechanisms of simple locks allow stored energy to be released suddenly: they are, in principle, the origins of the idea of batteries. Water lifted to a height, like my bespoke shower in the woods, is an energy-storage mechanism. Trees make such hydraulic efforts look puny by comparison.
Wells are such a crucial feature of agricultural subsistence in the fertile crescent of the Middle East that it’s hard not to think that another form of mechanical advantage, the windlass, did not develop there. But actually the idea of reeling in a rope or twine about a drum is another of those moments that does not, perhaps, require the inspirational Gestalt ‘aha!’ moment but is probably an innate skill: it may have been universal. Once string or rope was invented—and any child will independently make some sort of twine just by twisting a blade of grass, a hair or a tuft of wool—the thought of winding a length of it over a stick when fishing is almost subliminal. One might also argue that the biting fish, in fighting against the hook and drawing the line out from its spool, should be credited with the invention of the axle, even before the bow-drill. But I may be giving the fish too much credit. It’s one thing acting; quite another to see its significance for the range of adaptations it spores. Nevertheless, the axle is integral to the windlass, as it was to the later pulley, wheel and mill.
The wheel is not a natural, single-step progression from the axle. But it does seem to have been invented independently in more than one place. Before 2002, the oldest wheels that we knew about belonged to the early civilizations of Mesopotamia and the Indus Valley. Now there is an outstanding, well-dated candidate from a lake dwelling near Ljubljana in Slovenia, dated to about 3130 BC. It was made of two eighty-year-old ash planks battened together, with a rectangular hole cut in the centre to hold the oak axle. Since the three components were held together with wedges it is not clear how the bearings of this probable hand cart were mounted; but it was evidently the product of more than a single technological leap. And since this is a relatively sophisticated artefact, we ought to expect there to have existed something simpler even earlier.
The problem of the wheel is that it requires a fundamentally flat surface upon which to operate to its best advantage. Humans must manipulate their landscape. The indigenous peoples of South America independently came up with the idea of a wheel, but, partly because of the terrain of the Andes and partly because South America h
as no natural draught animals (the llama is not a cooperative beast), the early wheel there never developed a use beyond children’s toys. That excuse does not apply in the Banaue municipality in the Philippines. In a landscape equally precipitous but long-ago converted to rice-paddy terraces, the locals spend much of their time walking up to their fields (inclined planes again); but here they value the reverse effect of the incline: they make fantastic-looking scooters wholly out of wood and ride them back down to their homes. Every three or four years, in a great cultural celebration, they race them at crazy speeds downhill as part of the Imbayah festival.
The wheel is not, therefore, a foregone conclusion in societies that have discovered, by one or other means, the windlass and the axle. But the axle, once invented, stays, and even nomadic societies, constantly on the move, have uses for it. Along with the bow-drill comes the less glamorous but equally important spindle. Without the spindle there is no weaving, only the beaten wool fabric we call felt. Weaving requires thread, and thread must be spun to give it its strength. The drop spindle, no more than a notched stick with a weight attached (in effect the invention of the flywheel, did those early weavers but know it), is a truly wonderful thing: mesmerizing to those of us who haven’t been taught how to use it, and an elegant, efficient and portable means of living for those who have. One of my students brought hers in once, and while I was almost setting fire to the classroom (over-enthusiastic demonstration of a bow-drill) she had us all enchanted with the process of drawing out her wool, imparting spin to the spindle, then dropping it so that the twist was taken up by the thread. All much harder than it looked. Imparting twist to yarn is a possible derivation from the many ropes and twines that the natural world has inspired us to develop. Honeysuckle and hemp were certainly used early on in the British Isles for binding, towing, lashing and fixing: the gaffer tape of the prehistoric world. Without rope, the woodsman is one hand short.
Hydraulics
Shifting water and sugar around an organism as large as a tree requires some serious engineering. Four separate forces are at work: water adhesion, transpiration and surface tension all working one way, with gravity acting against them. Water is sticky, more sticky than any other non-metallic liquid. Its hydrogen and oxygen atoms bind together very effectively. But if the forces working on water appear simple, they operate together in ways that are not. First, water in leaves is drawn out by evaporation; since water sticks to itself (surface tension, or cohesion), the evaporating water pulls other water molecules along in its wake. So long as there is no gap in the column, water in the roots, connected by surface tension with water at the leaves, is drawn up continuously. As all children know, even if there is a gap the effect of sucking (negative pressure) caused by transpiration at the top of the column ought to be able to keep that column rising.
Well, no actually. Engineers tell us that there is a limit to the power of suction, a rather precise limit: 33 feet 10 inches, to be exact; or about 10 metres. No pump can work beyond that limit with suction alone because the effect of gravity on a column of water eventually overcomes the forces of suction and surface tension. The column breaks irrevocably: water snaps. In plants, the breaking of the water column is called ‘cavitation’, which is like an air embolism (often fatal) in animals. In summer, when it is very dry, you can sometimes hear the cracking sound from a tree trunk in which the column suddenly breaks. New research seems to show that beech trees are peculiarly vulnerable to long dry spells. Yet trees can grow much taller than 33 feet 10 inches, and in any case cavitation in trees is generally caused not by suction but by drought or frost. So how does a tree get water up so high?
There are two possible answers: one is that the engineering of the cells that transport water and sugars around the tree brings a third force into play: water adhesion. Water coheres to itself, but it also adheres to other materials. Its adherence to the walls of the woody cells, combined with surface tension in the water column, seems to allow the tree to supplement the pull of transpiration and keep that water column from cavitating. There is also another force at work, one which we do not yet understand properly: the ability of many trees to accumulate positive pressure in their roots and force the water column upwards. This is partly a response to cavitation, and it happens in spring. Think birch wine and maple syrup: that’s positive root pressure.
This is all very well and it reminds one just how brilliant trees are. Inconveniently for us, scientists can’t agree with one another on how trees draw water to so great a height. There is no current plausible explanation for their ability to create positive root pressure. It will remain, for the time being, a wonderful mystery.
Bodgers
The spindle and the bow-drill enact the idea of changing reciprocal motion (the back-and-forth action of a saw) into rotary motion (the wheel). Many of us remember grandmothers pedalling away on a treadle-operated sewing machine—or even a spinning wheel, from which the term spinster derives. The male equivalent of the spinster is the turner, he who operates the treadle lathe, which is one of the earliest complex machines. Perhaps the first lathe was operated by the use of a bow. I have seen pictures of Egyptian wood-turners of the present day with a billet of wood held between two spikes, one at either end, on a simple frame, the turner holding his chisel against it while he operates the bow with his toes, back and forward like a saw. The pole-lathe is a lot less difficult. When I built my first pole-lathe in the wood we had a lot of friends visit who became hooked on it and stayed long beyond their original intention. It is addictive.
The traditional name for a pole-lathe turner, especially one who makes chair legs, is ‘bodger’. The pole-lathe is a simple frame, holding two horizontal, parallel wooden bars a couple of inches apart at waist height. Between the bars are wedged two upright stocks of wood, and each of these holds an iron or steel spike (bronze would do; or antler). The spikes hold the billet of wood in place, under just enough pressure so that it won’t rattle but will turn freely: a simple axle. The lathe is set up next to a lean, springy tree—a rowan or ash tree about twelve or fifteen feet tall. I know turners who set their lathes up in the garage and use bungee cords instead to provide the elastic spring. A cord (I use window-sash cord) is tied to the top of the tree, wound twice around the billet and attached on the ground to a forked branch which looks like a wishbone. The two branches of the fork are attached to a flat board by leather hinges, and the board lies flat on the ground, while the other end of the branch is tied to the rope. Push down on the forks with your foot and the billet is rotated towards you; let go and the spring in the tree rotates it the other way. Being reciprocal in motion, the lathe only cuts on the down stroke—it takes a little getting used to, but becomes perfectly natural in a very short time. Very soon one is imagining chair legs, tool handles, candlesticks and bowls. Keep the tools sharp, get some nice greenwood to work with (seasoned wood will not turn anything like so easily), and you will wonder why you ever watched television.
MAPLE
The field maple does not shout its existence to the world; it has showier cousins. But it produces lovely red timber, it is very hardy, and the leaves are a source of fodder for browsing animals.
Bodgers who are members of the Association of Pole-lathe Turners and Greenwood Workers have their own website (bodgers.org.uk), which is full of ideas. They even hold a Bodgers Ball.
How high can a tree be?
A tree’s ability to draw water to a great height is evidently a factor in determining how tall a species might become. The battle between gravity and the effects of water adhesion, transpiration and surface tension means that the taller the tree, the harder the pull. But that’s not the whole story. In the very tallest trees stomata are smaller and with smaller gaps between them, leaves are smaller and the photosynthetic potential decreases to a point of theoretical maximum tree height, balanced by the chances of cavitation and the limits of positive pressure. The tallest possible tree would be about 126–130 metres or 420–433 feet, just a littl
e taller than the current record-holder, a coastal redwood (Sequoia sempervirens) in California called Hyperion, which stands at 115 metres or 379 feet: four times the theoretical maximum height of a suction pump (and just taller than the dome of St Paul’s Cathedral at 113 metres or 376 feet). You might think that the world’s second-tallest tree would be found in the same forest; but you couldn’t be more wrong. It has recently been found on the other side of the world, in Tasmania’s great, unique forests. Nicknamed Centurion, the giant swamp gum (Eucalyptus regnans) stands 101 metres or 331 feet tall—and may once have been taller before it was damaged. It is the tallest hardwood and the tallest flowering plant anywhere; and the eucalyptus, being highly fire-adapted and opportunistic, is the fastest tree to get to that height.
Standing up straight
Like humans standing on two legs, trees like to live in a perpetual state of balance, with a centre of gravity running more or less down the middle of their trunk. Trees, along with an assortment of man-made structures—one thinks of bridges, cathedrals and sports stadia—deploy counter-balancing levers to maintain stability, stay as light as possible and yet flex. Anyone with a bad back knows that if the counterbalancing forces are compromised, the whole edifice suffers.
