by Max Adams
1. Erect solar panels as high as possible to ensure a good supply of sunlight (the power source)…
2. …but also draw water and minerals from as large a volume of soil as possible. (You already see the problem: Nos 1 and 2 are mutually improbable).
3. Combine with other machines to replicate over a wide area for best survival chances (tricky, if the machine can’t move).
4. Find a way of bridging that gap between the sky (No. 1) and the ground (No. 2). Subsidiary tasks consequent on 1–4 include storing energy because of the variable supply of sunlight and the time it might take to process the raw materials; and protecting the mechanism from mechanical failure and attack.
5. Have a Plan B in case it all goes horribly wrong.
6. Enable the machine to seek help from a third party (or parties).
That is all there is to it: solve those few key engineering problems and you have… a tree.
Trees with latitude
Trees, rather obviously, have a relationship with light: they need it to provide their energy. But not all trees deal with light in the same way, as anyone will know who has noticed how many different types of leaves there are. Conifers have very narrow leaves, often in pairs or trios of thin, dark-green waxy needles. Most conifers keep most of their needles over winter when they can still photosynthesize, albeit very slowly. Each needle requires a small investment and pays it back over a long time, so slow light-gathering is a reasonable way to go about the coniferous business. Conifers do not produce costly flowers, so there is an economy in everything they do. The larches, rather unusually for conifers, shed their needles in autumn like a broadleaf tree, and grow a new set every year. It’s more expensive to do that, energy-wise, but it has its advantages—a larch is less likely to be damaged by heavy snow in winter; and there may be some value in not needing to use much winter groundwater in cold, dry continental climates where, during the coldest months, that precious water is frozen solid. Most broadleaved trees shed most of their leaves in autumn most years (few things are absolute in the world of trees); but oak trees, for example, can shed their first set in June and grow a second set if there is a sustained insect attack on the first lot. The further south you go, the more likely you are to see broadleaved trees keeping some or all of their leaves all year round; and some trees shed leaves not because of impending winter but because of drought and heat.
Although trees cannot move—actually, there are exceptions, but they prove the rule—they can move their leaves to face the sun and, naturally enough, they grow their twigs and branches to procure the most advantageous position in a wood. It’s obvious that trees in woods grow rather differently from the great spreading beasts of hedgerow and park; but it may come as a surprise to know that many trees also grow more than one type of leaf. Holly trees, an evergreen broadleaf, have more prickles on their lower leaves, where most of the animal browsers are, than on their upper branches, where only giraffes and elephants would be able to get at them. Many trees have sun leaves and shade leaves, with different patterns and numbers of light-absorbing cells (chloroplasts) and different types and intensities of chlorophyll, the green pigment that absorbs light in the blue and red parts of the spectrum. These variations mean that trees don’t expend too many resources on leaves that will gather less energy. Oddly enough, shade leaves seem to be just as efficient as sun leaves, and recent scientific studies seem to show that sunlight in the form of photons can actively be diverted to find the best route to the light-gathering chloroplasts. Biologists are beginning to talk as if this is an example of ‘quantum’ biology: all very exciting and tantalizing.
Not surprisingly, leaves come in all shapes and sizes to cope with different conditions. Trees that are happiest in wet, temperate forests tend to have drip tips and shinier leaves to get rid of rain quicker. Trees in areas prone to drought slow down the shedding of rain to allow water to soak more gently and profitably into their roots. Different-shaped leaves can cope with high winds or soaking up more, or less, water. There is no perfect solution to all the problems tackled by leaf design—there are many solutions, and most lines of engineering seem to have been explored. The trees don’t know they are doing it, you understand... they just do. Organisms that have taken 200 million years to adapt and have survived have had long enough to get most things right; this fools some people into thinking that there is planning involved. If there is a God, one must allow for her or him to have been omnipotent indeed in the few days it took to think up those sixty-thousand tree species with all their brilliant variations.
LARCH
The larch is that rare beast, a deciduous conifer. Its wood is wonderfully light and a beautiful red in colour, but sawyers say that it turns their blades and is hard to mill.
The relationship with light leads to some radical differences in tree shapes. So conifers are generally tall and pointy, and broadleaves are more often domed or, in extreme cases, flat-topped. It doesn’t take more than a moment’s thought to realize that this has to do with latitude. Trees that thrive in northern climes, on thin soils and with a sun that does not climb very high in the sky, are tall and thin to maximize light coming from the side. Oaks, beeches and other trees of the temperate latitudes are generalizers, doing well across a broad range of latitudes. Baobabs and dragon trees are equatorial specialists, presenting a large proportion of their leaves like a flat tarpaulin spread out in the high sun of the equatorial regions. But, trees being pretty adaptable, you will find conifers quite far south in Europe—Corsican pines are a great sight on their native mountainsides—and quite far north in the southern hemisphere, such as the famous pines of Norfolk Island in the southwest Pacific. Likewise, many broadleaved trees will survive in surprisingly high latitudes: one thinks of the southern exotica, including some semi-tropical trees, at Poolewe Gardens (also called Inverewe) on the Scottish west coast, a little to the north of the Isle of Skye, where they benefit from milder air warmed by the Gulf Stream.
Solar panels
A tree’s primary job is to place its solar panels where they will have the best chance of soaking up light. The leaves are small miracles of physics, chemistry and microscopic construction. I don’t want to make trees sound simpler than they are, because they are tremendously complex in the fine detail of their chemistry, but here is a basic sketch. The task is to make glucose from a simple recipe of water, air and light. The numbers involved for one single molecule of glucose are six carbon atoms, twelve hydrogen atoms and six oxygen atoms: C6H12O6 in chemical shorthand. Carbon doesn’t hang around the atmosphere or in the ground in pure form, so the leaf must take it in doses of carbon dioxide—CO2—so that for every carbon atom acquired, a pair of oxygen atoms comes free. The leaf needs six of those molecules for every molecule of glucose it will make. Hydrogen, similarly, is not to be found on its own, but it comes conveniently enough in water—H2O—offering one extra oxygen atom for every two of hydrogen. By simple arithmetic one can see that the leaf requires six water molecules to get enough hydrogen for one glucose molecule. It’s obvious that there’s going to be some surplus oxygen. So this small but successful factory is drawing water from its roots up through the trunk to the leaf, pulling in carbon dioxide through minute gas-exchangers called ‘stomata’ on the underside of the leaf, and taking in sunlight at the top.
The pigment in the leaf, chlorophyll, acts as a receptor for the energy from sunlight (billions of photons) to carry out its magical synthesis, and the fundamental sums work like this: 6 × CO2 + 6 × H2O = 1 glucose molecule, plus 6 spare molecules of pure oxygen (O2), which are breathed out through the stomata. The oxygen-breathing creatures of the planet are the obvious beneficiaries of this arrangement, although the tree needs a little of this excess to power some of the reactions in its cells. In practice, trees take on more water than is required just to make glucose, and get rid of the excess by ‘transpiring’. The limit of glucose production is much more likely to be determined by the amount of water available to the tree than by the
amount of sunlight. Regardless of how sunny it is, if the roots can’t supply enough water, alarm bells go off and the system slows dramatically to prevent the tree from dehydrating. Water is, so to speak, the bottleneck in the factory production line.
Leaves are not only complex, but also vary enormously in design, adapting to all sorts of varying conditions of shade, sunlight, wind, rainfall and the suite of predators and spongers who are after a free meal or a free ride. But the engineering of the leaf can also be simplified. At the upper surface there is a transparent waxy cuticle, which prevents scorching and desiccation. Beneath that, a spongy layer of chloroplasts contains the receptors for the sun’s photons; these are held within a lightweight vascular structure, whose larger veins are superficially visible to the naked eye. They brace and hold the leaf in place and contain the plumbing to shift all those gases, liquids and sugars around. The stiffness of the leaf is dependent on positive fluid pressure: lose that pressure and the leaf sags, as every gardener knows. On the underside of the leaf, out of direct sunlight, is another cuticle layer perforated with the stomata that let in carbon dioxide, exhale oxygen and allow water to evaporate. They can open and close to control the rate of photosynthesis, so responding to—and setting the rate of flow of—water being drawn up from the roots. In case you think it worth peering at all these chloroplasts under a magnifying glass to see what they look like, forget it. Chloroplasts come at the rate of about half-a-million per square millimetre; stomata occur at a rate either side of two-thousand per square centimetre. A large tree might display in the region of three-hundred-and-fifty square kilometres of solar collecting cells. I hope the ambitious human engineers I have mentioned are beginning to think that it would be easier just to go out and plant trees; but I fear they may continue their experiment for some time.
HORSE CHESTNUT
The horse chestnut is a relative of the maple, not the sweet edible chestnut; a native of Eastern Europe, its opulent creamy candlestick flowers are hard to reconcile with its autumn bounty of conkers.
Among the repertoire of leaves, perhaps the strangest you are ever likely to see is that of the maidenhair tree (Ginkgo biloba), one of nature’s living fossils and at least 270 million years old as a species. Its veins all radiate from the base of the leaf, or petiole. It is now commonly planted as an ornamental, but it survives in the wild in a couple of small reserves in China, where it may live for more than two-thousand years. It is one of the very few trees known to have survived as close as a mile from the atomic blast at Hiroshima in 1945. To say that ginkgos are tolerant of urban pollution is the ultimate understatement.
Collecting autumn leaves to put in a scrapbook, or to frame as nature’s art, is a fine thing to do for children up to the age of about a hundred. The best way to preserve leaves with all their divine colours is to gently rub moisturising cream into them before they dry out; or, if you prefer, you can soak them in glycerine.
Family tree
Trees, like all other life forms on our planet, evolved ultimately from very simple chemical associations, single-celled organisms, and bacteria. The billions of years that it took to develop the sophisticated organisms we call trees and the super-organisms we call forests can be crudely condensed into a few key stages. We might, if we were in the American state of Massachusetts, go down to Walden Pond, where Henry David Thoreau lived among the woods in the middle of the nineteenth century, and see some of these key stages at first hand. Actually, almost any freshwater lake will do. In summer you will see algae growing where the waters are still and there is plenty of sunlight. Algae are very simple compared to trees: a billion-year-old experiment in single-celled life, but still with us after all these years because there is not a lot to go wrong with them. They might get in the way of a perfect pond; and some of them, like the cyanobacteria of so-called blue-green algae, are toxic. But they, the early oxygen-givers, are the progenitors of all land plants.
Look beneath the deep shade of a forest tree overlooking the banks of the pond and, especially in winter, you will see mosses in all their amazing variety. They represent the next stage in plant development. Modest in their way, they cannot stand upright to compete for light and are heavily dependent on local moisture for their success. Even so, they are small marvels. All mosses live in alternate generations: the velvety mat growing on the tree or damp rock is the gametophyte. It produces second-generation sporophytes, which you can see if you get up very close: these are the little antennae-like wands, which stand proud of the mat and which make and distribute spores to make the parents of the next generation.
But mosses did not lead to trees, because they remained on a two-dimensional path. The way to trees was signposted by the first vascular plants, the trachaeophytes—the plants that can stand up for themselves. If you see a horsetail, one of those spiny dark green clumps clinging to the banks of rivers and ponds, you can see how the stem of the plant is made of single vessels stacked one above the other. Between the vessels are semi-permeable membranes that allow fluids to pass up and down. Without this brilliant innovation, there’s no getting anywhere near the idea of a tree. This little miracle of engineering happened somewhere in the region of 400 million years ago. There is, evidently, a limit to the height that a single pipe of vessels can attain, although it’s quite a limit: horsetails, bamboos and other single-tubed plants will compete with many a shrub. But it is the woody plants that dominate the canopy around Walden Pond and in almost every other forest around the world, because wood, in which bundles of vessels are bound together and stiffened, allows plants to build year on year into structures of immense height and strength of a kind that could not grow in a single season. They are the architects of their own world. Wood—that is to say, cellulose cells strengthened with the polymer ‘lignin’—does not rely on water pressure to keep it stiff, which is how herbaceous plants make a living. Woody cells—the ‘xylem’—in trees are dead; they no longer contain living tissue. But they are not discarded. They support the expansion of new growth outwards from the cambium layer of a tree and, along their now hollow length and via a network of medullary rays (most easily seen in a section through an oak log), allow excess fats and sugars to be transported where they are needed. The whole is knitted brilliantly into a structure of admirable strength and suppleness. Woody plants are like cities, with bundles of arteries linking production and consumption; power stations collecting and transporting energy; manufactories churning out raw materials and bespoke engineering solutions; communications hubs and invisible infrastructures held in a tight network, fantastically interconnected and unimaginably functioning as a single organism.
From the horsetails and their like emerged two… um… branches: the gymnosperms (naked-seeded plants such as conifers, ginkgos and cycads) and the angiosperms, or broadleaves. All these are seed-bearers engaged in sex, although they can practise other forms of reproduction too. Seeds, an innovation dating back to about 350 million years ago, enable plants to reproduce with greater certainty: they contain not just genetic information but emergency rations and a self-assembly starter kit for the plant to get growing fast. Fifty million years after the seed comes the first conifer, during the Carboniferous period. Flowering plants only arrived for the closing ceremony of Jurassic Park, 150 million years ago. One of the most ancient survivors of this ancestral line is the magnolia, older than the bee for which, some scientists argue, it specifically evolved its exotic, exuberant, irresistible flowers—producing one of nature’s charming love matches. Within 50 million years from the first flowering plants a great expansion of angiosperms, including the ‘modern’ broadleaves like oaks, led to their overwhelming dominance of the tree world—at least in terms of the diversity and range of the trees and shrubs which they have produced. Latest on the evolutionary scene are relative youngsters like the beech and ash, birch and hazel, all belonging to the last 2 million years or so, each exploiting the myriad niches available, and each adapting to new challenges and competition. Or not. Any number
of tree species and proto-trees did not make it past the fossil record. Yet, sixty-thousand species later nature has still to explore all the avenues of the diversity maze.
TREE TALE
The Rowan
Because it does not grow large enough to make a commercial timber tree, the lovely rowan (Sorbus aucuparia—aucuparia meaning ‘bird-catching’) is neglected by foresters. The rest of us know it as that graceful garden ornamental or as the modest sentinel on a steep rocky hillside by a rushing burn. The tallest in Britain grows near Cheshunt in Buckinghamshire, less than a hundred feet tall; the oldest and most venerable grow in the Scottish Highlands: one at Knappach, Kingussie, has a trunk about five feet in circumference. Growing slowly there as it must, it may be hundreds of years old.
At all seasons the rowan has an understated beauty. The trunk and branches are a smooth, slender silver-grey, often dappled with lichen. In winter, the buds are slightly hairy and dark purple. As the leaves emerge they have a silvery tint, as if cocooned in gossamer, before opening to reveal delicate leaflets, slightly serrated, on dark red twigs. The flowers are clusters of creamy-white five-petal drops, strongly scented, which attract many a pollinating insect: bees, butterflies, wasps and beetles. These transform into trademark postbox-red berries, which signal the onset of autumn when they stand out against the darkening foliage, before even the leaves pass from green through orange to brown and sometimes the deep red of the blood orange. The fruits, which like the apple must pass through the acid gut of a bird (often blackbirds, waxwings and bullfinches) before the seeds can germinate, are an astringent and can be made into a distinctly tart jelly rich in vitamin C, but are slightly poisonous if uncooked. Because of its berries and its tolerance of altitude—up to three-thousand feet in Britain—the rowan is a valuable source of winter food for birds. Deer will eat the foliage and bark too, once they have munched the lichen off it. Ironically, the berries used to be crushed, fermented and mixed with oil to make birdlime, a sticky glue most attractive to birds, which was spread on twigs to which the bird would be inescapably stuck. During the Second World War birdlime was adapted to the manufacture of the experimental and not very successful sticky bomb, an anti-tank grenade. Even birdlime would not adhere to the dusty, muddy plates of a tank.
