The Tree (10 page)

Present-day horsetails are modest plants often found on waste ground, where they resemble the swagger sticks that indeed are carried with considerable swagger by sergeant majors, but with rings of needle leaves at intervals along them, like tutus. Their stems have ridges, like Ionic columns, and along the ridges are spicules of silica. In earlier times, when people made use of whatever grew, horsetails made excellent pan scourers. Only about fifteen species are known, all placed in the single genus
Equisetum,
but in Carboniferous times in particular some of the horsetails grew into fine trees.
Calamites
is among the best known. It could be up to 10 meters tall, shaped like a torch, with a straight, thick stem and a crown pointed like a flame.
Calamites
grew like irises—or, indeed, like modern horsetails—from thick, creeping underground stems (known as “rhizomes”).

So the monilophytes invented the form of the tree at least twice: tree ferns and tree horsetails. Only one group of spore-bearing trees, the tree ferns, is still with us, but we should be very grateful to the extinct types
—Calamites
and
Lepidodendron
and their relatives. In fossil form, the horsetail and lycophyte trees formed much of the coal that gave rise to the Industrial Revolution. Indeed, it was mining that made them known to the world. Worldwide, in the deep past, those spore-bearers were very significant players.

But now we will put them, and the tree ferns, to one side. It was left to the seed-bearers to produce the world’s grandest trees in the greatest variety, and they must dominate the rest of this book.

TRANSFORMATION 7: PLANTS WITH SEEDS

A little more than 360 million years ago, in the late Devonian, there appeared the first plants that reproduced not by spores but by seeds. Seeds were, and are, a marvelous innovation. Spores obviously do a good job. The plants that make use of them include many that were and are hugely successful. But although it has become politically correct to argue that there is no progress in evolution, there very clearly is, of a technological kind; and seeds, beyond doubt, are a technological improvement. Spores are little more than groups of relatively undifferentiated cells wrapped in a protective coating, light enough to be carried away by wind or water. Unless they land somewhere very favorable indeed (and, in particular, very damp), they perish. Spores are like children setting out on a wild adventure with nothing but high spirits and a bag of toffees. Seeds, by contrast, contain embryos that have already developed significantly while still attached to the parent plant, and they are equipped with a food store of carbohydrate, protein, and fat. The embryo and its attendant hamper are encased within a coat (a “testa”) that is custom-built for the circumstances the seed is liable to meet and commonly contains (chemical) instructions on when to germinate (sometimes, both in trees and herbs, including devices to delay germination for several years, for not every season is favorable). To continue the metaphor, seeds are like commandos, beautifully equipped with iron rations—in some cases able to grow for weeks after germinating before receiving any fresh nutrient from outside—and with a well-worked-out survival strategy to boot (the strategy being encoded within their DNA).

There is one final subtlety: alternation of generations. This occurs not only in mosses but in
all
plants. In ferns and horsetails, the plant you see all the time is the sporophyte, the generation that produces the spores. The spores then germinate to produce a small gametophyte (which typically resembles a liverwort), where sexual exchange takes place, producing a new sporophyte generation (a new fern or horsetail).

In seed plants too the main plant is the sporophyte, but instead of spores it produces small collections of cells that represent the entire gametophyte generation. In the male flower (or the male part of a hermaphroditic flower) this collection of cells is contained within a protected package, the pollen. The pollen is then carried to the female flower by wind, animals, or water. The female gametophyte remains within the ovary and manifests as the ovule. I like the whimsical notion that since pollen contains the entire male gametophyte it is, botanically speaking, flying moss.

S
O THAT’S IT.
By the time we have seed plants, all the transformations required to take us from inchoate clouds of noxious gases to plants that can manifest as oaks and redwoods have occurred. There were many refinements still to come, including the evolution of flowers. But the basics were in place at least 150 million years before the time of the first dinosaurs. Such antiquity is hard to comprehend; yet, botanically speaking, it was the beginning of modernity.

Many lineages of seed plants have appeared during that long, long time. Most are long extinct. But five are still with us. Two of them—the conifers and the flowering plants—dominate the terrestrial ecosystems and account for at least 99 percent of all trees. These two occupy all of the rest of this book. But the other three remaining lineages also contain trees, including some highly attractive and sometimes magnificent ones. They deserve passing mention.

CYCADS, THE GINKGO, AND THE MYSTERIOUS GNETALES: THREE NOBLE ALSO-RANS

Of the five remaining lineages of seed-bearing plants the most ancient is that of the cycads—the Cycadales. Beyond doubt you must have seen them on your travels—although you may have mistaken them for something else. Some have thick wooden trunks like giant woody pineapples, with a mop of spiky dark green leaves at the top. Others have somewhat more cylindrical trunks and superficially resemble palms. They are widely cultivated in warm countries for their exotic beauty.

The cycads first came into being around 270 million years ago in the early Permian, the age just before the dinosaurs appeared. But they became most various and abundant in dinosaur times, and were doubtless staple dinosaur fare. About 130 species are left to us.
¹
They have many unusual features. For one thing, they have spherical seeds—often large, and with a fleshy, colored coat. Individual cycads are either male or female (known as “dioecious”). Their reproductive apparatus is neither a cone like a conifer’s, nor a flower. It is a “strobilus.” The female strobili bear the seeds, and the males bear pollen. Male or female, the strobilus is often very large, like the head of a drum major’s mace, and sometimes brightly colored. Strobili function as flowers, but they are not homologous with them: they are a separate invention. Like flowering plants, present-day cycads employ insects to effect pollination and various animals to help scatter their seeds. Indeed, the first ever symbiosis between plants and insect pollinators was probably between cycads and beetles; and the flowering plants, which evolved later and independently, would have cashed in on the beetles that had already evolved to service cycads. In nature, one thing leads to another. Evolution is opportunistic, and everything builds on what was there before.

Another way to be a tree: cycads look like palms but are quite different.

The pollen of cycads is odd. It invades the would-be seed by sending out a multitude of “roots” like a fungus; and then at the end of this invasion, and quite unlike a conifer or a flowering plant, the pollen produces a giant sperm—a sperm with many tails. Both these features may be primitive—they possibly represent the way that very early seed plants in general conducted their affairs.

You cannot miss cycads as you stride along the avenues and promenades of Florida or California or Spain—that is, unless you mistake them for palms. It is worth looking closely. It would be a shame to miss out on life’s subtleties.

Second most ancient of the surviving seed plants are the Ginkgoales, which first appeared in the fossil record around 260 million years ago, again in the Permian. In the past there have been many species, which were highly various. But only one is left to us: the ginkgo, or maidenhair tree (
Ginkgo biloba
), with its curious and absolutely characteristic, fan-shaped leaves. The ginkgo too may be extinct in the wild, but human beings cosset and cultivate it for its physical beauty and curiousness—around temples in China, and in gardens, parks, and avenues in all the temperate world. The outer layer of the skin that surrounds its seeds is fleshy and smelly, and the Chinese gather the seeds for cooking (as indeed they were doing in New York’s Central Park the last time I was there).

It is lucky that ginkgoes are so quaint: humanity has driven many other, less striking trees to extinction. Peter Raven, director of the Missouri Botanical Garden in St. Louis and one of botany’s most original thinkers, says that if you want to save a plant from extinction, you should put it into the horticultural trade; the ginkgo is a case in point. This option does not work quite so well for animals. No one could give a satisfactory home to a blue whale.

The third of the five remaining groups of seed plants is the Gnetales. They are, taken all in all, seriously weird. The whole group contains only about seventy living species in three genera—which look nothing like one another to the untrained eye but seem, nonetheless, to be truly related. One is
Welwitschia,
which grows in the extremely dry coastal desert of Angola, Namibia, and South Africa. Most of the plant stays buried in the sand. The bit that shows is a massive, woody, concave disc bearing two enormous leaves that are never shed and never stop growing, are ragged at the ends with wear and tear, and look permanently dead.
Welwitschia
is a wan creature: botany’s answer to A. A. Milne’s Eeyore. But, like Eeyore, it endures, and indeed does quite well. The remaining members of the Gnetales belong to the genus
Ephedra
(mostly shrubs, highly branched, with small, inconspicuous, scaly leaves superficially like horsetails) or to
Gnetum
(some of which are vines—but others of which are trees, with big leathery leaves).

Ginkgoes, too, were once diverse. Now only one species remains.

The Gnetales is now a minor group (if very curious), and so far as can be seen, it always has been. While other kinds of plants took over vast stretches of the globe, the Gnetales just jogged along.

This leaves just two main groups of seed plants, which between them contain well over 99 percent of all trees. Before we survey them all (Part II) and look at the ways they live (Part III), we should look more closely at the wondrous material that they have in common and that enables them to grow so big and live so long, and has allowed them to occupy a third of all the world’s land: wood.

4

Wood

I
F ALL THE GREATEST
aesthetes and engineers that ever lived were assembled in some heavenly workshop and commissioned to devise a material with the strength, versatility, and beauty of wood I believe they would fall far short. Wood is one of the wonders of the universe. Of course, human architects create structures that are bigger than any tree and sometimes, like the great cathedrals and mosques, are of great beauty. But a cathedral or a mosque is built; it does not grow. Until it is complete it is useless, and probably unstable. It must be held up by scaffold. When it is finished it remains as it was made for as long as it lasts—or until some later architect designs it afresh, and rebuilds.

A tree, by contrast, may grow to be tall as a church and yet must be fully functional (apart, perhaps, for the business of reproducing) from the moment it germinates. It must fashion and refashion itself as it grows, for as it increases in size so the stresses alter—the tension and compression on each part. To achieve hugeness and yet be self-building—no scaffold or outside agencies required—and to operate for good measure as an independent living creature through all phases of growth (first as seedling, then sapling, then young tree, then mature tree) is beyond anything that human engineers have achieved. After the tree is cut we see that the wood, of course no longer increasing in size, is the ultimate composite: remarkably complex chemistry (cellulose, lignin, tannins, resins, and often much else besides), minutely structured for maximum strength and functionality, lovely to look upon, and infinitely various. Great human craftspeople, from Grinling Gibbons to Henry Moore, can create artifacts to show off wood at its best. But the wood itself, on which they work their creativity, is nature’s invention.

A young yew—with perhaps another two thousand years to live.

The terms “wood” and “timber,” like the word “tree,” tend to be used in different ways. Some people define wood loosely, and some more narrowly. Loosely, the word refers to the hard skeletons of conifers and flowering (angiosperm) trees. But some botanists and foresters don’t like to think of monocot wood—the kind that comes from palms and bamboo and so on—as “true” wood because it has a quite different structure. For them, true wood comes only from conifers and broad-leaved angiosperms—the broadleaves being all the dicots, from magnolia to oak and teak.

This description focuses on “true” wood, as in conifers and broadleaves. In both cases, the basic component of the wood, which makes it both functional and strong, is the conducting tissue, the basic plumbing. This tissue is of two main kinds. On the inside is the xylem, a mass of tubes that carry water with dissolved minerals up from the roots to the leaves. In broadleaves, most of the xylem tubes are open all the way along, but in conifers they are interrupted by perforated plates (and this is the chief difference between the two types). The second group of conducting tissues form the phloem, strings of cells that carry the products of photosynthesis (organic materials of various kinds, which basically are variations on a theme of sugar) out from the leaves, downward and outward to the rest of the plant. The tissues of the phloem are on the outside. Collectively the phloem forms a cylinder, enclosing the solid column of xylem within.

So all in all, you can imagine wood as a close-packed bundle of straws, bound tightly together into a solid whole. But now add one more element to the image. Imagine that swords were thrust into the bundle from the outside, slicing between the bundles—running from the outside toward the middle. The “medullary rays” run in just this fashion from the center to the outside. These blades of tissue provide some linkage between the different elements of the xylem and phloem and also act as a food store for the whole trunk. By carrying material inward and outward, the rays enable the trunk to increase in diameter as the tree grows. More generally, they help to ensure that the trunk is itself a larder, to be drawn on as required.

But where does the growth come from? How can the trunk increase in thickness and yet be continuously functional? Here is where the subtlety really begins. Between the xylem and the phloem is a thin layer of tissue known as cambium, which forms a sheath, running from roots to leaves. The cambium is stem-cell tissue: the kind whose job it is to generate more tissue. It generates more xylem vessels on the inside, and more phloem vessels on the outside. So the tree grows thicker year by year—and yet the trunk is always functional. Always, fresh xylem and phloem are coming on line. Herbaceous plants and young trees, of course, have some thickness to them from the outset. A tomato stalk grows thicker as the season wears on—more and more cells are produced, all puffed up by water pressure within the cells. But only conifers and broadleaves have the complete sheath of cambium, not far from the surface, that allows the tree to go on getting thicker year by year, perhaps for centuries. This is the phenomenon of “secondary thickening.” Other trees that are
not
conifers or broadleaves may practice secondary thickening up to a point. Cycads do. The lycophyte tree,
Lepidodendron,
apparently did. (Palms don’t. In general, they begin life short and fat and stay at the same thickness until they are twenty or so meters tall.) But no trees apart from conifers and broadleaves have taken secondary thickening to such a peak. It is the final requirement and accomplishment of true treedom (at least up to now).

The cells that form the tubes of the xylem soon die. In fact, in order to become fully functional, they
need
to die. They lose their living cytoplasm; all that is left is the cell wall, cellulose stiffened with lignin. However, as time passes, the cells both of the xylem and phloem not only die but lose their function as conducting tissue. Clearly, in any one tree trunk, the xylem closest to the center is the oldest; it may have been laid down ten, a hundred, even a thousand years earlier. But xylem that is more than a decade or so old tends to be increasingly blocked, not least with tannins. So the center of a tree becomes increasingly solid. Not only are the individual cells dead, the whole structure loses its ability to transport water. Phloem is the mirror image of xylem: its oldest vessels are on the outside, and they become crushed as new phloem tissue is laid down inside them.

But although their days as conducting tissues are over, the very dead xylem in the core of the tree and the crushed phloem on the outside do not cease to be functional. The very dead, commonly tannin-soaked xylem within becomes the “heartwood”; and the newer xylem outside it, still serving as plumbing, forms the “sapwood” (because indeed it is full of sap). The heartwood truly provides the skeleton of the tree; it is what enables it to become big. The crushed phloem, outside, becomes incorporated into the bark, providing essential protection. “Bark” in general means everything that lies outside the cambium; the inside layers consist of living phloem, but the layers beyond that are dead. We get a hint of the life that lies just beneath the surface of the tree through the phenomenon of “cauliflory”: the way in which many tropical trees in particular, including the cacao tree, produce flowers and then fruit straight from the trunk or biggest branches.

In trees that grow seasonally, the addition of xylem and phloem is intermittent. In a typical temperate tree, the new xylem laid down in spring is wide but thin-walled, while the summer xylem is narrower but thicker-walled. The differences can be seen clearly and result in a series of concentric “growth rings.” Typically there is one growth ring per season, and so the age of the tree can be gauged. In good growing years the growth rings are broad. In bad growing years, they are close together. Thus, knowing the age of the trees, it is possible to work out the climate of past years. If we cut a mature tree in, say, 2006, we can see what the weather must have been like in, say, the 1850s. Some growth rings might be particularly far apart, and some particularly close together. If we have a piece of timber we know was cut sometime in the late nineteenth century but we don’t quite know when, we can see which of its growth rings correspond in width to the ones of the tree felled in 2004—and which, therefore, correspond to the 1850s. We can then count back and work out when the tree was planted. Then we can overlap that older tree with one that is even older, and so on back. This is the principle of dendrochronology—judging past climates, and the general ages of things, by examining the growth rings of successively older trees. Dendrochronology has provided some remarkable insights in archaeology. (Tropical trees in places where there are distinct wet and dry seasons also show growth rings. Trees where the climate is constant do not.)

Many trees have a layer of secondary cambium, outside the principal cambium layer, with the specific job of producing cork. Cork cells (like xylem cells) are born to die; they finish up small, with thick, impermeable cell walls. Cork is wonderful material: it is light; it is waterproof (hence preventing excessive water loss); it helps repel pests; and it is relatively fireproof. All trees have some corky cells in their bark, and some have a great deal of it. Trees that are most likely to be exposed to fire tend to have the thickest cork—like, of course, the beautiful cork oaks (
Quercus suber
) of the Mediterranean and the baobabs of Madagascar, Africa, and Australia (which are also used for cork). The one snag from the tree’s point of view is that cork is also airproof, and thus prevents exchange of gases. But it tends to be interrupted by passages of only loosely bundled cells, known as lenticels, which let air through.

Bark too, of course, compounded from formerly functional phloem and custom-built cork, is highly evolved and adapted. Much of the variation is not explicable in terms of function; it just is the way things have turned out. It can be used (by experts) to identify species, just as the pattern of the timber itself can. But bark does have many adaptive features. Some bark, for example, is highly impregnated with tannins to repel pests. The bark of redwood trees is not corky like cork oak but is fireproof nonetheless; it is fibrous and up to nearly a foot thick. Others, like
Enterolobium ellipticum
(it has no common English name), which must endure periodic fires in the dry forest of the Cerrado in Brazil, have huge ridges of corky bark. I suspect the ridges help to create an updraft, which carries the heat up and away from the trunk.

Many trees shed their bark, sometimes in great swaths, which can be helpful in various ways. Some (especially tropical forest trees) seem to shed it in an attempt to get rid of epiphytes, which can grow in great abundance on their trunks and branches and so weigh the tree down and block its light. The bark of eucalyptus is rich in oils and resins and burns quickly and fiercely. Oddly, this is an antifire device. The bark is shed, commonly in shreds, and builds up around the tree as litter. Other plants find it difficult to grow through the chemically rich, dark brew, and so there may be little or no undergrowth. When the bushfires rage they race quickly through the oily, resiny tinder on the ground—and a quick, hot flame is far less damaging than a cooler but slower one. The bark beneath the wisps that are shed is smooth and iron hard, difficult for fire to take hold in. By shedding their bark, London plane trees shrug off the polluting city soot, so they do well as urban trees. This cannot have been an adaptation—the parent species of this hybrid evolved long before cities did—but it is a good example of “preadaptation”: a feature that evolved earlier in some other circumstance, coming by chance into its own.

Clearly, different species produce different timbers. Some are very light and fast-growing. Some are very dense and on the whole tend to grow more slowly. Quite a few are heavier than water, such as lignum vitae and various species of
Olea,
the genus of the olive. Some timbers are black, some creamy white, some yellow, and some distinctly red.

These are the broad differences. To some extent they seem easily explicable. For instance, pioneer trees—those that invade newly available space quickly—need to grow fast. But since they are soon likely to be overtaken by other trees and will then be overwhelmed, they do not need to be strong enough to endure for a long time. So their timber, typically, is strong and light. A classic pioneer of this type are various species of
Cecropia,
whose big, silvery, horse chestnut–like leaves are such a feature of tropical forest that has been opened up by storms or logging. But nature cannot be second-guessed—we cannot assume that it will always follow our logic. So it is that some pioneer trees endure the later invasions of other species and are extremely long-lived—like redwoods; and some are not only long-lived but also have very hard timber—like mahogany. The baobab tree of Madagascar (and Africa and Australia), on the other hand, has extremely soft wood, like a classic pioneer, but commonly lives for five hundred years or more. Many other trees begin life in the shade as part of the understory, grow slowly up to the canopy (or wait for a gap to open), and then endure perhaps for centuries. Their wood is likely to be dense and strong, to enable them to live a long time. Some long-lived trees bend with the wind; others outface it. In Britain, the flexible ash and the resilient oak have become symbols of different life strategies. But other differences—including, perhaps, color—seem mostly down to chance. The prime requirement is to produce an organism that works. Many of the genes will have odd effects in addition to the ones that seriously contribute to survival. It is hard to see how it matters to a tree whether its timber is black or white or red or a pleasing buff, but the genes that influence color may be doing something truly useful as well—for example, repelling pests. Or they may not. Provided their side effects do no serious harm, these genes will be passed on through the generations, with whatever eccentricities they bring with them.