What Is a Tree?

Animal, mineral, or vegetable? Whenever our parents bundled us into the car for a long journey my sisters and I kept ourselves occupied with that guessing game. At its heart is the puzzle of how things should be classified, the more ambiguous the better. My inventive youngest sister came up with “a cow’s moo.” Through its astonishing revelations about what is related to what in the plant world, Colin Tudge’s The Tree reawakens the pleasure of those childish games. But The Tree is a far deeper book than this might suggest, for its author has a remarkable ability to ask fundamental questions about trees and their world—questions that, much to our detriment, most of us stopped asking as we grew up.

Humans are innate classifiers, and our earliest efforts were doubtless classifications of convenience: edible and inedible, for example. Despite this predilection, our workaday world is filled with appalling classifications. Consider the forester’s venerable division of “softwoods” for the conifers (including the remarkably tough parana pine of South America) and “hardwoods” for the broad-leaved trees (which include the very soft balsa). Despite its obvious potential to mislead, it’s so convenient for common lumber that it’s used by almost everyone.

And yet we instinctively recognize a good classification. Perhaps it comes from our sense that the natural world has a true orderliness. Birds, for example, are instantly recognizable as such, as are mammals and frogs; and so with smaller groups like kingfishers, hawks, and doves. In pre-Darwinian Europe natural philosophers hoped that by comprehending the “true classification” of nature they might glimpse the mind of the Creator. Yet in the absence of divine revelation, how could they hope to discern the one and only “true” classification from less perfect models?

In the mid-eighteenth century the Swedish botanist Carl Linnaeus provided natural philosophers with a hierarchical scheme that classified living things into kingdoms (of which he had just two—plants and animals), classes, orders, genera, and species. It is these final two categories that provide the scientific name. Linnaeus, for example, classified himself and other humansas Homo sapiens (though at first he dubbed us the rather less appropriate Homo diurnis—daytime man). The first part of the double-barreled name is the generic part. It is shared with a group of similar species, and so acts rather like a surname in a family. The second—the species name—is unique in the genus to that individual species, and so acts like a Christian name.

Linnaeus’s scheme was brilliant in its simplicity, and it is the basis of the universal scientific classification used by all taxonomists (classifiers of living things) today. When it came to higher levels (the classes and orders) of plants, Linnaeus turned to vegetable sex. And this scheme, carried on by botanists down the centuries, really has, to borrow from Andrew Marvell, grown “vaster than empires, and more slow.” Linnaeus’s highest plant categories—the classes—were arranged according to the nature of the plant’s male genitals (stamens), while his next level down—the orders—was based on the female genitals (pistils). This “male on top” approach had some unfortunate consequences: the castor bean tree (a flowering plant), for example, ended up in the same class as the pines (non-flowering plants), and his scheme did not account for the “plants” that had no obvious sex organs at all, such as algae and fungi.

All of this, however, might have passed as a minor flaw had the Swede shared the prudery of his age. Instead he was quite the opposite. We still have a genus of creepers called by him Clitorea (the beautiful blue flowers of one species really do bear a remarkable resemblance to female human genitalia), and botanical classification is besieged by copious “phallus” something-or-others (though here the resemblances are I think more imaginative). In Linnaeus’s day botany was considered a fit occupation for young ladies, and such provocatively named plants caused professors throughout Europe to obfuscate, or—as Linnaeus’s contemporary Johann Siegesbeck did—condemn the entire Linnaean scheme as “loathsome harlotry.”

The scheme’s misfortunes only increased when it was introduced to the English. The man who undertook the translation was Erasmus Darwin (the grandfather of Charles), a ponderous, pockmarked, and ungainly man whose principal interests were plants, poetry, sex, and ingenious mechanical devices. Darwin was a wild romantic, and despite his Johnsonian appearance and stammering speech, women loved him. To court his second wife (who was married when they first met) Darwin landscaped an entire valley, damming streams and planting bowers of exotic trees and meadows of flowers. It evidently worked: from his two marriages (and a governess in between), he fathered twelve children.

Darwin reveled in sex in all its manifestations—from marital to masturbatory—and perhaps homosexuality as well (he certainly had many homosexual friends, whom he never condemned). Indeed, he believed that sex was health-giving—it cured hypochondria, for example. One wonders, incidentally, whether this had anything to do with the first Mrs. Darwin being sickly, overly fond of the bottle, and inclined to smoke opium.

Because of his interest in poetry, it seemed entirely natural to Darwin to translate Linnaeus’s dense scientific work into rhyming couplets, so as to create a romantic epic. The Loves of the Plants was published in 1789 by the radical publisher Joseph Johnson. One of the most unusual scientific tracts ever written, its botanical classification is enlivened with alarmingly anthropocentric descriptions of the goings-on of male and female genitals in their “nuptial bed,” as Darwin refers to the calyx of the flower. Because some flowers have many male parts (some of which can be sterile—and thus liable to be characterized as “beardless youths”) and they share the nuptial bed with just one female part, some of Darwin’s “scenes” resemble the interior of a sultan’s bedroom—or a Roman orgy—more than they do a violet or pansy.

Erasmus Darwin was working at the dawn of the modern era of classification, and throughout the late eighteenth and first half of the nineteenth centuries interest in botany grew apace. It was during this period that many of the world’s great natural history museums and herbaria were founded, their collections forming a vast system that allowed the classification of the world to go on ever more expeditiously. And yet still no one could demonstrate that their particular classification was nearer the “true classification” than anyone else’s. It was Erasmus’s grandson Charles who provided the answer to that age-old riddle. Evolution by natural selection helped explain the orderliness of nature, and more importantly, it provided the mechanism that had given us the family tree of life. Yet for a century after the Darwinian revolution, most taxonomists continued with their task of classifying the world without fully absorbing the message evolution held for them.

What many taxonomists did was to say that if two organisms looked similar, then they must be related. It took a German entomologist, Willi Hennig, to change that. He saw that only some similarities tell us who is most closely related to whom. To illustrate the nature of Hennig’s breakthrough, we can imagine a family whose members have always had black hair. But then a child with red hair is born, and she gives birth to more redheads. These offspring are more closely related to their black-haired cousins than they are to more distant black-haired relatives. Yet anyone using black hair to understand relationships might misclassify all black-haired individuals as close relatives, and the redheads as more distant. Red hair, however, providing it breeds true, would be an accurate indicator of relatedness, at least within the redhead family. Hennig’s work gave rise to a new scientific method called cladistics, and it is this method that underpins Tudge’s book.

Scientific classification had come a long way between Linnaeus and Hennig, yet despite the great advances, before the 1980s many botanical classifications bore little resemblance to the true evolutionary relationships of plants. One final discovery was required before such classifications could be improved—the deciphering and use of DNA. Is the mushroom animal, mineral, or vegetable? Most nonscientists I’m sure would place it in a classification of convenience—perhaps with the cauliflower in the vegetable tribe. Indeed traditional classifications place the fungi alongside the plants. Yet modern evolutionary studies using DNA reveal that mushrooms are more closely related to human beings than to the cauliflower. Given just the three choices the child’s game allows, the mushroom is an animal.

It’s not that DNA provides some sort of divine revelation about who is related to whom, for it too is prone to convergence in evolution and errors of interpretation. But as botanists combine the results of DNA studies with more traditional methods such as the study of wood structure, and seed and leaf type, ever more robust classifications (ones that are increasingly difficult to find fault with) are emerging, and they reveal just how far astray our classifications of convenience can be. Who, for example, would ever have guessed that plane trees (sycamores to many Americans) and members of the protea family, such as South Africa’s proteas and Australia’s banksias, are close relatives? Even more mind-stretching is the newly published finding that the mighty teak is intimately related to herbs such as mint, oregano, and basil. Even as I recount it, I’m astonished that botanists now believe cucumbers are close relatives of oaks and beeches—far closer, indeed, than oaks are to sycamores.

This new knowledge, along with an ever-improving fossil record, reveals that the evolution of trees has been more epic than Erasmus, Darwin, or Linnaeus could ever have imagined. As Tudge puts it:

It is a wonderful thing to contemplate a living tree, or a fossil one, or any other creature. It is even more moving when we add the fourth dimension, of time, and see in our mind’s eye how the ancestors of the tree that grows in the field next door first saw the light in some remote corner of the globe millions or hundreds of millions of years in the past, and floated on its respective bit of continent as the continent itself circumnavigated the globe, and skirted around the glaciers of the ice age, and perhaps sweated it out in some primeval, long-gone swamp, with alligators around its feet and the world’s first hawks and kingfishers scouting from its branches.

As rich as the new discoveries are, The Tree is about far more than plant classification, and it begins with a question: “And what, pray, are trees, that anyone should presume to write a book about them?” As you might have already guessed, one of the most challenging lessons of The Tree is that the entire concept of “tree” as a classificatory category has no evolutionary reality whatever, for the things we call trees have arisen multiple times from humbler vegetation, and trees have repeatedly been transformed into vines, shrubs, and herbs. Some tree species, indeed, can exist as either shrubs or trees. As Tudge says, “Nature was not designed to make life easy for biologists.”

At this point, rather than becoming bogged down in elaborate evolutionary explanation, Tudge gives us a child’s definition of a tree as “a big plant with a stick up the middle,” and the “stick up the middle,” it turns out, is one of the most fascinating and useful objects in nature. Some woods can be made as sharp as steel and cannot be worked except with tools of tungsten and diamond. Others, such as the Japanese cedar, can be buried in the earth until they become deep green in color, and are then regarded as a kind of semiprecious stone.

The key ingredient in wood is lignin, a chemical compound that binds together wood’s celluloid fibers. Plants without it, which are called herbs, use water pressure to stay upright. Real wood only exists where the lignin is laid down in an intricate and meticulous manner; and wood, according to Tudge, “is one of the wonders of the universe…remarkably complex… minutely structured…lovely to look upon, and infinitely various.” “If humanity had only one kind of timber to draw upon it could think itself blessed…. But in practice we have many thousands—a tree for every job, and for every decorative caprice.”

Tudge’s documentation of the specialist uses of timber opens a world of craftsmanship and acute observation which is unknown to most of us. Why, one wonders, is the katsura tree of East Asia especially suitable for the manufacture of pencils and Japanese shoes? And who discovered that the lignum vitae of Central America makes splendid rollers and wheels in pulleys that are hard to get at, because the wood is self-lubricating; or that abura wood is excellent for battery boxes, because it resists acid? There is something about the timber of the coachwood of New South Wales that suits it to the manufacture of gunstocks and musical instruments, while tropical American snakewood makes marvelous violin bows and umbrella handles. Snakewood would not do for xylophones, however: for them one must seek out the wood of a Dalbergia tree. Presumably someone knows why the wood of the African ekki tree was used to support the tracks of the Paris metro. In view of the present sad state of Africa’s rain forests, one wonders where replacement timbers will come from should the existing ones wear out.

Familiar trees sometimes have mysterious uses. Who for example would have guessed that elm is favored in the manufacture of “buttock-moulded seats,” or that basswood was the best timber for the fronts of pulpits? And why is it that the jacaranda is favored for making pianos—but only in Egypt? Tudge has seen cogs made of hornbeam at work in a century-old brewery near his home, where they perform better than cogs of iron; such sights perhaps inspired him to write: “The intricate knowledge that our forebears had of each kind of plant and its caprices and possibilities never ceases to astonish me.” It is “knowledge now largely lost, or at least confined to academic tracts of whimsical accounts like this one. Maybe when the fossil fuels run out and heavy industry has run its course, such wonders may be rediscovered.”

Of course trees have mysteries far deeper than the functional qualities of the material comprising their “stick up the middle.” Despite all of his botanical knowledge Tudge does not know why the Indian rain tree goes out of its way to encourage epiphytes—plants that grow on other plants—when most trees seek to discourage them. Some indigenous people possess enormous botanical knowledge. Tudge writes of Brazilian Indians known as mateiros whose expert knowledge of tree species and their uses outstrips that of the wisest professor. Not all indigenous knowledge of trees, however, may be correct. Tudge is unable to confirm whether the asoka tree (a close relative of the Indian rain tree) does in fact blossom more vigorously if kicked by a young woman, as reputed in Indian folklore. In the botanical world, however, scientific facts can be more fascinating than folklore: Who would have imagined that alder trees accumulate gold in their tissues, or that their near relative the birch accumulates heavy metals in its leaves, and so can be used to clean up toxic mine sites? The Tree is full of similarly wonderful scientific facts and folklore.

After leading us through such diversions Tudge asks again—but this time from a functional point of view—what is a tree? His answer, which pertains to all life including ourselves, is profound. Living tissue, he says, “is constantly replacing itself, even when it seems to stay the same. It is not a thing but a performance.”

The performance that is a tree is in fact an interaction between the four elements recognized by the pre-Socratic Greeks: air, fire, earth, and water. Air is the principal ingredient, for trees are quite literally made of air, or at least the carbon dioxide (CO² ) it contains. For this gas, combined with the hydrogen wrested from water (H² O) by photosynthesis, is what makes those great trunks, arching bows, and leafy canopies. (Photosynthesis occurs when the green pigment chlorophyll assists in combining carbon dioxide, water, and light, thus transforming the sun’s energy into a chemical form.) Next time you look at a tree, think that it was, not so long ago, CO² wafting about in our atmosphere. It’s a thought that has great import for our battle to control global warming. For all our ingenuity, humanity has never devised a machine that can so efficiently and elegantly convert greenhouse gases into the most wondrous natural sculptures known.

There are “around” 60,000 species of trees and still counting, which prompts Tudge to ask, “How on Earth can anyone—the most astute of hunters and gatherers, or the most learned of professors—keep tabs on 350,000 or so species of plants, including around 60,000 trees?” The answer—indeed a sort of salvation—comes in the form of D.J. Mabberley and his wondrous Plant-Book. From Aaron’s rod to the obscure Zyzyxia, it lists them all in alphabetical order, and with such wit, elegance, and compression as to be breathtaking. Indeed I suspect that it is the source of many of the more intriguing facts enumerated by Tudge.

The Plant-Book is, being all encompassing and indispensable, the botanical equivalent of Johnson’s Dictionary. In the acknowledgments Mabberley provides the astonishing information that “the first edition…was typed with the author’s right index finger on a Brother electric typewriter.” The undertaking of this Herculean effort clearly required fortification, and Mabberley records his “appreciation of the work of Philip Glass, Malcolm McLaren, Franz Schubert (1797–1828) and Carl Maria von Weber (1786– 1826), which has kept me sane during some of the more tedious episodes.”

A part of just one of the hundreds of thousands of entries that comprise this work must suffice to testify to its excellence:

Cannabis…more northerly cult. (in China for 4500 yrs, obligatory crop in Eliz. times in GB, where illegal since 1951) for fibre (hemp used for ropes, fibre-board, paper etc. since 4000 BC esp. in N & NE China where form. only fibre available, prob. used in first paper (AD 105) there) & subsp. indica (Lam.)…more southerly cult. principally for psychotropic drugs (marijuana, marihuana (Mex.), pot (US, where allegedly the biggest cash crop worth $32 billion), dagga (S Afr.), kif (Morocco)), cannabis resin, which exudes from the glandular hairs & is used like opium (effects described 2736 BC by Chinese Emperor Shen Neng). In India, 3 common forms: ganja (dried unripe infrs), charras or churras (resin knocked off twigs, bark etc.) & bhang (largely mature lvs of wild pl). Smoked (“weed”) with or without tobacco (“skunk”) in cigarettes (“joints”) or taken as an intoxicating liquid formed from it (hashish; Arabic for “hashishtaker”=root word of “assassin”), in food or drink (e.g. in comm. beers in the Netherlands) it has a stimulating & pleasantly exciting effect, relief from muscular sclerosis, cerebral palsy & glaucoma, though addictive & in excess can cause delirium & “moral weakness and depravity” (Uphof). Seeds the source of hemp seed oil used in varnishes, food, soap, lip balm & fuel in Nazi tanks etc. & used as birdseed & to attract fishes.

Such are the uses of a single species, whose only near relative is the hops vine that gives us good beer. After reading The Plant-Book and The Treeit is plainly obvious that the extinction of even a single tree species, out of the 60,000 known, would be an immense tragedy for humanity and the natural world.