by Colin Tudge
Evolutionarily speaking, bryophytes may be seen as a dead end. The ancestors of modern trees are not to be found among their ranks. The option of being big was left to other lineages, which did develop plumbing.
TRANSFORMATION 6: PLANTS WITH “VESSELS”—AND THE FIRST STIRRINGS OF WOOD
Some time around 420 million years ago, in the late Silurian, other groups of land plants emerged that did solve the problems of being big. These were the first “vascular plants,” with columns of cells that act as conducting vessels, providing them with a plumbing system—comparable with the bloodstream of animals—that allowed them to grow big, and for different parts of them to become specialized without losing touch with one another.
The early vascular plants also invented lignin. Chemically speaking, lignin is not spectacular. It is a fairly small molecule, but it serves to toughen the cell walls of plants, which are made of cellulose. Pure cellulose is flexible—it is the stuff of cotton—but cellulose spiked with lignin is tough and hard. Lignin, in short, is what turns floppy cellulose into wood. Plants that lack lignin (or have only small amounts) are called “herbs.” They can grow fairly tall, like the stems of tulips, say. They can stay upright because each of their cells is filled with water under pressure, and this water pressure (“turgor”) gives them resilience, like a well-inflated football. But such plants wilt when their water supply fails. Plants with lignin can outride dry periods and can grow far bigger than any herb. Many herbs have some lignin that toughens them here and there, yet they remain primarily herby. Bona fide wood requires special architecture—the lignin-toughened cells meticulously stacked and interlaced. With lignin and appropriate architecture, then truly we have wood. Although we may admit bananas as honorary trees for the purposes of discussion, in truth it is wood that makes trees. In practice, it is mainly the cells of the conducting vessels that become lignified, and they and their surrounding, supporting cells are the main stuff of timber. Creatures like us have a blood supply to carry water and nutrients around the body, and a separate skeleton to keep us upright. The woody plumbing system of trees serves both purposes.
Full-blown treedom, though, took a long time to achieve. The very earliest vascular plants were little bigger than matchsticks (and not as stiff), as they emerged from swamps. Among the oldest of them are the rhyniophytes (named after the Scottish village of Rhynie, where their fossils were first discovered), which date from around 420 million years ago. They and their various successors are long gone, but shortly after they first appeared, one of their number gave rise to the two great lineages that are still with us today, which between them include all the living plants that are larger than moss. Both of these lineages, quite independently, invented the form of the tree—and one of them, at least, reinvented the tree form several times.
THE TWO GREAT LINEAGES OF BIG LAND PLANTS
The first of these two great lineages are the lycophytes (Lycophyta). The surviving types are small—club mosses, selaginella (also mosslike), and quillworts (which look like sprouting onions). But in the deep past, spanning the Carboniferous period and lasting well into the Permian (from about 360 million years ago to around 270 million years ago), the lycophytes produced a range of forest trees. Their architecture was primitive: their roots and branches divided simply, each into two equal parts, like a Y. But some of these ancient trees were magnificent. Lepidodendron could be up to forty meters high—as tall as most modern forest giants, and as high as a twelve-story building. The straight columnar trunks of Lepidodendron, patterned all the way up with leaf scars shaped like diamonds, could be two meters across at the base. They formed great swampy forests. Among the strange animals that roamed within them were eurypterids, like giant scorpions—some aquatic, some land-bound, and some more than two meters long, the size of a small rowboat. The ecology of those lycophyte forests was doubtless as intricate as that of modern forests, and doubtless played out by much the same rules—and yet the cast list of players was utterly different. Some of those early forest creatures have left descendants, but others (including the eurypterids) have not. They have had their hour upon the stage.
So it was among the lycophytes, plants that are now known only to botanists as also-rans, that some of the world’s first trees emerged—perhaps the very first—and some of them were magnificent. Yet, like the bryophytes, the lycophytes lack true leaves. In lycophytes, the organs that resemble leaves are really just scales. It was left to the second great group of vascular plants to invent true leaves. These were, and are, the euphyllophytes (“good leaf plants”), which contain all our living trees. The euphyllophytes, like the lycophytes, had a magnificent past. Unlike the lycophytes, they also have a magnificent present.
The earliest euphyllophytes, like the bryophytes and the lycophytes, continued to reproduce sexually by means of eggs and sperm and asexually by means of spores. But somewhere around 400 million years ago (by now in the Devonian) the euphyllophytes divided again into two great groups. One group, now known as the monilophytes, continued to reproduce in the traditional way—a generation that produces eggs and sperm, then an alternate generation that produces spores. The other group gave rise to the spermatophytes—the group that reproduces by seeds. Both groups independently gave rise to trees—and, indeed, in both groups the form of the tree arose several times.
The monilophytes include present-day ferns and horsetails. Ferns nowadays are hugely various and include many tree-like forms. “Tree ferns” form significant forests in much of the tropics and subtropics (which I have been privileged to walk among in New South Wales—a must for all connoisseurs). More accessibly, they also turn up in botanic gardens throughout the world—even in England (as in Cornwall’s Lost Gardens of Heligan).
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 adve
nture 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.
SO 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
IF 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.
