The Tree (9 page)

Yet we see a far more spectacular illustration of nature’s collaborativeness within the fabric of the eukaryotic cell itself—the very structures of which we ourselves are compounded. For the eukaryotic cell is a coalition. It was formed initially by a combination of several different bacteria and archaea that hitherto had led separate lives (and others are probably involved, besides the proteobacteria and cyanobacteria). Over the past two billion or so years the eukaryotic cell, innately cooperative, has proved to be one of nature’s most successful and versatile creations. There could be no clearer demonstration that cooperation is at least as much a part of nature’s order as is competition. They are two sides of a coin.

The ancestors of today’s plants arose from the ranks of the general melee of eukaryotic cells. These first ancestors contained chloroplasts and were green, and these can properly be called “green algae.” Many single-celled green algae are still with us (they often turn ponds bright green).

It’s a reasonable guess that the first green algae appeared on earth about a billion years ago. Thus it took about 2.5 billion years to get from the first living things to single-celled green algae, and only another one billion to get from single-celled algae to oaks and monkey puzzles. Still, we tend to think of algae as “simple” and primitive. If we took the long view and considered all that life entails, we might rather argue that by the time the first green algae evolved, it was all over bar the shouting. (Though there was still an awful lot of shouting to be done.)

TRANSFORMATION 4: ORGANISMS WITH MANY CELLS

Organisms that have only one cell are doomed to be small. There are many advantages in smallness: there is more room for small organisms than for big ones, and a virtual infinity of niches to exploit. Single-celled organisms are easily the most numerous and always have been—living free wherever there is moisture, in oceans and lakes and soil, as inhabitants of bigger creatures’ guts, and as parasites of bigger creatures.

But there are advantages in being big, too. A whole range of ways of life are open to big creatures, whether trees or people, that small ones cannot aspire to. To become large, organisms must become “multicellular.” Creatures like oak trees and us have trillions of body cells.

Multicellular organisms must originally have arisen from single-celled organisms. At its simplest, a multicellular “organism” is little more than a collection of cells that have divided, but failed to separate. The real transition comes about when the different cells in the bunch begin to take on specialist functions—some producing gametes, some not; some photosynthesizing, some not; and so on. Then we see real division of labor, and real teamwork. Then you have what the great English biologist John Maynard Smith was wont to call a “proper” organism, with each cell dependent on all the rest, and groups of cells cooperating to form organs, such as lungs and livers or leaves and flowers. This degree of collaboration requires enormous self-sacrifice: to be a member of a bona fide organism, each cell must give up some of its own ability to live by itself. Each cell has to trust the others, so to speak. Any cell in the organism that goes berserk and tries simply to do its own thing destroys the whole, and ultimately destroys itself. In medical circles, such cells are said to be cancerous.

In fact, there is a spectrum of compromise positions between cells that can live perfectly well by themselves (as single-celled organisms) and cells that are utterly dependent on those around them (like human brain cells). Thus many cells from many organisms (including many of ours) can be grown indefinitely in special cultures. Many cells from many plants, once cultured, can then be coaxed to develop into whole new organisms. Indeed, many plants (including many of the most valued trees, such as coconuts and teak) are now cloned by cell culture. On the whole, though, the generalization applies. True multicellularity is possible only because the individual cells give up their autonomy, each relying on the rest for its survival and for the replication of its genes.

TRANSFORMATION 5: PLANTS COME ONTO LAND

The first plants that can loosely be called “algae” ventured onto land around 450 million years ago. On land they faced, for the first time, the problems of gravity and desiccation. Some of the earliest algal pioneers evolved into mosses, liverworts, and hornworts, known collectively as “bryophytes.”

None of the bryophytes has ever come properly to terms with the special difficulties posed by life on land. They duck the issue of gravity by staying squat and small, and hence extremely lightweight. They never solved the desiccation problem. They remain confined to damp places—but because there are plenty of damp places, they are extremely successful. Mosses in particular abound on damp walls and rocks just about everywhere. They are a huge presence in forests, as epiphytes. Some, particularly the sphagnum, or peat, mosses, form vast swards in the wet tundra and tend to prevent other plants from growing there. Mosses in general overcome desiccation not by resisting it, as a leathery-leaved holly tree or a spongy, water-packed baobab will do, but by putting up with it. They can be dried to a virtual crisp and yet spring back to life.

An aside is called for on the reproduction of mosses—for they illustrate one of the fundamental phenomena of botany, and without some inkling of it, we cannot properly understand the reproduction of the plants that mainly concern us in this book: the conifers and flowering plants. The phenomenon is known as “alternation of generations.” The moss that is a permanent presence on walls and tree trunks is called the “gametophyte generation” because it produces eggs and sperm (gametes), which fuse to produce embryos, which grow into the “sporophyte generation.” (It is odd to think of plants producing eggs and sperm, but that is what the primitive types do.) The sporophytes appear among the general background of “leafy” moss as little upright structures that commonly resemble tiny lampposts—the “lamps” at the top contain spores. Spores are little more than packets of unspecialized cells, encased in some protective coating. They are dispersed by various means (not least by water), and if they land in some comfortably damp spot, they multiply and differentiate to produce new mosses of the gametophyte type. The sporophytes, which produce the spores, cannot live independently. They depend entirely upon the gametophyte.

Thus the gametophyte practices sexual reproduction, while the sporophyte practices asexual reproduction. Both ways of reproducing have their advantages and drawbacks—and plants practice both, in alternate generations. In this they are ahead of us. We (together with most but not all large animals) reproduce only by sex.

Bryophytes could never have given rise to trees. Their overall body structure is too simple. They have no proper roots, merely anchoring themselves by projecting “rhizoids,” which have no special role in absorbing nutrients and water. Most mosses look as if they have leaves, but they are not true leaves, just green scales. Most important, bryophytes have no proper, specialist conducting tissue within them, to fast-track water and nutrients from one part of the plant to another (or at best they have very rudimentary conducting tissue). Lacking specialist plumbing, they are bound to remain small.

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.

T
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WO
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INEAGES OF
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LANTS

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).