by Colin Tudge
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. Singlecelled 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 singlecelled 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 labour, 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. Any cell in the organism that goes berserk and tries simply to do its own thing destroys the whole, and ultimately destroys itself. Such cells in medical circles 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 ON TO LAND
The first plants which can loosely be called ‘algae’ ventured on to 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 lamp-posts – 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 practises sexual reproduction, while the sporophyte practises asexual reproduction. Both ways of reproducing have their advantages and drawbacks – and plants practise 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 importantly, 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 – which allows 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 to help them 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), whic
h date from around 420 million years ago. They and their various successors have 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 40 metres high; as tall as most modern forest giants, and as high as a twelve-storey building. The straight columnar trunks of Lepidodendron, patterned all the way up with leaf scars shaped like diamonds, could be two metres 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 metres long; the size of a small rowing boat. The ecology of those lycophyte forests was doubtless as intricate as that of modern forests, and doubtless was 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 reproduce 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 that are often to be 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 metres 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 – Catamites 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 marvellous 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 favourable 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 are equipped with a food store of carbohydrate, protein and fat. The embryo and its attendant hamper is encased within a coat (a ‘testa’) that is custom-built for the circumstances that are liable to be met, 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 favourable). 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 which represent the entire gametophyte generation. In the male flower (or the male part of a hermaphrodite 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 taken place. 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 ecosy
stems, and account for at least 99 per cent 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 for their exotic beauty in warm countries.
Another way to be a tree: cycads look like palms but are quite different
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, coloured 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 coloured. 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.
