by Colin Tudge
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 – or at least you could, if you did 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, half-moon-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).
Ginkgoes too were once diverse. Now only one species remains
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 Gardens 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: and 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, or are, 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 each other 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 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).
The Gnetales taken all in all are now a minor group (if very curious) and so far as can be seen, they always have 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 per cent 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 which enables them to grow so big, and live so long, and has enabled them to occupy a third of all the world’s land: wood.
4
Wood
A young yew – with perhaps another 2,000 years to live
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 suggest 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 artefacts to show off wood at its best. But the wood itself, on which they work their creativity, is nature’s invention.
The terms ‘wood’ or ‘timber’, like the word ‘tree’, tend to be used in different ways. Some 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 broadleaved angiosperms – the ‘broadleaves’ being all the dicots, from magnolia to oak and teak.
This description focuses on ‘true’ wood: as in conifers and broad-leaves. In both cases, the basic component of the wood, which makes it both functional and strong, is or are the conducting tissues, the basic plumbing. These tissues are 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, downwards and outwards 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 towards the middle. The ‘medullary rays’ run in just this fashion from the centre 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 inwards and outwards, 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 online. 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 b
roadleaves 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 broad-leaves may practise 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 20 or so metres 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 they lose their function as conducting tissue. Clearly, in any one tree-trunk, the xylem closest to the centre 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 centre 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 cocoa, 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 clear to see, 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, 2004 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 is a distinct wet and dry season 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 cells walls. Cork is wonderful material: it is light; waterproof (hence preventing excessive water loss); it helps to 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 air-proof, and prevents exchange of gasses. 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. Of course, 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, 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: fibrous, and up to 30 centimetres thick. Others, like Enterolobium ellipticum (it has no common English name), which must endure periodic fires on the dry forest of the Cerrado in Brazil, have huge ridges of corky bark. I suspect the ridges help to create an up-draught which carries the heat up and away from the trunk.
Many trees shed their bark, sometimes in great swathes, 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 anti-fire 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 ‘pre-adapation’: 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 Olea laurina (a heavy-timbered 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 they are soon likely to be overtaken by other trees and will then be overwhelmed, so 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 is 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 500 years or more. Many other trees begin life in the shade as part of the understorey, 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, colour – 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, and the genes that influence colour may be doing something truly useful as well, for example repelling pests. Or they may not. Provided their side effects do no positive harm, then these genes will be passed on through the generations, whatever eccentricities they bring with them.
By the same token, different species have different patterns of grain and ‘figure’: grain being the narrow stripes that run along the length of the wood and appear when the growth rings are cut across, and figure being the general appearance, whether the growth rings are cut across or not. These variations represent differences in micro-architecture. Clearly, it is vital to a tree that its wood should be functional. Equally clearly, the very fine details of structure do not matter too much – especially in the heartwood, whose only tasks are to provide strength and bulk. We can imagine, then, that trees contain a variety of genes that in some way or other influence grain and figure – to the extent that experts can (usually, and at least in theory) identify the species of any kind of timber from these patterns. The small genetic variations that cause these differences do not matter to the tree.
