Tree is not a distinct category, like ‘dog’ or ‘horse’. It is just a way of being a plant. The different kinds have much in common and is good and necessary to have some feel for what is essential. But the essences of nature will not be pinned down so easily. In the end, all definitions of nature are simply for convenience, helping us to focus on the particular aspect that we happen to be thinking about at the time. There is no phenomenon in nature – whether it’s as simple as ‘leg’ or ‘stomach’ or ‘leaf’ or more obviously conceptual like ‘gene’ or ‘species’ – that does not take on a variety of forms, and which cannot be looked at from an infinite number of angles; and each angle gives rise to its own definition. … The way we define natural things influences the way we treat them – whether we speak of wild flowers or weeds, of Mrs Tittlemouse or of vermin. But in the end nature is nature, and we must try with different degrees of feebleness, and for our own purposes, to make what sense of it we can.
Metabolism – the basic business of staying alive – is half of what living things do. The other half is to reproduce. It is not vital to reproduce in order to stay alive – indeed, reproduction involves sacrifice; reproduction, as we will see later in this book, is often the last fling: many a tree dies after one bout of it. But it is essential nonetheless. At least, all creatures that do not reproduce die out. However successfully an organism may metabolize, sooner or later time and chance will finish it off. Everything dies. Only those that reproduce endure – or at least, their offspring do. All individuals are part of lineages – offspring after offspring after offspring.
All of life’s requirements – metabolism, reproduction and the business of getting along with others - are difficult. Each creature must solve life’s problems in its own way. There is no perfect, universal life strategy. Each has its own drawbacks and advantages.
So it can pay a creature to be very small; or it can pay to be big. Each has its pros and cons. A plant this is big like a tree can stretch further up into the sky, and so capture more of the sun’s energy; and reach further down into the earth, for water and minerals. This is the upside. But it takes a long time to achieve a large size, and whether you are an oak tree or an elephant or a human being, the longer you take to develop the more likely you are to be killed before you reproduce.
… German Alexander von Humboldt who, together with the French physician and amateur botanist Aime Bonpland, travelled 10,000 kilometres in South America between 1799 and 1804, on foot and canoe. They collected 12,000 specimens of plants, including 3,000 new species, and hence doubled the number known from the western hemisphere. On their return they published the thirty volumes of Voyage aux regions equinoxiales at von Humboldt’s expense (it cost him his entire fortune), of which Bonpland contributed just one, although von Humboldt insisted that they share the authorship of the whole.
The bark of the 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 the fire to take hold in.
Best known of all Lauraceae is the avocado, Persea Americana, native to Central America. It has more protein than any other fruit and is 25 per cent fat. It also has a wonderful strategy to prevent inbreeding. As with other Lauraceae, it is pollinated by insects. It has two kinds of flowers, inventively dubbed A and B: some individuals have A flowers and some have B flowers. The stigmas of A flowers are receptive to pollen only on one particular morning, while the anthers of A flowers do not release their pollen until the afternoon of the same day. In B flowers, the stigmas are receptive on one afternoon, but do not release, while pollen is not released until the following morning. So A flowers can only be pollinated by B flowers, and B flowers can only be pollinated by A flowers.
Organisms like us, which need their food ready made, are called ‘heterotrophs’. But the buck has to stop somewhere – and in most earthly ecosystems, it stops with plants. Plants make their own carbohydrates, fats, proteins, and everything else they need from raw materials – simple chemical elements, and the simplest possible chemical compounds. They obtain their energy to do this from the sun. They are ‘autotrophs’: self-feeders.
Water molecules cling tightly together. Their cohesive strength is prodigious. Were it not so, trees could not pull water from below, and could not grow so tall; but in practice the forces are such that a tree could grow to a height of three kilometres if the tensile strength of water was the only constraint on growth. Even as things are, the threads of water may sometimes break – an accident known as ‘cavitation’ - leaving a space in the vessel that a plumber would call an airlock… Given time and favourable conditions, plants can eventually fill this space again, and normal service is resumed. Otherwise, if cavitation is too great, the tissues that depend on the vessel may die.
There are many more twists as the game of chemistry unfolds. An array of plants has been shown to produce various terpenes only after insects have begun to feed on them: another economy. These terpenes discourage the invading pests from laying their eggs. But in addition they also attract the natural enemies of the pests, so that they fly in from far and wide to see the pests off: swarms of parasitic wasps or ladybirds to feast on aphids. So far such effects have been shown in maize, cotton and wild tobacco. I know of no specific examples in trees – but again it would be very surprising indeed if they were not to be found.
…when elephants in Africa feed from the mopane trees of Africa…, they take just a few leaves before moving on to the next tree. Furthermore (so it is claimed) the elephants move upwind to a new tree. Evidently the mopane increases its output of tannins as the elephants browse, so the leaves become less and less palatable. Evidently, too, they emit organic materials (…perhaps the tannins themselves) that act as pheromones, and so warn other mopanes downwind that an attack is imminent and they too should produce more tannins. Some acacias are said to behave in the same way, in response to giraffes.
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