The Tree Read Free Page A

a nursery, you need some guidelines. But guidelines are not definitions. They are ways of helping practical people do practical things. They do not—and are not intended to—capture what Aristotle would have called the essence of nature.
    For many trees grow big when conditions are favorable, and stay small when they are not. An oak is a noble tree in a forest or a park, but an acorn that falls in a fissure in some Scottish crag may spend a couple of centuries in bonsai mode, never more than a twisted stick. Yet it may turn out acorns that, if they should be carried to some fertile field, could again produce magnificence. Is the twisted stick less of an oak because it fell on stony ground? And if it remains an oak, is it not still a tree? Then again—a different kind of case—the world’s many kinds of birches form the genus
Betula.
None are as huge as an oak may often be, but most are perfectly respectable trees. Yet there is one,
Betula nana,
that is adapted to the tundra of the north of Scotland and mainland Europe and is very small indeed. Do we say that all birches are trees except for the tough little
Betula nana
? Or do we say it’s a dwarf tree?
    What of the stick that runs up the middle, the “trunk,” which holds the “crown” of the tree aloft? Should there be just one, a solitary pillar, or are several allowed? Many a gardener and forester has insisted that plants with a lot of supporting sticks should be called shrubs. Again, for practical purposes such distinctions can be useful. If Alice’s Queen of Hearts had instructed her long-suffering gardeners to plant her an arboretum and they’d come up with a shrubbery, their heads would surely have come off. But wild nature is not so easily pinned down. In the Cerrado of Brazil—the vast, dry forest, about the size of France, in the middle of the country to the south and east of Amazonia’s rain forest—there are trees that form bona fide, big, one-trunked trees when they grow along the banks of the occasional rivers but become multistemmed, short shrubs where it’s drier. The shrub is not merely stunted, like the oak in the rock. It is a discrete life-form. Many organisms exhibit what biologists call “polymorphism,” meaning “many forms.” Many kinds of fish, for example, have dwarf forms and full-size forms; some butterflies and snails are highly variable. Here we see a polymorphic tree—one form for the forest, another for the open ground.
    Then again, many big trees, including some cedars, many a mulberry, and the beautiful blue-flowered jacaranda, may grow from ground level with several solid trunks of equal magnitude. Each may be as big as a respectable oak. Are they trees or big shrubs? The family of the heathers, Ericaceae, also includes the rhododendrons from the Himalayas and the madrone trees of the United States, with their beautiful flaky, yellow, pink, and gray trunks (which add yet more color to the already wondrous hills of California). Rhododendrons tend to have many stems, while madrones are commonly content with one. But the rhododendrons can be just as big and solidly wooden as the madrones. In nature, in short, trees and shrubs are not distinct. Why should they be? Nature was not designed to make life easy for biologists.
    Must the central stick be of wood? That, after all, is what we generally mean by “stick.” How, then, should we categorize banana plants? In general shape they resemble palm trees, with a thick central stem and a whorl of huge leaves at the top. But the stem of the banana plant is not made of wood. Its stem is formed largely from the stalks of the leaves, and its strength comes from fibers that are not bound together, as in pines or oaks or eucalypts, to form true timber; its hardness is reinforced, as in a cabbage stalk, by the pressure of water in the stem. So botanically the banana plant is a giant herb. But it looks like a tree and competes with trees on their own terms, as a big plant seeking the