What Is a Tree?

Animal, mineral, or vegetable? Whenever our parents bundled us into the car for a long journey my sisters and I kept ourselves occupied with that guessing game. At its heart is the puzzle of how things should be classified, the more ambiguous the better. My inventive youngest sister came up with “a cow’s moo.” Through its astonishing revelations about what is related to what in the plant world, Colin Tudge’s The Tree reawakens the pleasure of those childish games. But The Tree is a far deeper book than this might suggest, for its author has a remarkable ability to ask fundamental questions about trees and their world—questions that, much to our detriment, most of us stopped asking as we grew up.

Humans are innate classifiers, and our earliest efforts were doubtless classifications of convenience: edible and inedible, for example. Despite this predilection, our workaday world is filled with appalling classifications. Consider the forester’s venerable division of “softwoods” for the conifers (including the remarkably tough parana pine of South America) and “hardwoods” for the broad-leaved trees (which include the very soft balsa). Despite its obvious potential to mislead, it’s so convenient for common lumber that it’s used by almost everyone.

And yet we instinctively recognize a good classification. Perhaps it comes from our sense that the natural world has a true orderliness. Birds, for example, are instantly recognizable as such, as are mammals and frogs; and so with smaller groups like kingfishers, hawks, and doves. In pre-Darwinian Europe natural philosophers hoped that by comprehending the “true classification” of nature they might glimpse the mind of the Creator. Yet in the absence of divine revelation, how could they hope to discern the one and only “true” classification from less perfect models?

In the mid-eighteenth century the Swedish botanist Carl Linnaeus provided natural philosophers with a hierarchical scheme that classified living things into kingdoms (of which he had just two—plants and animals), classes, orders, genera, and species. It is these final two categories that provide the scientific name. Linnaeus, for example, classified himself and other humansas Homo sapiens (though at first he dubbed us the rather less appropriate Homo diurnis—daytime man). The first part of the double-barreled name is the generic part. It is shared with a group of similar species, and so acts rather like a surname in a family. The second—the species name—is unique in the genus to that individual species, and so acts like a Christian name.

Linnaeus’s scheme was brilliant in its simplicity, and it is the basis of the universal scientific classification used by all taxonomists (classifiers of living things) today. When it came to higher levels (the classes and orders) of plants, Linnaeus turned to vegetable sex. And this scheme, carried on by botanists down the centuries, really has, to borrow from Andrew Marvell, grown “vaster than empires, and more slow.” Linnaeus’s highest plant categories—the classes—were arranged according to the nature of the plant’s male genitals (stamens), while his next level down—the orders—was based on the female…

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