No wonder that the earth expresses itself outwardly in leaves, it so labors with the idea inwardly. The atoms have already learned this law, and are pregnant by it…The whole tree itself is but one leaf, and rivers are still vaster leaves whose pulp is intervening earth, and towns and cities are the ova of insects in their axils…The Maker of this earth but patented a leaf.
– Thoreau, Walden
Again and again in the time of Thoreau, the world saw itself in the shape of a leaf. In the days after Enlightenment but before Darwin, the obscure unities behind the Earth’s great multiplicity held an allure that verged on the mystical. Thinkers as disparate as Goethe (‘All is leaf’) and Spencer, Hegel and Whitman, seized on foliage as an epitome of the organic plan that, according to their various visions, generated all form. The same process of evolution (in its literal sense of ‘unfolding’) that carved the veins in a blade of grass, perhaps, guided just as surely the heaving of mountains and the march of civilization–and if one were to study the former with enough care, who’s to say one couldn’t anticipate the latter?
Ironically enough, one of Darwin’s broadest impacts was to nip such notions in the proverbial bud. By proposing a unity that seemingly relied on the absence of all plan or direction, the Origin turned the central Romantic questions on their head. Multiplicity could not be explained by positing a God who reveled in diversity, or some vital drive of all matter to differentiate and rearrange itself constructively. Instead, the question became: if all life on Earth is the product of the same ruthless, pointless reproductive tournament–a process that seems, if anything, guaranteed to destroy rather than construct–then why are there so many different kinds of things? Why, so to speak, have the winners not taken all? The problem of diversity, of the biosphere’s seemingly gratuitous extravagance in spinning off new varieties and varieties upon varieties, was a lifelong thorn in the side of Darwin himself, and filling in its solution has been a preoccupation of Darwinists ever since.
With the workings of natural selection laid bare, the same phenomena that had furnished proof of a congenial Creator, or of an irresistibly fertile life force–the nightingale’s song, or the rainforest’s kaleidoscopic lushness, or the riotous profusion of beetles–now seemed like glaring violations of Nature’s decree against waste. And what better demonstration of this than leaves themselves? Maple and oak, elm and walnut and beech: what had once served as a textbook example of the universe’s creativity, its skill in rendering artful variations on a simple, elegant theme, now read as an awkward footnote to a story about function. Especially awkward, in fact, because if anything ought to be purely and straightforwardly functional, surely it’s foliage. Right? Trees are not like beetles, dividing their labor in the diverse economy of nature. They’re not like songbirds, needing to brand themselves visually or to impress their neighbors through performance. Leaves are all employed in the same blue-collar business, as eaters of light, and they all do it in essentially the same way, with essentially the same toolkit. Right? So after four hundred million years of refinement, why is every forest not now a monoculture, with every tree wielding the same streamlined, optimized leaf?
To wrestle with this question is to confront our kingdom-based chauvinism. The life of a plant, so simple from afar, is in fact no less dynamic and no less challenging than our own. Leaves are far more than light eaters–and even that business is far from a simple one. Light is not just universally there for the taking, and just as the search for carbohydrates has variously shaped us as animals, the fight for light–in different quantities, at different times, under different pressures–has driven a spectacular diversification of form in our vegetable neighbors.
First: leaves are expensive. They have to be gotten to where the light is, arranged so as to best intercept it, and then held there until conditions change. The constraints of support favor certain leaf shapes over others–notably arrowhead shapes, heart shapes, and other base-heavy designs, in which the bulk of the leaf is concentrated near the stem. These shapes are most common in plants for whom leaf support is especially challenging. Take vines, which have given up their robust limbs in favor of climbing the limbs of others, but which nonetheless must stretch their light-eaters as far as possible to avoid being shaded by their hosts. The ideal vine leaf, therefore–think of grapes and morning-glories and ivy–has an unusually long petiole (leaf stem) and a leaf blade with counterweights (such as the ‘lobes’ of the heart) for balance. (This association is strong enough, in fact, that when these shapes happen to arise in non-vines, they may put their owners on the fast track to the climbing life. Among the ubiquitous aster family, there is only one common vine species in much of North America–Mikania scandens, whose closest non-vine cousin is the distinctively arrowhead-ish Ageratina.) Similar shapes often occur in emergent aquatic plants (such as the aptly named arrowheads, Sagittaria), whose petioles must both stretch above the surface and hold their positions against a current.
That all leaves are not spades or wedges suggests that there is a price to be paid for this approach. The nature of that price is perhaps best appreciated by considering a leaf at the opposite end of the shape spectrum: the colossal blade of the bigleaf magnolia, Magnolia macrophylla, with its bulk concentrated towards its great, floppy tip. Support is far less of an issue for a magnolia, with its stout twigs that are obliged to carry only a few leaves apiece. What is an issue is light, which is at a premium for a tree that makes its home in the shadowy understory of lush hardwood forests. In such an environment, if you want to most efficiently harvest what little light filters through the canopy, it’s imperative not only to minimize shading by neighbors but also to minimize the risk of shading yourself–that is, to arrange your leaves so that each is as unlikely as possible to obscure another. This criterion favors a shape that expands outwards, collectively forming a fan of foliage with maximum coverage and minimum overlap.
Most plants, of course, feel neither of these pressures quite so disproportionately. Most plants have struck a compromise between balancing their leaves and stretching them toward the light. That is why, if you were asked to draw a ‘basic’ leaf, you would likely draw a simple elliptical shape, widest at the middle. The notion that this shape really is somehow basic, the ur-leaf, is encouraged by the fact that it is so prevalent in the tropics–where much of early plant evolution took place, and where the environment is in many ways least rigorous. Tropical plant identification is not for the faint of heart, and not just because there are so many species; as I myself complained upon first setting foot in Costa Rica, they all look the same! Elliptical, smooth-margined, medium-sized, leathery; these leaf characteristics, which would eliminate all but a handful of trees or shrubs in Pennsylvania, are no help at all in Peru. Here is a welcome dose of perspective for us spoiled, Peterson-armed temperate botanizers. And here, too, is a clue to the next big question to ask.
Instead of imagining the ur-leaf, picture the most distinctive leaves you can think of. White oak. Sugar maple. Sycamore. Sweetgum. Hickory. Buckeye. Tuliptree. Locust. (Those of you from outside the eastern United States, bear with me.) Is it a coincidence that all of these are deciduous trees of temperate forests? Probably not, but the answer to this puzzle is complex and controversial. To get at it, we’ll need to understand the particular pressures that the deciduous lifestyle imposes on its followers, including the urgency of beginning photosynthesis in the spring, and the annoyance of herbivory. But that–as well as other fascinating phenomena, like the strange shapes of underwater leaves, and the use of leaf shape as “camouflage”–will have to wait till Part Two of this article.