The Life of a Leaf

The University of Chicago Press

Contents

Chapter One

WHERE TO S TART? Maybe before reading further, you should glance out the nearest window. Unless you're stuck in a prison cell or high-rise apartment, you can probably see vegetation, green stuff you ordinarily ignore. It's just life's wallpaper, something that provides a comfortably neutral foreground and softens the starkness and angularity of distant land. Those unassuming bits of vegetation, leaves in particular, provide our present protagonist. I intend to celebrate them, not as poet (Joyce Kilmer comes to mind) or novelist (think of Joseph Conrad), but as scientist. I'll try to convince you that looking at a leaf on a tree from the perspective of a scientist enhances rather than detracts from the aesthetic experience.

I mean to do more than that, however. If the story goes as I intend, you should begin to look with different eyes at your immediate surroundings, seeing not just leaves but yourself and everything around you as reflections of the physical situation here on solar planet number four. Too often we imagine science as a body of facts, growing breakthrough by breakthrough the way a pile of pancakes rises as each new one comes off the pan. At its core, though, science is not the facts but a way of thinking; not a body of knowledge but a way of knowing; a particular and peculiar way of looking at the world. And by "world," we scientists mean more than moons and molecules. We include all the immediate and mundane, things like liquids, lions—and leaves.

As part of this attempt to alter attitudes, I have organized this book in a somewhat eclectic way, so its arrangement asks for a little explanation. Lots of people find numbers and, worse, equations at least off-putting and maybe even indigestible. Other people see them as intrinsic and unavoidable. Since almost all science is inescapably quantitative, we get a severely bowdlerized impression from any account that eschews numbers. While I mean to introduce quantitative arguments as gently as I can, I do mean to include them. Along with numbers (oops), you'll run into equations (horrors). Don't let that bother you, if it's not your métier—the basic text tells the basic story with the sequential linearity necessary for a proper narrative. (Okay, the sequence may be slightly contrived, but after all, this isn't some historical account.) The off-putting details and almost all the quantification have been piled into the footnotes. They're linked by superscripted symbols to the text, so they work rather like hyperlinks on a computer. As a result, you can ignore the formalities in the footnotes without losing the thread of the story. Recognize, though, that graphs and equations provide an economical and effective way of expressing things that torture the tongue. If you read the words and then look at the equation, you'll recognize that they say the same thing. Pretty soon you might start ignoring the words as mere cumbersome redundancy.

I want to encourage the reader to be a player as well, with emphasis on the "play" in player. That's the special advantage of asking about matters close to home. So, embedded in the text from time to time you'll find suggestions for things you might do to get a more perceptual feel for what I'm talking about, or to explore beyond what's explicitly mentioned. These "do-it-yourself " interpolations are enclosed in boxes. Again, skip over them if you wish, with no fear of losing the main thread. Finally, to minimize clutter, mention of sources, for both what's here in the text and what's not here but might be of interest, will be relegated to endnotes in the back of the book and indicated by superscripted numbers in the text.

Introducing the Protagonist

The leaf will play a particular—and peculiar—role. It represents a biological everyman, an ordinary and ubiquitous living thing that provides the subject for an exploration of our immediate physical world. We'll look into all (or most, to be honest) of the different physical matters that it has to get right in order to work properly. These are the ordinary phenomena that confront all of us, our domesticated plants and animals, and our mechanical devices. I'd allude to the "cheap physical stuff," except that in my youth that referred to some less savory aspects of human mating behavior. Nonetheless, the word physical should be taken more literally than usual. We might look at leaves with biology in mind, asking questions about ecological relationships or about ancestors and lineages. Or we might look at their molecules, at the chemistry of photosynthesis or the genes directing their formation. Here the context will instead be that of more mundane phenomena. Put a bit pretentiously, biological and physical sciences will be inextricably intertwined, as they are in reality as opposed to their dichotomization in high school and college courses.

After all, only in the nineteenth century did scientists adopt the attitude that it wasn't necessary or expected that an investigator be familiar with areas of science in which he or she didn't work. We lost any concern that a well-educated physical scientist might not casually converse with a biological scientist. Curiously, that acceptance of intellectual fragmentation arose at about the same time as the very word scientist, originally a replacement for natural philosopher, which reflected the earlier fragmentation of philosophy itself. Here I want to revert to the less specialized style of the eighteenth century. In particular, I'll not worry a bit about drawing on not just biology and physics—as currently practiced—but physical chemistry, mechanical engineering, and whatever else puts paint of a pretty color on the canvas.

The best editor with whom I've ever worked (he'll know who he is) advised me to start a book or chapter with a teaser and then move from the specific to the general—not, as in a textbook, from a principle derived up front to examples further along. Teasers, then ...

• Intercepting light. On a summer's day, a sunlit open field feels hot; by contrast you're pleasantly cool in the shade of a forest—even though air moves faster in the field. The difference speaks directly about the effectiveness of light interception by the array of leaves that form the forest canopy. We might take a lesson when designing gazebos, as well as realize how proper eaves and covered porches can improve the comfort of a house in the summer.

• Not overheating. Leaves have to absorb sunlight, and they use it inefficiently. So a broad, sunlit leaf in nearly still air can get surprisingly hot. They don't just hang in there, though, but employ a host of devices to keep cool—or at least to keep from getting hotter. Both the devices and the underlying schemes matter to us when we choose cookware, bake at least one kind of pizza, arrange clothing, or pick roofing material.

• Not being too draggy. Most of the drag of a tree comes from its leaves. Fluttering things like flags suffer lots of drag—and in the process, as you may notice, fray. But leaves do better by curling and clustering in high winds. We once built large-bladed windmills that permitted some air to pass directly through their blades to reduce their drag when winds got too strong, but we've made little recent use of flexible structures that reconfigure in strong winds or water currents.

• Getting water up. Leaves lose lots of water, which the tree must extract from the ground and lift far upward. They use pumps with no moving parts at all. Their scheme pulls water from the top rather than pushing from the bottom. Despite spectacular sucking, they manage to keep air from getting into the system and wrecking everything. We understand their wonderful trick reasonably well, but we've never managed to do much with it in our own technology.

To focus our inquiry, we might put the leaf 's basic game in a single (if legalistic) sentence: it uses energy obtained by intercepting sunlight to convert the carbon of the atmosphere's carbon dioxide into larger molecules that can provide material, and, in turn, energy, for growth and reproduction of the plant. The process, as you almost certainly know, goes by the name photosynthesis. We know quite a lot about the basic process and its variations; I mention it here so I can get away with largely ignoring it hereafter. Just don't forget that the criterion for quality—or, we might say, success—for each item that follows boils down to its efficacy in aiding this basic game.

It's a remarkably multifaceted endeavor, this business of doing a leaf 's business in a physical world, even if directed at a single end. Assuring access to light, providing mechanical support, coping with heat, deploying from a bud, dealing with wind, getting atmospheric carbon dioxide into the cells, extracting water from soil and raising it upward, deterring herbivores—lots of functions have to be decently done. The diverse devices for doing them can't fail to interact and force compromises, which must be a major reason why the leaves we encounter are so diverse. A list of the physical factors that bear on the leaf 's life gets dauntingly formidable: density of plant material, water, and air; viscosity of water and air; mechanical properties such as strength, extensibility, the elastic moduli, and others; thermal capacity, conductivity, and expansion coefficient; surface tension; wind speed; diffusion coefficient; osmotic and hydrostatic pressures—and some others. Every one of these factors bears on your life as well as on that of a leaf—some perhaps less, but most at least as strongly.

Such a complex business doesn't lend itself to a cold plunge into the particulars. It needs some context setting, so here are a few words about each of three nearly independent contexts.

About Science in General

As put a century ago by French mathematician Jules-Henri Poincaré (1854–1912), science isn't about the things but about the relationships among the things. Science tries to see order in the world around us by our best alternative to mutually accepted revelation or mythology. Sometimes that means organized catalogs, things arranged in some arguably natural hierarchy rather than some order-of-convenience-and-convention such as an alphabetical list. More often, and more powerfully explanatory, are rules that apply to a wide variety of overtly disparate and diverse items. The simpler the rule and the wider the range of things it encompasses, the greater its value. The search for predictive and explanatory general rules—that's the crux of our game.

Most often—but certainly not inevitably—our rules involve stepping down in organizational level (or moving up in sophistication, some would say). Thus we explain the motion of the planets, physical phenomena, with mathematical rules; we explain how some substances (visible powders, say) combine with molecular rules; we explain why portholes and aircraft windows have rounded corners with a general explanation of crack initiation and propagation. Reductionism describes the scheme, and it has a long history of successes.

No certain sequence defines the reductionist path, though. Should we seek enlightenment by recourse to genetics, to chemistry, to mechanical engineering, to physics, to computational modeling, or to classical mathematics? In a sense, the further "down" the better, with mathematics constituting a kind of grail. In the end, we're Pythagoreans, engaged in a search for a mathematical order that we believe characterizes the universe. But that ideal provides only the coarsest of guides; were it rigidly prescriptive we'd skip the halfway houses and all become mathematicians. For better or worse, traditions take hold—traditions traceable to past successes, to educational inertia, to factors both savory and unsavory.

In biology the dominant tradition has been reduction to molecular chemistry, now including what's come to be called genomics. As an undergraduate, I was advised to take lots of chemistry courses, which advice I dutifully followed since I wanted to become a well-prepared biologist of the next generation. As a graduate student, I happened upon a project to which chemistry had little relevance—I was worrying about the peculiarities of flight in very small insects. Enlightenment came from fluid mechanics, something to which biologists rarely paid much attention. Most of us took a single year of college physics, but the traditional physics course then—and, I think, still—says almost nothing about moving fluids. I knew about viscosity, but I'd heard about it in a course in physical chemistry, not in physics. Step by step, the questions I asked led me into the world of mechanical engineering. A reductionist path, yes, but a different one.

This book intends to make the case for explanation by reduction to physics and mechanical engineering, to this alternative realm of explanation: not to alternative explanations but to explanations of phenomena with which the biologist's classical chemical reductionism just doesn't help. As we'll see, this realm not only explains different phenomena but provides information that makes wonderfully satisfying intuitive sense. Bending, tearing, shadowing, pumping are activities that form parts of our immediate world. When, though, did you last see a molecule? While we assume molecules aren't just polite fictions concocted by chemists, our personal experience doesn't help a lot in thinking about how they behave. Electrons and photons are still worse. In graduate school I roomed for a time with a particle physicist. He ended one attempt to explain the essence of an exciting lecture by admitting, with uncommon candor, that he could think of no explanation, not even an analogy, that wasn't unacceptably misleading. By contrast, I've had the great fortune of working on questions that could be described to just about anyone, from elementary school students to novelists.

About the Biological Big Picture

Evolution by natural selection forms the centerpiece of biology. It's neither physics nor chemistry, so people argue about its position in a reductionist hierarchy. Evolution by natural selection serves here as a background presence, underlying (or haunting) every argument or assertion about how some feature works. It operates this way:

• Reproductive success drives functionally consequential changes and thus much of the design of organisms.

• Reproductive success results from effective functioning of the organism—not just in the mating game but in acquiring resources, growing, and dealing with all aspects of its surroundings.

• Since better functional arrangements lead to greater reproductive success, these arrangements will be favored in the evolutionary sweepstakes.

I like to think of evolution by natural selection as an explanatory principle based on formal logic, an "if, if, and if, then" sequence, because its logical structure conveys the proper note of inevitability. Thus, with no claim of originality ...

• Observations:

1. Every organism can produce more than one offspring, so populations, if unrestrained, will increase steadily.

2. Every organism needs some minimum amount of material from the environment to survive and reproduce.

3. The material available to a population of organisms is finite in extent, restraining the population's increase.

• Consequence of 1, 2, and 3:

4. A population in a given area will rise to some maximum size.

• Consequences of 1 and 4:

5. For a population at this maximum size, more individual organisms will be produced than the environment can support.

6. Some individuals will not be able to survive and reproduce.

• Further observations:

7. Individuals within populations vary in ways that affect their success in reproduction.

8. At least some of this variability is inherited—individuals resemble their parents more than they do more distantly related individuals.

• Consequences of 6 through 8:

9. Characteristics that increase the number of an individual's surviving offspring will be more prevalent in the population in the next generation.

(Continues...)

Excerpted from The Life of a Leafby STEVEN VOGEL Copyright © 2012 by Steven Vogel. Excerpted by permission of The University of Chicago Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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