National Geographic : 2012 Oct
trees tend to evolve to be as tall as possible, given the limits of physics and precipitation. Without competition, every forest would be a thick lm of green life. e battles among plants have changed their stems and their veins. Leaves with more veins can carry more water to the chloroplasts, allow- ing the chloroplasts to make more sugar and the plants to grow faster. ese species in turn can hold their leaves alo to occupy more space in the sky and consume more sunlight before others get to it. rough time the plants that were able to produce more and more veins in their leaves won many battles and some wars. Leaves with densely branched patterns of veins are also able to grow more quickly. e veins of a maple leaf, for instance, are like the roads of a city; they go everywhere and o en intersect. ey tra c in nutrients and water. e maple leaf can quickly get what it needs to continue to feed from the sun. Other leaves are not so lucky. Amid the seething competi- tion for space in tropical forests, pity the single-veined leaf. Plants have more to cope with than competition from other plants. e evidence of animals eating leaves is almost as ancient as the evidence for leaves themselves. In fossil dinosaur poop one nds evidence of an- cient leaves. In fossil leaves one nds the holes made by ancient mouths. Nothing on life's menu is more popular. Moths, butter ies, beetles, fungi, monkeys, sloths, and great loping monsters like cows, bison, and gira es eat the hard-earned greenery of plants, which, for all of their ingenuity, have never gured out how to run away. So leaves resort to self-defense. Some plant leaves have become special- ists in deadly tricks. Grass blades evolved the ability to accumulate the silica from the soil---becoming like tiny glass slivers, which ruin the teeth of browsers like cows one bite at a time. Other plants use chemicals to make themselves unpalatable or even poisonous. Sometimes the weapons are visible: latex oozing out of a vein or tingly hairs projecting from leaf blades. Other times they lurk unseen, waiting for the unsuspecting victim, be it the larva of a moth or an undiscriminating sheep. Climate, competition, defense---these evolutionary saws and scis- sors can explain much of the diversity of leaves. Yet if you pick up two leaves in your backyard, most of what di ers between them---the details naturalists have spent thousands of years naming---remains unaccounted for. Evolution can whittle similar forms again and again when confronted with similar circumstances. But through innovation and chance, evolution can also work in the abstract: Jackson Pollock dashing paint on the canvas of life. We should not expect to understand every tomentose blade or arachnoid lobe. Sometimes it is enough to step back and know a master- work when we see one, whether it hangs in a museum or from its petiole on the branch of a park tree. Not that leaves care whether you notice; the blessing they convey comes each day with the rise of the edible sun. j Rob Dunn's most recent book is e Wild Life of Our Bodies. Illustrations are from Impressions of Nature: A History of Nature Printing, by Roderick Cave.