During the spring and summer, we pay attention to flowers; during the fall, we pay attention to leaves. Just in time for autumn, Rob Dunn has written a fantastic essay about leaves for National Geographic magazine. The shape and structure of a leaf, Dunn writes, can reveal a lot—about the climate it’s designed for, the evolutionary history of its species, even the size and vitality of the plant to which it belongs.
Photo: Jon Sullivan.
Different kinds of environments, Dunn explains, push evolution to favor different kinds of leaves. Rain-forest plants, for example, tend to have long, narrow leaves with “drip tips” on the end, to help drain away the deluge. If a plant shares an ecosystem with lots of leaf-munching animals, then, over evolutionary time, the textures of its leaves may change. “Grass blades,” Dunn writes, “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.”
The veins that line a leaf are a clue about the plant to which it’s attached. The veins carry water to the plant’s chloroplasts, the intra-cellular organs that power photosynthesis. The more water a leaf can transport, the faster it can photosynthesize; that, in turn, allows the plant to grow faster and higher. The more veined a leaf is, in other words, the bigger and taller its plant is likely to be. That’s why, Dunn writes, the veins of a maple leaf are “like the roads of a city.”
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Joshua Rothman is a graduate student and Teaching Fellow in the Harvard English department, and an Instructor in Public Policy at the Harvard Kennedy School of Government. He teaches novels and political writing.