Appears in
On Food and Cooking

By Harold McGee

Published 2004

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Leaves specialize in the production of high-energy sugar molecules via photosynthesis, a process that requires exposure to sunlight and a good supply of carbon dioxide. They therefore contain very little storage or strengthening tissue that would interfere with access to light or air, and are the most fragile and short-lived parts of the plant. To maximize light capture, the leaf is flattened out into a thin sheet with a large surface area, and the photosynthetic cells are heavily populated with chloroplasts. To promote gas exchange, the leaf interior is filled with thousands of tiny air pockets, which further increase the area of cells exposed to the air. Some leaves are as much as 70% air by volume. This structure helps explain why leafy vegetables shrink so much when cooked: heat collapses the spongy interior. (It also wilts the leaves so that they pack together more compactly.)