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In botany, a leaf is an above-ground plant organ specialized for the process of photosynthesis. For this purpose, a leaf is typically flat (laminar) and thin. As an evolutionary trait, the flatness of leaves works to expose the chloroplasts to more light and to increase the absorption of carbon dioxide at the expense of water loss. In the Devonian period, when carbon dioxide concentration was at several times its present value, plants did not have leaves or flat stems...

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