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Anyone awed by towering redwoods should offer thanks to stomata, the tiny pores on the leaves of all trees and other vascular plants. These microscopic mouths allow plants to grow tall and to regulate carbon dioxide intake and water loss. Stomata, in short, helped plants colonize the landscape and transform the planet. Now, molecular studies are giving scientists glimpses of the early days of stomata and how they have changed since then. They suggest complex stomata evolved to help early plants control moisture in their spore capsules and that other plants later exploited these pores to breathe in carbon dioxide and exhale water vapor. And hundreds of millions of years later, more sophisticated stomata evolved in grasses, enabling them to tightly control water loss—a feature that helped them dominate dry landscapes around the world.