Human beings grew up in forests; we have a natural affinity for them. How lovely a tree is, straining towards the sky. Its leaves harvest sunlight to photosynthesize, so trees compete by shadowing their neighbors. If you look closely you can often see two trees pushing and shoving with languid grace. Trees are great and beautiful machines, powered by sunlight, taking in water from the ground and carbon dioxide from the air, converting these into food for their use and ours. The plant uses the carbohydrates it makes as an energy source to go about its planty business. And we animals, who are ultimately parasites on the plants, steal the carbohydrates so we can go about our business. In eating the plants we combine the carbohydrates with oxygen dissolved in our blood because of our penchant for breathing air, and so extract the energy that makes us go. In the process we exhale carbon dioxide, which the plants then recycle to make more carbohydrates. What a marvellous cooperative arrangement - plants and animals each inhaling each other's exhalations, a kind of planet-wide mouth-to-stoma resuscitation, the entire elegant cycle powered by a star 150 million kilometers away.
- Carl Sagan in Cosmos