>>26545839"With cells similar to those of plants, it can perform photosynthesis.. ..When you see Leafeon asleep in a patch of sunshine, you'll know it is using photosynthesis to produce clean air."
Getting all 10th grade Biology up in here: Photosynthesis just means a plant takes carbon dioxide from the air, water, and sunlight, and converts all that into chemical energy, which it typically stores as sugar. In turn the plant disposes of unused oxygen as a waste product. That's clean air to us.
If humans had chlorophyll in their skin, the extra energy we would get from sunlight would be hardly noticeable. Animals, especially warmblooded animals, need much more energy than a stationary plant. We'll have to assume that the energy gain from photosynthesis is much greater in the Pokémon world, for it to be worth it.
It is possible that Leafeon's mammalian lungs inhale CO2 and pump out oxygen, rather than the usual way around. That would be cool but I doubt it's how it works, since Leafeon still needs to carry oxygen to its organs. It wouldn't necessarily have to eat all the time, though.
