A moderate amount of air pollution from nitrogen dioxide can make tobacco plants grow more leaves and heavier leaves, but it can also change the way their leaves work. After being exposed to this pollution for 15 days, the oldest leaves of the plants did better at turning sunlight into energy, while the youngest leaves did better at using that energy efficiently. The middle-aged leaves didn't do as well at either of these things. So, this pollution can boost one part of the leaves' energy-making process while slowing down another part.