In this enhanced image the ‘mouths’ are the pores on the underside of an elder leaf. The ‘lips’ are the guard cells that expand and contract on either side, closing or opening the pores between them to permit the exchange of gases. During the day, plants give out oxygen as a byproduct of photosynthesis, and they breathe in carbon dioxide, their only source of carbon from which to grow new cells. At night they exhale carbon dioxide and inhale oxygen. Trees are truly the lungs of the planet.

Photograph: Susumu Nishinaga/Science Photo Library