Like insects, plants must meet the opposing demands of water retention and gas exchange. The site of photosynthesis in plants, as well as the gas exchange site, is the leaf. This is what a section through a leaf looks like:

1. The mesophyll cells are surrounded by quite a lot of empty space for air to mingle around, providing plenty of surface area for gas exchange by diffusion.

2. Air with its carbon dioxide (necessary for photosynthesis) enters the leaf through the stomata. Stomata are holes on the leaf surface, made by the guard cells. They can open and close depending on environmental factors such as humidity, temperature and wind. This controls the amount of water loss. Oxygen, the byproduct of photosynthesis, also leaves the leaf through the…