Why Do Many Leaves Have More Stomata Underneath?

Stomata are more than simple “breathing holes”. Each pore is controlled by a pair of guard cells. Carbon dioxide enters for photosynthesis, but water vapour escapes through the same route. A leaf therefore faces a trade-off: it must acquire carbon without losing water too quickly.

Many horizontally held leaves of land plants have more stomata on the lower surface. That surface is often less exposed to direct sunlight and rapid air movement, and the air beside it is renewed more slowly. Placing more pores there can support gas exchange while reducing the risks associated with an exposed evaporating surface. This does not mean the underside is windless or that stomata simply “avoid light”; leaf shape, the boundary layer and water conditions work together.

However, “stomata are underneath” is not a botanical law. Upright leaves, needles and grasses may have stomata on both surfaces. Floating leaves often place them on the upper surface because the underside contacts water and cannot exchange gases directly with the air. Stomatal position is not one fixed answer, but an adaptation that arranges the exchange of carbon, water and heat under different conditions.

https://labs.biology.ucsd.edu/schroeder/clickablegc2/
https://pmc.ncbi.nlm.nih.gov/articles/PMC7065165/