Stomata play an important role in water balance and photosynthesis of a plant. Evaporation of water takes place through stomata and this has consequences for the uptake of water by the roots. The stomata also absorb the CO2 that is important for photosynthesis. 

Stomata in the leaf

Figure 1: Stomata in the leaf. Photo: BCC Bioscience Image Library

A stoma consists of two closing cells. Below the closing cells are cavities in the mesophyll. When the relative humidity (RH) in these cavities is lower than in the greenhouse air, stomata close because the cell tension, the turgor, decreases.