The waxy cutin also repels water. For this reason, many pesticides contain a spray additive to help the product adhere to, or penetrate, the cutin layer.
Special epidermal cells called guard cells open and close in response to environmental stimuli, such as changes in weather and light. They regulate the passage of water, oxygen, and carbon dioxide into and out of the leaf through tiny openings called stomata. In most species, the majority of the stomata are located on the underside of leaves.
Conditions that would cause plants to lose a lot of water (high temperature, low humidity) stimulate guard cells to close. In mild weather, they remain open. Guard cells also close in the absence of light.
Located between the upper and lower epidermis is the mesophyll It is divided into a dense upper layer (palisade mesophyll) and a lower layer that contains lots of air space (spongy mesophyll). Located within the mesophyll cells are chloroplasts, where photosynthesis takes place.