The epidermis of leaves and stems secretes a waxy cuticle, which is impermeable to water and thus helps minimize the loss of water from transpiration. However, the cuticle is also impermeable to carbon dioxide. The cuticle poses a dilemma: how can the plant balance its need to retain water with its need to obtain CO2 for photosynthesis?
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Stomata in leaves provide a balance between water loss and carbon dioxide diffusion into cells.
Environmental factors affect the opening and closing of stomata; numbers of stomata can also change if the plant sheds leaves or grows new leaves.