Epidermal cells secrete a waxy cuticle on their outer surface that limits water loss. Without a cuticle, the humidity within the internal air spaces would drop and the photosynthetic mesophyll cells would dry out. However, the cuticle prevents CO₂ from diffusing into the leaf even as it restricts water vapor from diffusing out of it.

As noted above, small pores in the epidermis allow CO₂ to diffuse into the leaf (Fig. 29.5a). These pores are called stomata (singular, stoma). Stomata can be numerous—there can be hundreds per square millimeter. Yet because each one is small, less than 1% to 2% of the leaf surface is actually covered by these pores. Thus, even with stomata, the epidermis is a significant barrier to the diffusion of CO₂ and water vapor.

The epidermis with its stomatal pores represents a compromise between the challenges of providing food (CO₂ uptake) and preventing thirst (water loss). However, because both the dryness of the air and the wetness of the soil are variable, leaves must be able to alter the porosity of the leaf epidermis to maintain a balance between the rates of water loss to the atmosphere and water delivery from the soil. They can do this by opening and closing their stomata.

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