If you’re lying on a beach, working in a field, or walking in the mountains, the sun’s heat can feel relentless. But we can seek shade when the heat becomes too intense and drink water whenever we’re thirsty. Plants don’t have these options. Consequently, they are perpetually at risk of losing too much water as it evaporates from their tissues. Because water is essential to nearly every chemical reaction in every living organism, water loss is a serious issue. Not surprisingly, plants have multiple adaptations that solve the “sun problem” and keep them from becoming dangerously dehydrated. We see three of these important adaptations right on the surfaces of most leaves (FIGURE 17-19).

Figure 17-19Plants have multiple adaptations that keep them from becoming dangerously dehydrated.

1. The cuticle. Dermatologists have instructed people for decades that good skin care and protection starts with maintaining the skin’s moisture. Many plant structures have evolved that are consistent with this advice. As we’ve seen, the top layer of most leaves, secreted by their epidermal cells, is the cuticle. This waxy, water-repelling substance keeps water inside the leaf from diffusing through the leaf surface and evaporating into the atmosphere. It seals in water almost completely. Carnauba wax, from the cuticle of Brazilian palm trees, is an ingredient in many sunscreen products for humans, as well as in a popular car polish.

2. Leaf hairs. Some of the surface cells on leaves become modified as tiny hairs. These hairs can reduce water loss by reflecting some of the sunlight (thus reducing the temperature inside the leaf) and by reducing the speed at which breezes move over the leaf’s surface, taking water with them—through evaporation.

3. Guard cells. The cuticle would solve the problem of water loss in plants almost completely if it covered the entire leaf. Unfortunately, the cuticle is also impervious to CO₂, which is an essential ingredient for photosynthesis. So, just as a castle needs a front door—which then becomes a major point of vulnerability to attack—leaves must have openings to the environment. These openings are the stomata. As many as 10,000 stomata per square centimeter are found on the underside of leaves (and, to a lesser extent, on stems). Their structure reflects an important water-conservation adaptation. A pair of guard cells (seen in Figure 17-6) surrounds each pore, one cell on either side. The guard cells can increase in size to create a small opening through which CO₂ can enter and water can be lost, or they can decrease in size to seal off the opening. The opening and closing of the stomata is controlled by osmotic pressure in the guard cells and appears to occur in response to changes in temperature, humidity, light intensity, or CO₂ concentration.

Stomata are the chief sites where plants “breathe.” If you were to do a cruel experiment and coat the underside of the leaves of some plants with Vaseline, what do you think would happen? With no way for air to enter, the plants would become starved for CO₂ and eventually the leaves would die.