Every time the temperature drops, a cloud passes overhead, or the sun sets, a plant makes a choice: Keep its microscopic pores, called stomata, open to absorb carbon dioxide and continue photosynthesizing or close them to protect its precious stores of water. That capacity to open and close pores re quires the plant to respond to subtle environmental changes by adjusting the pressure within the cells of the stomata—a complex ability that plants evolved over hundreds of millions of years. ...read more read less