متن دانشگاهی (Read an Academic Passage): از ایده‌ی اصلی تا استنباط

(1) Plants take in carbon dioxide through tiny openings in their leaves called stomata. Most plants open these pores during the day, when sunlight provides the energy needed to turn carbon dioxide into sugar. However, open stomata also allow water to escape. In a hot, dry desert, a plant that keeps its pores open at midday can lose a dangerous amount of water within hours. (2) Many desert plants, including most cacti, solve this problem with a strategy known as CAM photosynthesis. Instead of opening their stomata in the heat of the day, they open them at night, when the air is cooler and more humid. The carbon dioxide they absorb is stored in the form of an acid. When the sun rises, the stomata close, and the plant releases the stored carbon dioxide inside its own tissues, where it can be used to make sugar while very little extra water is lost. (3) This arrangement is remarkably efficient. Some CAM plants lose only a fraction of the water that ordinary plants of the same size lose. The strategy does have a cost, however. Because a plant can store only a limited amount of acid overnight, CAM plants usually grow slowly. This is why a cactus may take years to reach a height that a sunflower reaches in a single summer. (4) Interestingly, some plants can switch between the two methods. When water is plentiful, they behave like ordinary plants; during a drought, they change to CAM. Researchers hope that understanding this flexibility could one day help farmers grow crops in drier conditions.