Science Puzzle
Why Do Cacti Breathe at Night?
Plants make their food from carbon dioxide and water, using the energy in sunlight. Most plants open tiny pores in their leaves during the day to let the carbon dioxide in. Cacti do the opposite: their pores stay shut all day and open at night.
But sugar-making needs sunlight, and at night there isn't any. Why do cacti breathe at night, and how do they still make their food?
The Answer
To save water. An open pore in the desert sun loses water fast, so a cactus opens its pores in the cool night, stores the carbon dioxide it takes in and uses that store to make sugar the next day with its pores firmly shut.
Every open pore is a two-way door: carbon dioxide comes in, but water vapour escapes. In hot, dry daytime air, a plant can lose a lot of water this way. At night the air is cooler and damper, so far less water escapes through the same open pores.
The trick is storage. At night the cactus takes in carbon dioxide and locks it up inside its cells as an acid. When the Sun rises, it closes its pores, releases the carbon dioxide inside its own cells and uses sunlight to turn it into sugar as usual. The acid builds up overnight, and the leaves of some of these plants really do taste more sour at dawn than in the afternoon.
It's called CAM photosynthesis. Pineapples, aloes and many succulents use it too, and it lets them make food on a small fraction of the water ordinary plants need.
The principle: Saving water in photosynthesis. Desert plants open their pores at night, when little water evaporates, store the carbon dioxide as an acid, then use it for photosynthesis by day with their pores shut.