How do cacti and other desert plants hoard water?

Desert plants hoard water in three ways: they store it, they save it, and they breathe at night. A cactus is the classic water tank. Most cacti are stem succulents, which means the stem is the organ that stores water, and water may make up to 90 percent of a cactus's mass. A mature saguaro is said to be able to take in as much as 760 litres during a single rainstorm. Many cacti are round or columnar, shapes with little surface for their volume, which reduces water loss.

The cleverest trick is how they photosynthesize. Plants using the usual C3 method can lose as much as 97 percent of the water they take up through their roots to transpiration, because their stomata must stay open to keep carbon dioxide coming in. Cacti and many other succulents use crassulacean acid metabolism, or CAM. Their stomata stay shut during the day to cut water loss and open at night, when it is cooler, to collect carbon dioxide, which is stored as an acid and used in daytime. Keeping the pores shut through the heat of the day slashes transpiration, the water a plant loses through those pores. CAM is found in over 99 percent of cactus species. The price is limited growth, because it restricts how much carbon gets fixed.

The first clue was a taste. In 1812 Benjamin Heyne noticed that leaves of a plant in India were acidic in the morning and tasteless by the afternoon.

Another desert champion is Welwitschia in Namibia, which has only two leaves that keep growing for its whole life, and which can be thousands of years old, helped by groundwater and fog.