I recently visited Southern Arizona. It was my first visit to a desert so I was pretty keen to take a look at the local plants. The most conspicuous plants in the landscape were the cacti.
Cacti are flowering plants belonging to the family Cactaceae. There are 1200-1500 species of cactus, most of which are native to the Americas (1). Having never seen any cacti in their native environment before I was impressed at the size of the cacti in Arizona. The largest species, the saguaro cactus, can grow to be 12-18 m (40-60 ft) tall. Cacti look quite different to most other plants because of their adaptations to living in dry climates.
The biggest challenge facing desert plants is the shortage of water. In Arizona, it only rains for a few days a year so plants have to be able to capture and store as much water as possible. Cacti have large root systems that grow just under the surface of the soil to enable them to absorb water when it is available. Some cactus species, including the saguaro, also produce a taproot, which grows to around 5 metres deep to access water stored in the soil.
Cacti can store water in their stem tissue. In many species the stems are ribbed or fluted, enabling the stems to expand and contract depending on the amount of water they contain.
Cacti have several adaptations to reduce water loss, the most obvious being the lack of leaves, which reduces the surface area of the cactus. The primary function of leaves in most plants is to produce sugars by photosynthesis, so cactus stems are adapted to carry out photosynthesis instead. To further reduce water loss, the surface of a cactus is covered in a thick waxy layer.
A major route of water loss from plants is evaporation from pores (stomata) on the plant surface. The stomata enable carbon dioxide to be absorbed into the plant. Carbon dioxide is needed for photosynthesis, so the stomata are vital for plant growth and survival. Unfortunately, photosynthesis happens during the day, which coincides with when the rate of water loss from open stomata would be at its highest. The cactus family use a modified form of photosynthesis called CAM (crassulacean acid metabolism). In CAM, carbon dioxide is absorbed into the plant during the night and stored as malic acid. During the day the malic acid is then used in photosynthesis to make sugars. This mechanism enables cacti to open their stomata at night to acquire carbon dioxide when the rate of water loss through evaporation is lower, and then to close the stomata during the day.
Desert plants tend to be slow-growing and many produce thorns or spines to protect themselves from herbivores. Cacti produce spines from modified branch structures called areoles (the characteristic dips in the surface of a cactus), which are also responsible for producing branches (or arms) and flowers. Alongside deterring herbivores, the spines also help reduce water loss by reducing air flow close to the surface of the cactus and providing some shade. Also, when the air is moist, for example during fog or early morning mist, water can condense on the spines. The water then drips onto the ground and can be absorbed by the cactus roots.
1) Wikipedia: cactus (retrieved 19/05/14)
2) Arizona-Sonora Desert Museum: Saguaro cactus fact sheet (retrieved 21/05/14)