Cacti are a family of flowering plants known as Cactaceae and are only found in the Americas. They are sometimes confused with the similar Euphorbias, a plant from the African continent. Both have fleshy leaves and stems and often have spines. All cacti grow in desert environments and some, such as Rhipsalis, attach to trees.
Surviving in the desert
Photosynthesis is the process plants use to make energy from light. Cacti use 'CAM photosynthesis'. This is a special kind of photosynthesis, which minimises water loss. Carbon dioxide, CO2, enters through leaf pores during the night and is stored as an acid. The pores close in the day to stop water evaporating. The stored acid is then used in photosynthesis.
Stopping water loss
The globular shape of most cacti reduces surface area and so reduces evaporation. The waxy skin helps to reduce further water loss. The spines of some cacti collect dew, which is then channelled towards the roots. Surface roots are ideal for collecting short bursts of rainfall.
Invasive Opuntias (Prickly pears)
Opuntias were farmed in the new world for cochineal insects, which produce a red dye that was coveted by Europeans. When the secret of how to produce the dye was discovered, Opuntias were taken around the world for cultivation. Where they have been cultivated, they have spread into the surrounding countryside and become a pest. They prevent the land being used for grazing because of their large spines. As a result, Opuntias are the only cacti species that are not listed as threatened.
All cacti (apart from the Opuntias) are listed on CITES as threatened species. They cannot be moved between countries without a permit - this is to stop the collection of wild cacti and reduce the damage done to fragile desert habitats. The sale of cacti is strictly regulated.
Article Date: 23 July 2007