Most cacti are native to the Americas, and have highly evolved to tolerate hot and dry conditions, even the Atacama desert where some parts have not seen rain for more than 20 years! Their leaves have been replaced by spines in many cases, which helps conserve water and protects against predators, water is stored in the thick fleshy trunk of the plant, which also serves as the area for photosynthesis.
The only native cactus also found outside the Americas is the Mistletoe cactus which may have been carried by Ancient traders or birds to Africa and Sri Lanka.
Cacti range in size from over 20 metres to just over 1 cm, and many offer a spectacular display of flowers, to attract birds and insects in the brief periods of pollination usually after heavy rains. They absorb a great deal of water during these times into their fluted stems and store it for dry spells. A large Saguaro cactus can store over 200 gallons in one go!
The ground dwelling types usually have superficial roots that stay near the surface to absorb the brief downpours, before the water quickly evaporates in the hot desert, the very tall columnar varieties produce very deep tap roots to stop it falling over. Cacti can be easily propagated by seed or cuttings in species such as epiphytes