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 Their leaves have been replaced by spines in many cases, which helps conserve water and protects against predators, water is stored in the thick fleshy trunk of the plant, which also serves as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.
- Rhipsalis baccifera
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
Epiphyte cactus flowering
Cacti like a 50/50 mix of loam and grit and fed with a dilute organic seaweed fertilizer during the growing season every 2 weeks with rain water if possible. 
Some cactus fruits are edible, the prickly pear is one example, some can be fermented into alcoholic drinks as with the syrup from the enormous Saguaro. Some species have long been known to be psychoactive containing the chemical Mescaline such as the North American Peyote and South American san pedro cactus
Peyote cactus.
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