They are classified into three groups known as tribes, among which there is also a genus called Cactus . True cacti have areoles or warts, the nubbinlike structures from which cactus thorns emerge. About 1,500 species exist, and not all of them are spiny. The cacti evolved approximately 40 million years ago, their leaves becoming modified as spines to conserve moisture in harsh environments. Cacti are quite slow growing.
All cacti are native to the Western Hemisphere.
The most important arid varieties in the foliage trade are native to drier areas of Mexico and South America. Because of the strong seasonal changes in their native environments, cacti grow and rest in cycles.