Cactus have also developed spines, which allow less water to evaporate through transpiration by shading the plant, and defend the cactus against water-seeking animals.
Apart from a very few exceptions (for example, the genus "Rhodocactus"), all cacti are succulent plants, although not all succulent plants are cacti.
The popularity of cacti has led to instances of cactus poaching in which naturally growing cacti are dug up to be planted in gardens.
Some cactus flowers form long tubes, up to 30 centimeters (12 inches) so that only moths can reach the nectar and thus pollinate the blossoms.
By 1737, 24 species were known, which Linnaeus grouped together as the genus Cactus.
Regardless, cactus is popularly used as both singular and plural, and is cited as both singular and plural by the Random House Unabridged Dictionary (2006).
Some cactus species have been utilized by humans since prehistoric times, for various purposes including food and medicinal use.