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These plants are so well-adapted for arid habitats that they’re practically synonymous with deserts. This episode, we explore the diversity, special adaptations, and mysterious evolution of Cacti.
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Prickly Plants
Cacti are succulent plants of the family Cactaceae. They’re notable for their spine-shaped leaves, for being endemic to the Americas (except for one species in Africa/Sri Lanka), and especially for being extremely well-adapted to dry desert environments.
There are around 1,700 living species of cacti, ranging from not-very-cactus-like trees and shrubs with persistent leaves to the much-more-cactus-like prickly pears and saguaros.
Top right: The miniature cactus Blossfeldia liliputana. Image by Peter A. Mansfeld, CC BY 3.0
Middle left: Rhipsalis, aka mistletoe cactus. Image by Raffi Kojian, CC BY-SA 3.0
Middle right: A flowering cactus visited by a feathery pollinator. Image by rob Stoeltje, CC BY 2.0
Bottom left: Opuntia, prickly pear, in Mexico. Image by Juan Carlos Fonseca Mata, CC BY-SA 4.0
Bottom right: Carnegiea gigantea, Saguaro. Image by Tomascastelazo, CC BY-SA 4.0
Cacti are among the most specialized organisms on Earth for dry environments. Their spiny leaves not only provide protection but also prevent excess water loss, and – since they tend not to have broad leaves – they photosynthesize with their stems, which are constructed of fleshy tissue that’s great at storing water. Like many dry-adapted plants, cacti employ CAM photosynthesis, a strategy that allows them to take in CO2 at night to limit water loss.
Image: Pachycereus pringlei in Sonora, Mexico by Tomascastelazo, CC BY-SA 4.0
Right: Ground tissue (“wood”) of Opuntia, prickly pear. Image by Juan Carlos Fonseca Mata, CC BY-SA 4.0
Cacti are thought, based on genetic evidence, to have originated around 30 million years ago. Their diversification was most likely influenced by a global trend of cooler and drier climates since that time, not to mention the uplift of the Andes mountains which created new arid and high-elevation habitats.
The story of cactus evolution is made somewhat mysterious by a near-complete lack of a fossil record. The oldest known ancient remains of cacti come from packrat middens in the southwest United States, dating as old as 24,000 years. Those arid and high-elevation habitats that cacti are so well-suited to inhabit are not often great environments for extensive fossil preservation.
Learn more
Beyond aridification: multiple explanations for the elevated diversification of cacti in the New World Succulent Biome (technical, open access)
Fossil cacti and other succulents from the latest Pleistocene (technical)
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