Cactus

• 1. Introduction

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  During a rainstorm, a fully grown saguaro cactus can absorb as much as 200 US gallons of water in a single event. That capacity captures the cactus family in essential form. It is a lineage built around one central problem: the scarcity of water. Cactaceae holds roughly 127 genera and around 1,750 known species. The range runs from Blossfeldia liliputiana, barely 1 cm across at maturity, to Pachycereus pringlei, which stands 19.2 m tall. With one exception, these plants are native entirely to the Americas, ranging from Patagonia in the south to parts of western Canada in the north. Some grow in the Atacama Desert in South America, one of the driest environments on Earth. How did this family develop such extreme mechanisms for enduring drought? How did they become central to human food, ceremony, and trade over thousands of years? And why has placing them in a scientific classification remained contentious since the 18th century?

• 2. Water's Architecture

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  The ribs that line many cactus stems follow a Fibonacci sequence in their number: 2, 3, 5, 8, 13, 21, and so on. This is not incidental. The ribbed structure lets the stem expand rapidly when rain arrives and contract during drought without tearing. When a cactus is fully hydrated, water can make up as much as 90% of its total mass. At that point, the ribs may nearly vanish into the swollen surface.

  In Mammillaria rekoi, areoles produce both hooked central spines and straight outer spines. Spine form varies so widely across the family that it serves as a reliable tool for species identification. Areoles are tiny compressed shoots unique to the cactus family, and every spine as well as every flower grows from one. Spines are botanically modified leaves rather than modified branches; this distinction matters because spines evolved before leaves were fully lost.

  Ferocactus latispinus and other globe-shaped species combine maximum internal volume with the smallest possible outer surface, limiting the area through which moisture escapes. Spines reduce evaporation further by trapping a thin layer of still air directly against the stem. On foggy mornings or when humidity is high, those same spines condense water that drips to the ground and is absorbed by the roots.

  A young saguaro only 12 cm tall was recorded with a root system 2 m across and no more than 10 cm deep. That wide, shallow network is built to intercept brief, scattered rainfall before it sinks beyond reach. Ferocactus cylindraceus reportedly begins absorbing water within 12 hours of as little as 7 mm of rainfall, reaching full hydration within a few days. The outer skin of most cactus stems is sealed with a tough, waxy cuticle. That coating holds moisture inside and gives many species their characteristic grey or bluish tinge. That same stem, stripped of leaves, also carries out all of the plant's photosynthesis. It does so through a mechanism quite unlike that of ordinary leafy plants.

• 3. Breathing After Dark

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  Plants using the C3 mechanism for photosynthesis must keep their stomata open during daylight to draw in carbon dioxide. The process can cost such plants as much as 97% of the water they absorb through their roots. Cacti and other succulents avoid this through a process called crassulacean acid metabolism, or CAM, in which stomata open only at night.

  Malic acid is the form in which cacti store carbon dioxide taken in during the night, holding it inside cells until daylight returns. When daylight arrives, the stomata close and photosynthesis proceeds using only that stored supply. This shift dramatically reduces water loss without stopping the production of carbohydrates.

  Pereskia species, which sit near the base of the cactus family tree, show an intermediate version called CAM-cycling. Researchers can detect this by measuring the ratio of two carbon isotopes, carbon-14 and carbon-13, within the plant's tissue. That ratio reveals how much carbon was fixed at night versus during the day. Studies of Pereskia have found evidence of CAM-cycling even in their leaves, suggesting the adaptation predates the full shift to leafless, stem-based photosynthesis. Whether that shift happened once across all core cacti, or independently in the opuntia and cactoid lineages, remains unresolved. The origins of those earliest cacti, and the geography that shaped them, are better read from living plants than from any fossil record.

• 4. Ancient Origins

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  Molecular studies of living species suggest cacti first appeared around 35-30 million years ago, during the Late Eocene to early Oligocene. No cactus fossils exist to confirm or refine that estimate. The geographical distribution of living species provides the most useful evidence. With one exception, cacti are confined to the Americas, which means the family could only have evolved after South America and Africa separated. That separation took place during the Early Cretaceous period.

  The earliest diverging group, Leuenbergeria, is thought to have originated in Central America and northern South America. The caulocacti, whose stems delay forming bark and carry stomata, evolved later in the southern part of South America and then spread northward. Core cacti, with the most strongly developed stem succulence, are estimated to have emerged around 25 million years ago.

  A possible driver was the uplift of the central Andes, roughly between 25 and 20 million years ago. This created new zones of aridity across the continent. The diversity of cactus species alive today appeared far more recently; most of it is thought to have emerged within the last 10-5 million years. Succulent plant groups in South Africa, Madagascar, and the Americas all diversified at a similar time, coinciding with a global expansion of arid environments. The flowers that emerged from that diversification are as varied as the plants themselves, shaped by long relationships with specific animal pollinators.

• 5. Flowers, Bats, and Seeds

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  Lophocereus schottii, a columnar cactus of semidesert terrain, is pollinated exclusively by the moth Upiga virescens. That same moth lays its eggs among the developing seeds, which its caterpillars later consume. The flowers of this cactus are funnel-shaped, up to 4 cm long, and open only at night to coincide with the moth's hours of activity.

  Schlumbergera truncata and other hummingbird-pollinated species carry flowers toward the red end of the spectrum, with protruding stamens and nectar of relatively low sugar content. Bees are the most common pollinators of cacti overall, and bee-pollination is thought to have been the first system to evolve within the family. About a quarter of all cactus genera are pollinated by bats, a proportion exceeded among eudicots by only two other plant families. Carnegiea gigantea, the saguaro, is among the bat-pollinated species. Bats capable of traveling long distances can carry pollen between plants scattered widely across open desert.

  Giant tortoises distribute Opuntia seeds in the Galapagos Islands by consuming fruit and passing seeds in their droppings. Birds do the same for fleshy, colorful cactus fruits across the Americas. Rhipsalis baccifera, the one cactus native to both the Americas and Africa, has seeds adapted for bird dispersal. Old-world populations of this species are polyploid, supporting the idea of a long-established colonization rather than a recent spread. In a few genera, including Blossfeldia, ants carry seeds. When humans first arrived in the Americas, they encountered landscapes where these pollination and dispersal networks had been running for millions of years.

• 6. Five Thousand Years of Use

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  Cave paintings in the Serra da Capivara in Brazil document early human encounters with cacti. Seeds found in ancient middens in Mexico and Peru suggest use going back 12,000-9,000 years. Those early interactions were almost certainly for food; hunter-gatherers likely gathered wild cactus fruits wherever they found them.

  By the 14th century, the Aztec empire had woven opuntias into its symbolic core. The capital's name, Tenochtitlan, may derive in part from the Nahuatl word nōchtli, which refers to an opuntia fruit. Mexico's coat of arms, which originates in the founding myth of that city, depicts an eagle perched on a cactus while gripping a snake. The Aztecs linked the ripe red fruit of the opuntia to the human heart. Offering hearts to the sun god was thought to ensure the sun kept moving, just as the fruit quenches thirst.

  Peyote, the small cactus Lophophora williamsii, has a ceremonial history spanning more than 5,500 years. Dried peyote buttons from a site on the Rio Grande in Texas were radiocarbon dated to around 3780-3660 BC. Roman Catholic attempts to suppress its use after the Spanish conquest largely failed. By the middle of the 20th century, peyote was used ceremonially as far north as Canada. It is now formally recognized within the Native American Church. The San Pedro cactus, Echinopsis pachanoi, is native to Ecuador and Peru and also contains mescaline. Carvings and ceramic objects suggest its ritual use going back roughly 2,000-2,300 years.

  The nopal industry in Mexico, based on the pads of Opuntia ficus-indica, was estimated to be worth US$150 million in 2007. In 2002 in South Korea alone, 49 million cactus plants were propagated, with a value of almost US$9 million. Of those, 31 million were produced by grafting. The fruit of Carnegiea gigantea, the saguaro, has long supplied indigenous peoples of northwestern Mexico and the Sonoran Desert with food. It can be boiled into syrup, dried for storage, or fermented into an alcoholic drink. Selenicereus undatus, now widely grown in Asia as dragon fruit, provides pitahaya orejona across tropical parts of the Americas. Cochineal, a red dye from a scale insect that lives on opuntia species, lost its market when synthetic dyes appeared in the mid-19th century. Rising demand for natural dyes has since revived its production. By 1570, melocacti were growing in English collections, decades before the naming conventions that govern plants today were established.

• 7. The Naming Problem

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  Carl Linnaeus placed all the cacti he knew into a single genus, Cactus, when he published Species Plantarum in 1753. That work anchors modern botanical naming. Linnaeus had already grouped the same plants differently in 1737, using two genera; the consolidation was his own reversal. Philip Miller began dividing them again in 1754. By 1789, Antoine Laurent de Jussieu had placed the whole group in his newly created family, Cactaceae.

  Botanists at the 1905 Vienna botanical congress voted to reject Cactus as a valid genus name, finding the term too tangled in competing definitions. Mammillaria was declared the type genus of the family instead. The family Cactaceae now contains no genus called Cactus.

  Curt Backeberg, one of the most prolific cactus classifiers, named or renamed some 1,200 species without attaching a single name to a physical specimen. Part of the difficulty is that cacti resist standard botanical preservation. They evolved to resist desiccation, making them hard to dry and press for herbarium storage. David Hunt observed that Backeberg had left a trail of nomenclatural chaos that will probably vex cactus taxonomists for centuries.

  In 1984, the International Cactaceae Systematics Group was established to build a working consensus for the family. Their classification recognized four subfamilies. A 2011 molecular study found that only 39% of genera sampled within Cactoideae, the largest subfamily, were monophyletic. Monophyletic means the group includes all and only the descendants of a single ancestor. Classification of the cactus family currently remains unsettled. Legal protections for cacti under international treaties depend on stable species names, and those names, as yet, are not fully settled.

• 8. A Fight to Survive

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  All cacti are listed in Appendix II of CITES, the Convention on International Trade in Endangered Species. This listing makes international trade in most specimens illegal without export permits. Ariocarpus and Discocactus, held to the stricter Appendix I, may only cross international borders for non-commercial purposes and require both export and import permits.

  A dam built near Zimapan, Mexico, destroyed much of the natural habitat of Echinocactus grusonii. Urban expansion and road construction have reduced cactus populations in parts of Mexico, New Mexico, and Arizona. Agricultural conversion eliminated populations of Ariocarpus kotschoubeyanus in Mexico, where dry plains were plowed for maize cultivation. In Chile, valley slopes planted with vines reduced populations of Copiapoa and Eulychnia. Grazing by introduced animals, including goats, has damaged cactus populations in multiple regions. The Galapagos Islands and Browningia candelaris populations in Peru are among the documented cases.

  Near Miquihuana in Mexico, the type population of Pelecyphora strobiliformis was virtually emptied by collectors supplying the European market. Illegal collection from the wild continues to be a threat.

  Big Bend National Park in Texas, Joshua Tree National Park in California, and Saguaro National Park in Arizona all protect significant cactus populations. Seeds of cacti and other succulents are held in long-term storage at the Desert Botanical Garden in Arizona. This reserve exists partly to support future reintroduction efforts. In Australia, Opuntia species introduced in the 19th century became serious agricultural weeds, eventually controlled by the moth Cactoblastis cactorum. In April 2012, the Australian Weeds Committee declared all opuntioid cacti except Opuntia ficus-indica to be Weeds of National Significance. That designation remains in force.