Images of cactus plants compete with cow skulls and evil-faced suns for Most Often Used Desert Symbol. Most of the 87 or more genera and over 2,000 species in the flowering plant family Cactaceae live in deserts and are adapted for dealing with aridity, dry air, extreme heat, and, in some areas, winter or high-altitude cold. Nevertheless, not all cactus live in deserts. Some can be found as native plants in tropical forests in the form of epiphytes, plants that grow on tree branches but are not parasites. Other cactus species are equipped to survive in the high Andes Mountains, exposed alternately to intense sunlight and freezing cold.
One can still find prickly-pear cactus, Opuntia vulgaris, growing wild in the eastern U.S. as far north as Connecticut. These are remnants of a swath of semi-desert land and climate that once reached across the continent from the American southwest to Martha's Vineyard, during the waning centuries of the last Ice Age, when dry conditions prevailed in the mid-U.S.A.
Most cactus are stem succulents, the trunk and stems, which were woody in the ancestors, having become fleshy and water-hoarding. Photosynthetic plant tissues, which use green chlorophyll to capture sunlight and use the energy for synthesizing food, are ordinarily found in the leaves of plants, but in the majority of cactus have become a green skin on the main body and branches. A waxy exhudate covers the leathery green hide, and in most species, the infamous spines, evolved from leaves, efficiently arm the plant while doing extra duty as dew collectors. Root systems of most arid-land cactus are shallow and fan out in a net extending far from the plant, to trap rare rainwater as efficiently as possible.
Not all cactus species bear the familiar features of swollen, succulent trunks, as in the saguaro cactus, the fat paddle-shaped lobes of opuntia, or the bulbous barrels of cholla cactus. Genus Rhipsalis species look like dangling wads of stringy or ribbony green pasta. Species in the genus Schlumbergera, whose most famous species is Schlumbergera truncata, the Christmas cactus, are superficially similar in form to Rhipsalis, and grow as epiphytes in Brazilian tropical forests.
The 25 species of the genus Pereskia bear large, true leaves, woody stems and trunks, and spines on the branches and trunks. Pereskia grandifolia, a small tree found from South America into Mexico, has prominent leaves, colorful red-pink flowers resembling roses, and spines along its branches. The main trunk bears bulbous, muffin-shaped outcrops, each fitted out with a starburst of long, deadly-looking spines. Opuntia and cholla cactus may produce tiny leaves, but these soon shrivel into uselessness. Some species in genera Maihuenia, Quiabentia, and Austrocylindropuntia have fat, bulbous, succulent leaves. Quiabentia verticillata, from the dry Gran Chaco region of South America, is a small tree, growing up to twenty feet tall, with a woody trunk and branches, and the small, succulent leaves that look incongruous on the woody branches. The tree is also deciduous, dropping its leaves in the dry season.
Some species of saguaro and cholla cactus still keep internal wood in their anatomy. A few cactus species on the Galapagos Islands have evolved woody, treelike trunks, making individual plants look like trees lopped off at their midpoints, then further insulted by having bunches of opuntia cactus stitched onto the long stumps.
All cactus bear flowers, and most of these are brilliantly colored though simple in design. More extravagant are the flowers of the famous night-blooming cereus species. That common name is applied across several genera of cacti, some of which live in deserts, others in tropical rainforests. Of the night-bloomers, the genera Cereus and Peniocereus are desert plants that live in the southwester U.S. and Mexico, while genera Epiphyllum and Hylocereus are rainforest epiphytes.
What all have in common are their flowers, which are big, bowl-shaped, and suggestive of water lily flowers. Most are a soft but conspicuous cream color that shows up with unnatural brilliance on moonlit nights. Some flowers are touched with delicate tints of pink and reddish. At the center of the flower is a brush of stamens, yellow with pollen. The flowers' large sizes, ethereal scents, bowl shapes, bushy wad of stamens, and ghostly coloring are designed by nature to attract bats as pollinators. In some species, a flower lasts but a single night.
The great majority of cactus species are New World plants, but a few species in the genus Rhipsalis can be found scattered about the Old World, in tropical forests and deserts, although the genus is most diverse in Brazil. There's an ongoing debate about whether they colonized the Old World naturally, or were brought by man in the past few centuries, or are an early form of cactus that dates from the early Cretaceous, around 150 million years ago, on the southern supercontinent of Gondwanaland before it split apart into today's southern continents. Most Rhipsalis species live as epiphytes, although some can be found growing on natural piles of rocks.
Hot and dry areas of the Old World have their own native plants adapted to those sorts of conditions, but most of them aren't cactus, although a fair number have come to closely resemble cactus through convergent evolution in response to similar conditions. There are also New World cactus that are known to have been brought to the Old World by man within the last 500 years, and in many places these have proliferated and become invasive pest species.
The Mexican prickly pear cactus, Opuntia monacantha, played a key part in biological warfare in the early part of the twentieth century, on the other side of the world, on the island of Madagascar. In 1768, the French Count de Modave established a settlement in Fort Dauphin, on Madagascar's southeastern coast. He imported specimens of Opuntia monacantha from the botanical garden on the island of Reunion (east of Madagascar). Modave knew that the plant, which bears five-inch spines, grows rapidly and forms impenetrable thickets, so he grew and trained them into a fortification barrier on the seaward side of his settlement village. From that site, the cactus spread almost audibly throughout southern Madagascar, most of which is dry-adapted forest and desert. Nothing in the native ecology, no diseases, no animals, could stop the spread or even slow it down.
The indigenous Malagasy (Madagascan) people of the south soon realized that O. monacantha was a blessing. The pads and fruit of the cactus are edible, and became a new, abundant source of food and stored water for people and cattle. There was a spineless variety run loose, while the spiny pads could be tossed onto fires and the spines burned off to make them safe for eating. The dominant group of the south, the tough and hardy Antandroy, or "People of the Thorns," named after the well-defended native vegetation, also used them as had Modave. They grew fences, walls, and barriers of the cactus, first to protect against cattle robbing, later against French invaders. France invaded Madagascar and captured the capital, Antananarivo, in 1895, annexed the island as a French colony in 1896, then set about affirming their rule. In the south, the Antandroy remained safe in their villages behind deadly walls of opuntia, which only they could find their way through by means of narrow, secret passages, with many blind side corridors to trap invaders. The French Army began invading southern Madagascar in 1900, and found themselves helpless against hostile Antandroy guerillas and spiny walls up to twenty feet high. For a quarter of a century, the Antandroy remained independent of foreign conquest.
The French, in time, found the weapon of choice to bring down the opuntia walls. O. monacantha is native to Mexico, where a natural control on its wild populations is the cochineal beetle, whose larvae feed upon the cactus. The French gathered up some live cochineal beetles, imported them to the town of Tulear, on Madagascar's southwestern coast, and let them loose in 1925. They multiplied and gorged on the endless larder of O. monacantha. Clouds of the insects in flight made traveling difficult. In a few years, the cactus all but vanished, bringing mass starvation in its wake, since the people had become so dependent on the cactus for food; and the French colonizers moved in.