The cactus family
Cactaceae is defined by one small structure that nothing else on earth has, and everything else about a cactus follows from what that structure does. It is also an entirely American family, which is why every cactus lookalike from another continent is something else wearing the same coat.
around 130 genera
roughly 1,800 species
The Americas, from Canada to Patagonia, with one genus reaching further
Everything in this guide that belongs to Cactaceae
Schlumbergera buckleyi and truncata
4-8 weeks in flower; plant lives decades
Look at any cactus and find the small round cushion the spines come out of, usually felted, often woolly, spaced out in a regular pattern over the plant. That is an areole, and it is the one character that defines this family. No other plant has them.
What makes it more than a spine holder is what an areole actually is: a branch, compressed to almost nothing. It is a complete shoot with its growing point intact, which is why everything the plant produces comes out of one. Spines come from areoles. Flowers come from areoles. New arms and new pads come from areoles. A cactus cannot put out a flower or a branch anywhere else, because there is nowhere else with the machinery.
This is why the test works so reliably against the imitators. A succulent from Africa with spines straight off the ridge of its stem and no cushions is not a cactus and is not closely related to one, however similar it looks.
If an areole is a shoot, the spines coming out of it are that shoot's leaves, modified past all recognition. The family gave up on leaves as solar panels because a flat leaf loses water faster than anything else a plant owns, and handed the job of photosynthesis to the green stem instead.
The spines kept working, in several directions at once. They deter animals, obviously. Less obviously, a dense white coat of them shades the stem surface from the worst of the sun and traps a still layer of air against it, and in some species they collect dew and drip it down onto the roots. In the chollas the spines are barbed so a whole joint of the plant detaches onto a passing animal and is carried off to root somewhere else.
The ribs are the other clever piece. A ribbed stem is pleated like a concertina, so after rain it can take up a great deal of water and swell outward by flattening the pleats, then shrink again through a drought, over and over, without ever splitting its own skin.
For three hundred years the strongest red dye in the world came from an insect farmed on prickly pear pads in Mexico. Cochineal produces carmine, which is brighter and far more permanent than any red available in Europe at the time, and Spain kept the source secret and the trade monopolized for as long as it could. It colored uniforms, tapestries and paintings, and it is still in food and cosmetics today.
The plant itself went everywhere and in one place it went wrong. Prickly pear was introduced to Australia and by the 1920s it had taken something like sixty million acres of grazing country, in stands too dense to walk through, at a rate faster than it could be cleared. Poisoning and grubbing failed completely.
What worked was a moth from Argentina whose caterpillars bore into the pads. Released in the mid 1920s, it collapsed the infestation within about a decade and the land came back. It remains the most successful biological control of a weed ever attempted, and there is a town in Queensland with a memorial hall named after the insect.
The desert is where the family is most obvious, not where all of it lives. A whole branch of the cacti went the other way entirely and moved into wet tropical forest, where they grow perched on tree branches in pockets of leaf litter rather than in soil, with flattened leaf like stems and few spines or none.
They are still cacti in every technical sense, areoles and all, and they still run the water saving chemistry the family is built on, because a plant sitting on a branch in the canopy has no more access to reliable water than one in sand does. The problem is the same; only the address changed.
One more oddity is worth knowing. The family is American from end to end with a single exception: one genus of these forest cacti also occurs naturally in Africa, Madagascar and Sri Lanka, and how it got there, whether by birds or by people, is still argued about.