The stonecrop and houseleek family
Crassulaceae is the family that gave its name to a way of breathing. The trick it uses to survive on almost no water was worked out in these plants first, which is why the chemistry is called after them, and it is still the reason they are almost impossible to kill.
around 35 genera
roughly 1,400 species
Worldwide in dry and rocky places, richest in southern Africa and Mexico
Everything in this guide that belongs to Crassulaceae
An ordinary plant opens the pores on its leaves during the day to take in carbon dioxide, and loses water through them the whole time it does. In hot dry air that is a terrible bargain, and this family refused it.
Instead the pores stay shut all day and open at night, when the air is cool and damp and far less water escapes. The carbon dioxide taken in after dark is locked into an acid and stockpiled in the cells until morning, and then, with the pores sealed tight, the plant releases it internally and gets on with photosynthesis in the daylight using its own stored supply. Water lost is a fraction of normal.
It was worked out in these plants first and is called Crassulacean acid metabolism after them, though it has since been found in cacti, pineapples, many orchids and others. The most enjoyable part is that you can taste it. The stockpile is genuinely acid, so a leaf chewed at dawn is sour and the same leaf on the same plant chewed at dusk is not.
Break a leaf off almost any member of this family, lay it on the surface of some gritty compost and leave it alone, and in a few weeks it will have grown roots and a miniature plant from the broken end. No hormone, no cut surface treatment and no propagator. Very few plant families will do this so reliably from a bare leaf.
It makes sense for something living on a cliff or a roof, where the likeliest event in a plant's life is a piece being knocked off and landing nearby. The family treats an accident as a method.
Some members industrialized it. There are kalanchoes that produce rows of ready made plantlets along the notches of every leaf edge, each already carrying roots before it detaches, so the parent is permanently fringed with its own offspring and drops them by the hundred. They are charming on a windowsill and a serious weed in warm countries.
A great many of these plants grow as a tight rosette of overlapping leaves flat to the ground, which is a good shape for the job: it shades its own roots, it presents very little surface to the wind and it funnels any rain that falls straight to the middle.
The catch is that in many of them the rosette can only flower once. Sending up the flowering stem uses the growing point at the center, and once it has gone the rosette has no way of continuing, so it dies after seeding. This is not a failure and it is not something you did.
The plants get around it by having already produced offsets around the base long before flowering, which is the arrangement behind the old name hen and chicks. The hen flowers and goes; the chicks carry on. The houseleek genus is called Sempervivum, meaning always alive, which is a fair description of the colony even though it is untrue of any individual rosette in it.
The familiar members are jade plant, echeveria, aeonium, the houseleeks and the kalanchoes, and between them they cover most of the succulents sold as houseplants that are not cacti. The family also sits in the same order as the peonies, which is not a resemblance anybody would guess from looking.
One warning is worth carrying. Several kalanchoes, particularly the plantlet makers, contain compounds that act on the heart muscle in the same way the milkweed family's do, and they have killed grazing cattle in Australia and southern Africa in large numbers. They are a genuine risk to a dog or a cat that chews one.
That makes at least three separate plant families that arrived at heart acting compounds independently, which says less about the plants than about how few good ways there are to poison an animal.