The dogbane and milkweed family
Apocynaceae builds the most mechanically complicated flowers outside the orchids, runs poison through every part of itself, and produced the drug that turned childhood leukemia from a death sentence into a curable disease. Very little of that is visible in the small waxy flowers a florist sees.
around 400 genera
roughly 5,000 species
Worldwide, concentrated in the tropics
Everything in this guide that belongs to Apocynaceae
Only two plant families on earth package their pollen into solid parcels handed over whole to an animal. One is the orchids. The other is this one, and the two are not remotely related, which makes it one of the cleanest cases of convergence in botany.
In the milkweeds the parcel is a pair of waxy pollen sacs joined by a yoke with a sticky clip in the middle. The flower is ringed by narrow slits. An insect's foot slides into a slit, catches the clip, and pulls the whole assembly out as it lifts off, so the animal walks away wearing a pair of saddlebags it cannot remove. The fit is exact enough that insects of the wrong size are sometimes trapped and lose a leg getting free.
The other structure to know is the corona, an extra ring of fleshy lobes standing between the petals and the center. It is not petals and it is not stamens; it is a separate crown, and it is what gives the flowers of this family their curious machined look. Behind it the stamens are fused to the stigma into one solid head, which the orchids also arrived at independently.
Break a stem in this family and it bleeds white, the same as the spurge family does, and the two are worth telling apart because the danger is different. Spurge latex is caustic and burns on contact. This family's latex is not especially irritating to touch, and it is far more dangerous to swallow.
What it carries is cardiac glycosides, compounds that act directly on the muscle of the heart. Oleander is the household example and it is genuinely lethal, in small quantities, from any part of the plant, including dried leaves and the smoke of a fire made from its wood. It has no business anywhere near a food surface.
The same chemistry has been used deliberately for a very long time. African arrow poisons were made from members of this family, and one of those compounds went on to be used to treat failing hearts. It is worth noting that the foxglove, which is in an entirely separate family, arrived at the same class of heart acting compound on its own. Two unrelated lineages found the same weapon.
The Madagascar periwinkle is a small pink flowered plant sold as a bedding annual, and it is one of the most important medicinal plants ever found. It was being investigated in the 1950s as a possible diabetes treatment. It did nothing for diabetes. What it did do was collapse the white cell count in the test animals.
The compounds isolated from it, vincristine and vinblastine, went into use against childhood acute lymphoblastic leukemia and Hodgkin lymphoma. Survival for childhood leukemia went from a small minority of patients to the great majority within a generation, and those two drugs are a large part of the reason. They are still on the World Health Organization's list of essential medicines.
Another member of the family gave reserpine, which was among the first effective treatments for high blood pressure and opened the modern era of drugs acting on the brain. For a family this dangerous to eat, it has an unusually good record in a pharmacy.
The fruit is a pod that splits down one seam and releases seeds each carrying a tuft of fine silk. Milkweed floss is hollow, waxy and extraordinarily buoyant, and when kapok became unobtainable during the Second World War the United States filled military life jackets with millions of pounds of it, collected by schoolchildren.
The monarch butterfly depends on this family completely. Its caterpillars eat nothing but milkweed, and rather than avoiding the cardiac glycosides they store them in their own bodies, so the caterpillar and the adult butterfly are both poisonous to birds. The bold orange and black is an advertisement of exactly that.
At the strange end of the family are the stapelias, low succulents whose starfish shaped flowers are colored and textured like raw meat and smell precisely as bad as that suggests, because their pollinators are blowflies. Frangipani, hoya, mandevilla, desert rose and the periwinkles are all relatives, which is a range from perfume to carrion inside one family.