The spurge family
Euphorbiaceae is a family that gave up on the ordinary flower entirely and built a replacement. It also supplies natural rubber, a staple food for hundreds of millions of people and one of the most poisonous substances known, which is a range no other plant family comes close to.
around 220 genera
roughly 6,500 species
Worldwide, richest in the tropics
Everything in this guide that belongs to Euphorbiaceae
Euphorbia does not have flowers in the sense the rest of a flower shop does. What looks like one small bloom is a structure called a cyathium, and taking it apart is the most surprising five minutes in botany.
The cup is made of fused bracts, not petals. Inside it stand several male flowers, each one reduced so far that it consists of a single stamen and nothing else, no petals and no sepals at all. In the middle sits one female flower, reduced to a bare ovary on a stalk, which leans out of the cup as it ripens. Around the rim are nectar glands, often colored and shaped like little crescents, doing the advertising that petals do elsewhere.
So the yellow nubs at the center of a poinsettia are not flowers. Each one is a whole inflorescence, a cluster of flowers compressed until it reads as a single bloom. This is the deepest floral reduction in the plant kingdom, and the family repeated it in around two thousand species.
Break any stem in this family and it bleeds white. The latex runs through the plant in its own network of channels under pressure, so a cut does not just weep, it runs, and it goes on running for a while.
This deserves real care rather than a footnote. The latex of many euphorbias is caustic. It blisters skin on contact in sensitive people, and in the eye it can cause serious injury that needs a doctor rather than a rinse. Anyone cutting the succulent euphorbias or the crown of thorns should be wearing something over their eyes, and nobody should rub their face after handling them. The ornamental species sold at Christmas are at the mild end of the range, but the family habit is worth knowing before meeting the rest of it.
The same substance built an industry. Natural rubber is the latex of a Brazilian tree in this family, tapped by cutting a shallow spiral into the bark, and effectively every rubber tree in Asia descends from seed taken out of Brazil in 1876.
A great many euphorbias are fat, ribbed, spiny, leafless desert plants, and they look so much like cacti that they are sold as cacti and killed by cactus advice. They are not related. The resemblance is convergence: two unrelated families arriving at the same answer to the same problem, which is how to hold water in a dry place and not be eaten while doing it.
Telling them apart takes one second and one rule. Cacti are American and euphorbias of this type are mostly African, but you will not have the label. Nick the plant instead: a euphorbia bleeds white and a cactus does not. The spines differ too. A cactus spine grows from a small felted cushion called an areole, always in a tuft; euphorbia spines come straight off the ridge of the stem with no cushion.
Cassava is a euphorbia, and it is the third largest source of carbohydrate in the tropics, eaten by hundreds of millions of people daily as tapioca, garri and fufu. It is also cyanogenic. The roots have to be grated, soaked, pressed and cooked before they are safe, and the processing traditions that make that reliable are hundreds of years old for good reason.
At the other end of the family is the castor bean, grown ornamentally for its enormous leaves and pressed commercially for castor oil. The residue left after pressing contains ricin, one of the most toxic proteins known, at a dose measured in micrograms. The oil itself is fine, because the protein does not come through with it.
Also here: the biodiesel crop jatropha, the crotons grown for their painted leaves, and the pencil cactus that is neither a pencil nor a cactus.