The Euphorbiaceae, part 1


The spurge family is one of the largest in the world, though exactly where it ranks is difficult to say for sure. There are almost 2,000 species in 300 genera worldwide (except the arctic and Antarctica). Most are tropical, and many are succulent, resembling cactuses; others are typical annual or perennial forbs, and a few are woody. It’s a family of great morphological diversity.

It’s also a family of great economic importance. Poinsettias are one of the most widely grown ornamentals. Natural rubber is made from the sap of Hevea brasiliensis. Cassava isn’t much known in the US, except in the form of tapioca, but it’s a staple food in many tropical regions; it comes from Manihot esculenta. The roots are eaten as a vegetable or dried and made into a flour.

Castor bean, Ricinus communis, is a particularly interesting plant, appearing on (or topping) lists of most poisonous plants in the world. It’s grown as an ornamental for its foliage, but is considered invasive in many places, and is highly allergenic, rating 10 out of 10 on the Ogren Plant Allergy Scale. The oil extracted from the seeds has industrial and medicinal uses, but the raw seeds contain a potent poison (ricin). Castor oil is even used in the manufacture of a food additive, polyglycerol polyricinoleate.


Species in the genus Euphorbia tend to have minute flowers. An involucre contains all the reproductive parts, collectively referred to as the cyathium. The cyathium consists of one carpellate (female) flower and several or many staminate (male) flowers surrounded by bracteoles (miniature bracts, or modified leaves). In many species there will be lipped glands (extrafloral nectaries) on the outside of the cyathium. This closeup of a poinsettia shows more than a dozen unopened cyathia with their nectaries, surrounded by the highly colored bracts.

Another characteristic common to many (but not all) euphorbs is the presence of a milky sap (latex) in the stems. Generally it’s caustic, more or less so depending on the species. It’s one field test to use when trying to identify plants, but not definitive, as ten percent of flowering plants contain latex*. At any rate, don’t get it on your skin.

Of course there are euphorbs in the Maryland piedmont, otherwise I wouldn’t be writing about them. More on those next time.

*Latex: A Model for Understanding Mechanisms, Ecology, and Evolution of Plant Defense Against Herbivory

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