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Euphorbia (Apocynaceae).

Plant of the month    (©Pierre Laszlo, all rights reserved)
Euphorbia (Apocynaceae).

Euphorbs — some of which are commonly known as ‟spurges‟ — are remarkable plants, numbering about 2,000 species, spread all over the planet. It is the fourth largest genus of flowering plants. They owe this success to their adaptability, changing their morphology to better fit a location and climate. They range from herbs and dwarf shrubs, less than 15 cm tall, to large trees, up to 30 m high.

The most recognizable euphorb is E. pulcherrima (the Christmas poinsettia), the world’s most economically imported potted plant. It was thus named for R. J. Poinsett, the first United States diplomat to Mexico (1825-1829). Poinsett collected the progenitors of the over 300 varieties in global cultivation on an 1828 excursion to northern Guerrero State, Mexico. He was an amateur botanist sending some plants to his home in South Carolina for further horticultural development.

Hevea brasiliensis, that provides natural rubber, also belongs to the euphorb family. Likewise, cassava, a staple food in tropical countries, together with tapioca, obtained from the roots of species of manihot. Ricinus communis, from which castor oil is obtained, is yet another plant from the same group of euphorbs. 

Carl Linnaeus thus named these plants, celebrating a physician from Antiquity. We know of him from Pliny’s Natural History, dating from about 77 AD.

Euphorbus was originally a Greek herbalist and physician. But his practice was not in Greece. Rather, he was personal physician to a king, Juba II.

His father, Juba I, a Berber, ruled over Numibia, present-day Lybia. Juba II was brought to Rome as a child in 46 BCE. He married Cleopatra Selene II, the daughter of the famous couple, Anthony and Cleopatra. He inherited the throne of Numibia, and went on to become king of Mauritania — nowadays, northern Morocco, western Algeria and perhaps a part of Lybia, at that time populated by Berbers.

Juba II, was a remarkable scholar, with wide-ranging curiosity. By age 20 he had written a book on Roman archaeology. A prolific author, a patron of the arts and sciences, King Juba II wrote quite a few other books in Greek and Latin on history, natural history, geography, grammar, painting and theatre. For Romans, his guide to Arabia was a must read.

Pliny brings up Euphorbus’s brother, Antonus Musa, also a physician, who enjoyed a yet more powerful employer, the emperor Augustus Caesar.  After Antonu Musa cured him from a severe illness — Augustus orderedd the Roman Senate to reward him in 23 BCE with a bronze statue. 

Likewise, Juba II, in gratitude to Euphorbus, after he had been treated with a powerful laxative obtained from a Mauritanian plant, named it after him. And so Linnaeus, centuries later, would give it his lasting seal of approval. There is a teasing note to the name Euphorbus. Juba II gave it to his physician in jest, Euphorbus was quite corpulent and his name means ‟well-fed‟ in Greek. 

The defining morphological feature, that euphorbs all share, is known technically as the cyathium. This is an inflorescence reduced to the essentials and resembling a single flower. In developmental terms, it is intermediate between a single flower and an inflorescence. It consists of a cup-like involucre that surrounds multiple male flowers, each reduced to a single stamen, and a single female flower, reduced to a single pistil. 

The fruits are three (rarely two) compartment capsules , sometimes fleshy but almost always ripening to a woody container that then splits open explosively. The seeds are 4-angled, oval or spherical.

The adaptability of euphorbs is best exemplified by their evolutionary response to arid habitats. In order to conserve water, and given that leaves undego considerable evaporation, the photosynthetic apparatus moved from leaves to stems. These unusual growh forms are known, graphically, as pencil-stem plants. They are leafless and semi-succulent. Precipitation at sites populated by pencil-stem euphorbs is, on the average, half that undergone at sites populated by leafy species.

The spine-shield Euphorbia are thus called because of the hard pad of tissue at each leaf axil that bears several spines. This section of the Euphorbia genus is also named Euphorbia, it comprises about 360 species, predominantly African and Asian. They are often likened to cacti in a case of convergent evolution to extremely dry habitats. The Asian species are descended from two colonizations in the middle to late Miocene. As for the numerous African species, they diversified in response to the aridification of Africa from the middle Miocene on.

The common name, spurge, comes from the French espurge, ‟to purge,‟ from the use of their latex as a purgative and laxative — exemplified by Juba II’s treatment at the hands of Euphorbus. This milky latex is a chemical weapon against herbivores.  It is composed of diterpenes and triterpenes, some of which are powerful skin and mucosa irritants.