THE CROWN of Thorns is a native of Madagascar, an island nation in the Indian Ocean just off the southeastern coast of Africa  known as the  fourth-largest island in the world. The plant is part of the Euphorbiaceae (yoo-for-bee-AY-see-ee) family,  the genus being euphorbia (yoo-FOR-bee-uh ),  and the species milii (MIL-ee-eye ).
The Crown of Thorns is a woody, spiny (plimplerish) climbing  and succulent shrub, with shoots reaching a height of 6 feet, and attractive red flowers.
It was named after Euphorbus, the Greek Physician of King Juba II who lived between 50 BC to 19 AD in a country then named Numidia, which is present-day Algeria.
It was King Juba II who discovered the plant, and named it after his physician. The phorbus. 
The species name, Milii, is in honour of Baron Miliius, once Governor of the island of Bourbon, situated in the Indian Ocean off the East Coast of Africa (now called Reunion Island) who introduced the species into cultivation in France in 1821.
The Crown of Thorns is a cousin of the Poinsettia, which is also is a member of the Euphorbiaceae family.
The flowers are small and inconspicuous, but the brightly coloured modified leaves (bracts) found just beneath the flowers are quite attractive.
Most have red bracts, but there are some with yellow.
Among the plant’s common names besides Crown of Thorns are ‘Christ Plant’ and ‘Christ Thorn’.
Many say that the crown of thorns placed on Jesus’ head at the time of his crucifixion was made from stems of this plant. The stems of the plant are flexible, can be intertwined into a circle, and could have been used for this purpose.
Additionally, it is said that the species had been brought to the Middle East before the time of Christ.
The plant exudes a sticky white sap (latex) from any cut surface, and is reputed to have  been used medicinally circa 1AD  as a laxative.
This latex, however, can produce severe dermatitis (skin problems) in susceptible individuals, and is generally poisonous if ingested in large amounts.
The sole purpose of the sap, though, is to protect the plant  from its enemies, such as  plant eating animals like cows,  sheep and goats.