The cactus-like trees of the Euphorbiaceae

The Euphorbiaceae is one of my favourite plant families. Not only are many Euphorbia trees instantly recognisable by their candelabra-like growth form, but they also form unique and characteristic components of the Subtropical Thicket Biome in the region where I spent my childhood. These trees – especially Euphorbia triangularis – remind me of my original home!

The Euphorbiaceae contains approximately 90 native southern African tree species. Most of the species have succulent stems with (often absent) simple, alternate or spirally-arranged leaves. The flowers are usually small, yellow and bird pollinated. Although Euphorbia species (the largest genus in the family) are often cactus-like in growth form, Euphorbs and cacti are quite unrelated, being an example of remarkable convergent evolution to arid environments. True Euphorbias can be distinguished by the paired spines and poisonous, corrosive milky latex-like sap. I recall one incident from my childhood where I was dared by a friend to taste the sap from one of these plants growing on our school’s premises. I was tempted, made a gash in the side of one of the plants and licked some latex from my finger. A human would have to drink large quantities of the stuff to notice serious effects, but I can still remember the horrible taste and the way it left my mouth dry for a few hours.

Su Abraham’s beautiful illustration (above) shows quite clearly the characteristic traits of a true Euphorbia, including the succulent stem, paired spines, and reduced yellow flowers (with three-lobed fruit capsules).

Non-succulent Euphorbs

Not all Euphorbiaceae species are succulent. In fact, most Euphorbs are non-succulent: only two out of the thirty nine genera have succulent trees (in addition to Euphorbia, the other genus containing succulent trees is Synadenium). Some of the remaining thirty-seven genera contain some familiar trees, such as Tambotie (Spirostachys africana) and several Clutia species. I have a stink-ebony tree (Heywoodia lucens) growing in my garden.

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