If it has thorns, it must be a cactus...not
December 14, 2006
by Dr. Stephen L. Timme

Is it a cactus or not?

Occasionally, some of the chain stores (I won’t mention any names but one of them might have the acronym WM) will sell a species of plant they call a cactus. In fact, this species is not a cactus but a member of the euphorb family, the same family for poinsettias. Obviously they do not have a botanist on their staff familiar with plant names to ensure that correct names are used for the plants they sell, or if they do, he/she is a very poor botanist.

The species in the photo is a common house plant sometimes referred to as African Milk Tree or Candelabra Tree, depending on the species. It has thorns like a cactus and is somewhat shaped like some cacti...so, they say, it must be one!

The phenomenon in which two unrelated species look similar to one another is the result of what biologists call parallel evolution. This generally occurs when two evolutionary lines are found in similar environments of different areas (e.g., continents), and so both are exposed to the same kinds of selective pressures.

Cacti produce spines from specialized structures, while many euphorbs have branched thorns, or spines formed from modified stipules (structures associated with leaves). Many euphorbs are shrubs, including the one pictured here, while others are herbaceous (soft stemmed). In addition, all members of the genus Euphorbia produce milky latex.

This latex is known to cause a mild to severe dermatitis or blistering. In others a severe inflammation occurs if accidentally rubbed in the eyes. The latex of most is poisonous if ingested.

One species, Euphorbia antisyphilitica, produces a wax from the candelilla plant that is used for leather polishes and waterproofing. When mixed with rubber it can be used as insulation, sealing wax, and even for making dental impressions.

The flowers of euphorbs are in specialized structures. The parts that appear to be petals are actually glands. In cacti, the flowers have many sepals that look like petals and are, therefore, difficult to differentiate.

The genus Euphorbia has more than 1000 species and is the “cacti” of Africa and the Middle East because they occupy similar habitats as the cactus in the New World. The ornamental, Crown of Thorns or Christ’s Crown (Euphorbia splendens), is so called because it is thought to be the plant used to crown the head of Jesus. It is told that the flowers of this species were originally white but turned red from Jesus’ blood and produced red flowers thereafter (no doubt a myth).

And finally, Christmas is marked by another euphorb, the poinsettia. The Mexican people call it the “Flower of the Holy Night.” The myth goes that a young girl wanted to lay a gift before the baby Jesus in a church one Christmas but had nothing to offer. Angels appeared to her and told her to pick a bouquet of weeds (whatever a weed may be). When she walked into the church, everyone laughed at her until suddenly the weeds burst into beautiful flowers, the poinsettia. The bright red structures (white or purplish in some) are modified leaves and not flowers. The flowers are inconspicuous and yellowish to yellowish-red or yellowish-green. (See photo.)

But take heed! The poinsettia also produces milky latex (break off a leaf) that is poisonous, may be irritating, and can cause blistering.

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