Imagine some prehistoric plant, the common ancestor inhabiting that land mass. Individuals that wandered off with Africa became exposed to many of the same brutal climatic conditions as those remaining in America. Increasing desert conditions on both continents forced the ancestor to adapt to survive. This adaptability was contingent on traits in the common gene pool coupled with the occasional crap shoot of a beneficial mutant that lived to reproduce.
In Africa the plants evolved into a group known as the Euphorbia. This is a huge family that includes poinsettias. Most interesting are the giant tree-like succulent forms common in southern Africa. These unusual succulents are often visually confused with cacti, but are very different plants. Cacti are exclusive to the New World where they did not evolve into the family Cactaceae until millennia after the Gondwana break up.
The progeny of that common ancestor exhibit the same adaptive traits on both continents. They took upright columnar forms or candelabra-like growth to reduce horizontal surfaces exposed to the sun. For example, Euphorbia in the bush of South Africa appear similar in form to the armed Saguaro cactus of Arizona.
Leaves of both groups are small if existing at all, reducing moisture loss. To compensate for lack of leaves, photosynthesis is carried out through the green surfaces of stems. Many species of both groups are viciously spiked, a mutual defense against aggressively browsing wildlife seeking scant sustenance in brutally dry climates.
Both groups developed succulent bodies to retain moisture within the plant itself. This compensates for lack of moisture in the root zone during the dry season. When it does rain, this helps them to more quickly absorb copious moisture before it passes through porous sandy soils.
The similarity of euphorbia and cacti is perhaps our most clear example of convergent evolution. This term applies to scores of other plants and animals separated by continental drift, then evolving many of the same traits in isolation.
So how do we tell if a plant is an American cacti or an African euphorbia? Simple. Use a pin to prick the skin of the plant. If the plant bleeds clear or green liquid it is a cactus. If it bleeds milky white latex, then it's euphorbia or a close relative.
This latex is filled with all sorts of caustic chemicals. Most will cause skin irritation on contact. If you are pruning or propagating, it's a good idea to wear rubber gloves that won't absorb the latex like cotton or leather. In fact, many African tribes have traditionally stunned fish by adding latex of certain species to pools. It is wise to consider all parts of all euphorbia poisonous, so beware if they are to grow where pets and children play.
Like all succulents, the chief enemy of these euphorbia is rot, usually caused by inadequate drainage or over-watering. Use porous cactus soil mix rather than standard potting soil to eliminate risk of rot.
Whether you choose to grow euphorbia or cacti, remember that at one time they both shared a common ancestor. And like siblings in a family separated at birth, they add a new dimension to that age old dilemma of nature vs. nurture.
On the net: www.desert-tropicals.com, www.cactus-mall.com
(Maureen Gilmer is a horticulturist and author of Water Works and 13 other books.)
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