You can't milk a cactus

One of my favorite topics in biology is convergent evolution. While the most common examples of convergence are animals (koala "bears," marsupial "moles," etc etc), there are some fascinating cases among plants as well. One of the best known is the very strong convergence between the Cactaceae and the Euphorbiaceae. The two clades have developed extremely similar adaptations to hot, arid environments, and succulent euphorbs are often mistaken for cacti. Look at the picture below, can you tell which is NOT a true cactus? (See bottom of post for answer).


The Euphorbiaceae is a family within the order Malpighiales, which contains an eclectic collection of well-known plants such as St. John's Wort, cocaine, and mangrove trees. The most famous member of the Euphorbiceae is Poinsettia, a Christmas ornamental, but the order itself is huge and contains many diverse families. The genus Euphorbia is where we find most of the succulents. The most obvious adaptations they share with cacti are the large, thick stems that act as water storage structures. In both cases, also, the succulent stems act as the main photosynthetic organs of the plant, as opposed to having many leaves with lots of surface area, which would cost the plant precious water.

True cacti are found in the family Cactaceae, which is in Caryophyllales, a completely different order.. Despite their phylogenetic distance, the cacti and euphorbs both possess succulent stems and leaves modified into spiny projections. These spines have dual functions: they act as antiherbivory devices and can also shield the plants from harsh desert winds, reducing evaporative water loss. Their tall, skinny form is also a way to reduce evaporation by exposing as little surface area as possible to the sun overhead.

So, how do you tell a true cactus from a member of Euphorbia? One clue is that cactus fruits are usually leathery, fleshy berries, while Euphorbia produces dry fruits called schizocarps. One of the best giveaways is the milky, latex substance produced by Euphorbia. Watch out for this, though, many species are highly toxic! Many of them are used in traditional medicine practices, but they are also known to be involved in homicides, and are used to create poison arrow tips for hunting. So, a quick way to determine whether a plant is a cactus or a euphorb is just to make a small cut: if it exudes a clear substance it is probably a cactus, if a milky substance comes out then it is a euphorb.

That is the quick and dirty field guide to distinguishing between these highly convergent taxa. Lastly, the answer to quiz: TWO are Euphorbia, only the one on the far left is a true cactus.

See earlier posts on convergence:
Reptile placentas
Hyena-Primate Convergence: The Social Brain Hypothesis

(Image credit for Euphorbia, cactus, and quiz group shot)