Probably the most famous desert-dwelling plant family, the cacti family are a group of dicots native to the Americas. There are 127 genera in this clade with 1750 known species in total.
When we look at a cactus, it can be hard to decide which part is the leaf and which is the stem. Cactaceae plants have evolved to change up the roles of their parts somewhat: the green, succulent parts are actually stems (internodes), whereas the “spines” are the true leaves.
The stems fulfill the photosynthetic function while the spines ward off opportunistic grazers.
Description
Cacti have a unique structure called “areoles”, which are typically a woolly area where the true leaves, or spines, emerge out from. Flowers also emerge from areoles. This is unlike most plants, which typically have leaves and flowers emerging from an ordinary node which joins two internodes.
On this bunny ear cactus Opuntia microdasys (same genus as the prickly pear), each white dot represents an areole from where the true leaves (spines) come from. Image source
Spines and wool vary greatly between species so are a great method of identification between cacti, as are flowers and stems.
There are some look-a-likes in the euphorbia family Euphorbiaceae which have evolved a similar habit with succulent stems and spikes. The spikes on euphorbia look-a-likes are technically “thorns”, which are modified stems, unlike the “spines” (leaves) on cacti.
Some ways to tell the two apart are: euphorbias have a white sap whereas cacti are juicy with clear liquid. Euphorbia flowers tend to be less showy than cacti flowers, which are often very loud and decorative.
Flowers, Fruits & Leaves
Tepals: Sepals and petals look alike. Counting tepals may not help with identification with this family, as numbers may not be consistent even on the same plant. Modified leaves called bracts line the inferior ovary without showy colours.
Male: Many stamens are attached to the inner wall of the flower.
Female: Usually one pistile with many stigmas on a single style.
Fruit: Prickly pears and dragon fruit are typical examples of the fruit. A soft skin contains a juicy, fleshy inner with multiple seeds.
Seeds: Varying greatly between species in shape and size. May be black or reddish.
Leaves: Spines come out of areoles.
Edible Cacti
Cacti are generally safe to eat, though that doesn’t mean they all taste good. Because thier stems hold moisture they can be a life-saver if you’re ever lost in the desert without water.
Dragon fruit and prickly pear are popular examples of cactus crops. Cacti make great crops in dry areas because they make good use of the little water available.
Just watch out for the spines. Prickly pears in particular have super fine spines that irritate skin for days. I use some leather gloves and a clean pair of secateurs to remove the skin completely, washing as I go along to avoid eating the spines.
Euphorbia sap can be toxic, so varieties that look like cacti are not to be consumed. Don’t get euphorbia sap in your eyes, either!
Noteworthy Types
Prickly pears Opuntia ficus-indica are the largest crop in the family. Other Opuntia spp. are noxious weeds in Australia but still edible and delicious (if you don’t get stung by the prickly hairs, that is).
Prickly pear Opuntia ficus-indica fruits on stems. Image source
Close up of prickly pear Opuntia ficus-indica fruits. Image source
Hylocereus is a genus of epiphytic cacti that often have the habit of a vine. Dragon fruit are an example from the group.
Dragon fruit plant Hylocereus undatus with plenty of flowers. Notice the many stigmas on each pistil, and also the bracts that appear like sepals, but are in fact modified leaves. Image source
Cross section of a dragon fruit Hylocereus undatus showing the bracts around the outside as well as flesh and seeds inside. Image source
The saguaro cactus Carnegiea gigantea is a large cactus that can live over 150 years, which is unusual for the family.
A saguaro cactus Carnegiea gigantea standing tall and proud. Image source
Conclusion
Cacti are a great family to plant due to their ability to tolerate drought. Some varieties are pretty harmless, and some must be admired from a distance. Cacti spines are no joke at the best of times, and worst case scenario bacteria and fungal spores may be present and cause infections when they break our skin.
If you haven’t already read my articles on plant identification and scientific names, I recommend reading those to get a broader picture of the topic. Alternatively, you can browse some of my other plant families, subfamilies and genera below.