What Makes it a Cactus?


And, back to wildflowers.

The pictures in yesterday’s post were of beavertail cactus, Opuntia basilaris, one of the earliest blooming cacti in the Anza-Borrego region. It grows in many different habitats of the Sonoran and Mojave deserts, and is easily identified as an Opuntia by the characteristic flat pads that appear to be spineless. It’s further identified as O. basilaris by the pink flowers (other species’ flowers are yellow). About those spineless pads: they aren’t spineless. The spines are just very small. They’re also barbed. Since this is botany, there has be be a word for them. They’re called glochids.

Beavertail has a cousin here in the Maryland piedmont: eastern prickly pear (Opuntia humifusa).

Cactus is one of those botanical terms that’s often mis-used colloquially, referring to any succulent plant. Succulent just means that the plant has organs that store water, while a cactus is any plant that is a member of the cactus family, Cactaceae.  There are many succulent plant species in the world, but only some of them are cacti.

So what makes it a cactus?

A number of things, taken in combination. First, cacti have spines, which are modified leaves. Spines are not the same as thorns, which are modified stems, or prickles, which arise from epidermal tissue.

Since (with very few exceptions) cacti don’t have true leaves, photosynthesis happens via the stems, which are succulent and often cylindrical, globular, or pad-shaped.

the things that look like dots on the stem are the aureoles; zoom way in to see some glochids in the upper ones

The spines arise from structures called aureoles, which are a defining feature of cacti; no other plants have them. Aureoles also give rise to new stems (on branching species) and to flowers.

Cactus flowers are showy and usually radially symmetrical, with numerous petals and sepals (which can’t be distinguished from one another). They are also usually bisexual, with numerous stamens and a single pistil with an inferior ovary (which means that it is located below the petals and sepals).

With the exception of a single species, all cacti are native to the New World.

Weird cactus fact: there are cactus species native to the the rainforests of Central and South America. Yes, cacti grow in the rainforest, as epiphytes on trees where there’s little organic detritus to form soil and water doesn’t collect, a situation like an extremely localized arid-yet-humid microclimate. Isn’t that nifty?! The common household plant Christmas cactus (Schlumbergera species) is one example.

next time: more cacti

interesting reading
CactiGuide.com
Spines, Photosynthetic Tricks, and Other Marvels of Cacti Evolution

One thought on “What Makes it a Cactus?

  1. Pingback: About Yellow | Elizabeth's Wildflower Blog

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