Camouflaged Cacti in Central Mexico

How does a plant "know" what a rock looks like? It doesn't. Instead, its surrounding environment*selects* for it over gradually over lengths of time that far surpass a human lifetime (and far surpass human civilization itself).

There are any multitude of shapes and forms that the evolutionary lineage that produced Ariocarpus kotschoubeyanus could have taken, but since it evolved on slightly-metamorphosed calcium carbonate rock that fractures as it weathers here at 6,600 ft elevation, the tubercles on this plant (which is basically just a flattened stem, like a cylinder that's been smooshed in a vice) resemble the rocks.

In arid regions there is a lot more herbivore pressure on plants than there is in more mesic ones since there is much less on the menu. As a result any individual plant whose phenotype made it easier to spot got picked off.

We have seen the same thing happen most recently with the lily species Fritillaria delavayi in China, which has been subjected to increasingly more intense harvesting pressure by humans who pick it to sell in market as medicine. As a result, Fritillaria delavayi has survived by become much harder to spot in its rocky environment - measurably so, to the point that there was a paper published on it. Such is a prime example of natural selection at work in real time.

I've seen the same thing with pyramidalis in South Africa.

The genus Ariocarpus contains 8 or 9 species depending on which taxonomy you accept. All resemble rocks, indicating that the adaptive benefits of mimicking them have been in this lineage for quite

some time.