| “The Mexican Living Rock”--Ariocarpus
Of all the different cacti that have intrigued botanists and enthusiasts alike over the years, a relatively few species from the Chihuahuan Desert of northeast Mexico and its environs stand out. Ariocarpus, the largest and most diverse genus of these extraordinary cacti includes species that resemble crystalline minerals, crowns made of petrified leather, or tiny rosettes of one-inch, black little spikes.
The Chihuahuan Desert , home to these so-called Mexican Living Rocks consists in great part of a vast plateau, traversed by small systems of mountains and hills, with an eastern boundary that rises up into the complex ranges of the Sierra Madre Oriental . The exposed, blazing white limestone and gypsum hills favored by many species of Ariocarpus magnify the Chihuahuan Desert climate of fairly cold winters and hot summers. Though the desert, particularly in the south, has a higher total rainfall than many arid environments, the rains shut off quite suddenly as winter approaches, and for several months of the year the region is close to bone dry.
Ariocarpus, by far the most widely distributed and most diverse of these genera, inhabits a vast geographic expanse, from the Big Bend region of southern Texas to as far south as the Mexican states of Durango , Queretaro and San Luis Potosi . As is the case with many widespread cactus genera, a few of its species cover wide expanses of territory, while others are restricted to extremely localized habitats. The northernmost species, Ariocarpus fissuratus (spineless, as are all members of the genus) ranges over a good bit of the northeast corner of Mexico and reaches into the Big Bend region of southwest Texas as well. The most widely distributed, Ariocarpus retusus, first occurs not too far south of A. fissuratus, and extends over a domain that covers tens of thousands of square miles. A third species, Ariocarpus kotschoubeyanus shares some of the territory of the other two species, but as an environmental specialist it rarely occurs in the same kind of sites as the others.
Both A. fissuratus and A. retusus prefer inhospitable substrates that at their most extreme can consist entirely of chunks of limestone or shattered beds of splitting, alkaline shale. Typical forms of A. fissuratus (the “Texas Living Rock”) grow almost flush with the surface, with rock-hard, gray bodies made up of prismatically triangular, flat-topped spineless tubercles that really do resemble a rough semi-crystalline mineral rather than organic matter. Even though the plants grow to a reasonable size (as much as six inches across), with dark coloration that strongly contrasts with the blinding white limestone in which they often grow, they still are almost impossible to see. I personally experienced the classic method of ariocarpus discovery years ago when I suddenly realized that I was actually standing on a here-to-fore invisible plant. At the northern edge of its range, in the Texas Big Bend, A. fissuratus tends to cluster around the southern edges of limestone ridges, where both sunlight and drainage are maximized. Mature plants persist by growing in a matrix of organic matter, the residue of their own old decaying tubercles, which can give the visual effect of a sharply prismatic smallish plant growing in the middle of a drying cow-pie. Farther to the south, the plants may grow on slightly less bleak slopes, sometimes in the company of the spiny terrestrial bromeliad, Hechtiatexensis, or that omnipresent Chihuahuan Desert resident, Agavelechuguilla. At the far southwestern corner of their range, in the Mexican state of Durango , the plants are distinctly less flattened, with their tubercles forming low ridges that radiate out from their center. This form of A. fissuratus was once known as Ariocarpuslloydii, but now it’s considered at most a variety of the species, if not merely a distinct looking population, the southern extreme of a continuum of growth forms.
If A. fissuratus resembles a mineral, then A. retusus might be compared to a stylized crown or a multi-armed starfish. This extremely variable species, with its enormous range, has tubercles that extend into upright or outward-facing points, sometimes blunt tipped and pyramidal, no more than a half inch long, sometimes two inches long or more, sharply angled or curving to a slightly rounded tip. The plants have attained the form of a typical leafy rosette succulent, with their elongated tubercles substituting for leaves. A. retusus can become quite large, close to a foot in diameter, and will sometimes slowly cluster, forming ancient multi-headed clumps a couple of feet in diameter—in a few places weirdly twisting crested forms also occur. They may live on low hilltops in the company of numerous other dry-growing cacti and succulents, or on barren flats covered by thin, alkaline, gray soil and rock. Not only widely distributed, in some areas the plants grow in abundance and may make up the most visible portion of the local cactus population. Though A. retusus, with its upright tubercles, always protrudes above soil level, much of its substance remains underground in the form of a thick, tuberous stem.
Observing either of these two Ariocarpus species makes one wonder how the plants can germinate and survive in their extremely harsh settings. A seedling may find shelter in the relative shade and moisture provided by a Selaginella (fern allies that resemble sunbursts of moss and curl up into crisp, dead-looking balls during the dry season) functioning as a nurse plant. More, however, get their start when an old plant dies, as seeds typically stay embedded in the dried remnants of their fruits at the bases of the tubercles where they remain viable for many years.
In contrast, A. kotschoubeyanus the third widely distributed species, generally lives buried in the cracked silt of dry lake beds, rarely protruding above the soil level and almost invisible except when it displays its bright magenta to pale pink-white flowers. The plant’s name commemorates a 19 th century Russian prince and cactus connoisseur, famous in cactus lore for paying more for the plant than its weight in gold. Flat topped and with a thick, tuberous, underground stem, A. kotschoubeyanus never rises above soil level, and in the wild only the symmetrical configuration of its tubercles distinguish the plants from the cracked, dry mud in which they grow. These geometric patterns resemble the tubercle arrangement of A. fissuratus, but the plants remain tiny, no more than an inch and a half across. A very few other small, flat topped cacti show up in this habitat, but other than that the dried lake beds are almost completely barren. Old plants of A. kotschoubeyanus rarely reach as much as an inch and a half across, with underground stems greatly exceeding their surface dimensions. The enormous dry lake beds of the Mexican state of Coahuila are home to most of the plants, but a population of whitish-pink flowered ones has found a home in a couple of tiny mud-flats (hardly bigger than vacant lots) hundreds of miles to the east. Intermediate geographically between these two forms, the variety elephantidens, up to twice the size of the typical plants, lives in a completely different kind of habitat, in rocky soil where it grows among other Chihuahuan Desert cacti, Dasylirion and sparse brush.
With some confined to habitats consisting of only one or two low hills and ridges, or desert patches totaling no more than a few acres, all the other Ariocarpus species are truly rare plants in the wild. Consequently they are all at least potentially endangered, whether from over-collection, habitat degradation, or encroachment from nearby towns. These plants include some miniature species extremely difficult to find and much larger plants whose displays of tubercle crowned rosettes comprise some of the more striking features of the barren, sandy flats which they inhabit.
Somewhere in the middle of these two extremes, Ariocarpus scaphirostris looks like a smaller version of A. retusus with magenta flowers. Its native territory within a single valley in Nuevo Leon consists of a couple of bleak ridges covered with pencil sized and shaped fragments of shaley limestone, of a shape, color and size very similar to the elongated tubercles of the plants. The main body of the plant remains embedded in the rocky matrix of the soil, and though little else lives on the ridges to provide distraction from the plants, they nonetheless are fairly difficult to see. Their valley home lies on the east edge of the mountains that mark Chihuahuan Desert boundary, where canyons and arroyos swiftly fill with raging flood waters during the late summer rains, only to return to a state of utter dryness by the following morning.
Native to a few valleys farther to the east and lower in altitude, the pale yellow-flowered Ariocarpustrigonus also develops elongated, upward-reaching tubercles and looks a bit like a brown hard-leafed Echeveria. Large plants reach nearly a foot across, and occasionally may produce an offset or two. In its best known locality, in eastern Tamaulipas, the surrounding country consists of fairly barren, sandy flats, interrupted by sporadic clumps of sad-looking agaves, and thorny scrub bushes that provide shelter for a few other small cacti. In this spot, however, A. trigonus tends to grow in the open, mostly buried in the soil but with concentric rings of tubercles fully exposed to the sun. Several years ago A. trigonus was merged into A. retusus as a subspecies, this taxonomic change caused by the discovery of several populations of a plant in some ways midway between the two species, and in some ways different from both. Although currently considered Ariocarpusretusus ssp. trigonus, a number of people familiar with the plants remain unconvinced, and whatever its taxonomic status, the name “trigonus” still signifies a distinct looking cactus.
Another veteran of taxonomy wars, the tiny (rarely over an inch in diameter) Ariocarpus agavoides originally had the genus Neogomesia all to itself. Sunken without controversy into Ariocarpus several decades ago, the species was named in recognition of its extremely long, narrow tubercles, like the leaves of a miniature agave. This is another extremely rare plant, with a significant part of its tiny habitat under severe threat from the expansion of local towns. The recently discovered A. bravoanus (collected almost out of existence shortly after being found in the 1990s) somewhat resembles a miniature, gray A. scaphirostris, with slightly shorter, deeply grooved tubercles. In contrast, its also recently discovered subspecies hintonii, similarly severely endangered, seems like a miniaturized A. fissuratus, with all the crystalline marking of its larger cousin also in place on its equally flat top, no more than an inch or two across. A. bravoanus ssp. hintonii grows in stony limestone soil that resembles the habitat of A. fissuratus, while A. bravoanus ssp. bravoanus, growing somewhat farther south, is found in deceptively verdant-looking circumstances amid terrestrial bromeliads (Hechtia), dense, billowing clumps of thin leaved Agavestriata, and tall single stemmed yuccas.
As often is the case with genera of sporadically distributed cacti, the degree of differentiation among different populations of plants can be difficult to determine. Speciation is an event at least partly controlled by time. Left in isolation for a few dozen more millennia, geographically isolated plants such as A. fissuratus var. (or forma) lloydii and A. bravoanus ssp. hintoni will probably evolve into clearly defined species; in contrast the many forms of A. retusus intergrade and mix throughout their range, and whether some of these distinctively long-tubercled, yellow-flowered forms deserve the distinct name of A. trigonus is not entirely clear. Appearing like odd mineral formations in their rocky or mud-flat homes, Ariocarpus, even more than most cacti, provide an endless display of individual differences that transcend taxonomy.
Though far from the easiest cacti to grow, Ariocarpus adapt well to cultivation when given the correct treatment. They need very bright light, which for anyone living near San Francisco Bay translates into the brightest light possible. Potting soil for these plants should, above all, be very fast draining, and they do best with little if any organic matter in their soil mix. Many of these cacti grow in alkaline soils or limestone rocks, and, though not really necessary, a little lime in the mix won’t hurt.
Summers in the Chihuahuan Desert can bring sudden downpours, and when in full growth Ariocarpus do need water, though less water than average for a cactus. For coastal central California , water every two weeks is about right; adjusted accordingly for climates with more or less sunshine and heat. In winter in our area the plants do best with total dryness. In the southwest or southern California , however, a light watering once or twice in winter shouldn’t hurt. As with most cacti, the plants can withstand considerable cold if dry. Ariocarpus agavoides tends to begin its growth a bit later than most of these plants, and may rot if given water too early, and all the species can rot if watered too early in the year.
In the past an Ariocarpus offered for sale were almost certainly collected from the wild. Even today, despite laws intended to protect them, some of the species with extremely limited ranges remain in real danger of extinction. Fortunately, a number of growers have accepted the challenge of raising them from seed, often grafting them to increase their rate of growth, and these seed grown plants are making their way into the trade. With this treatment, after five or six years an Ariocarpus will be a reasonable looking, if fairly small plant, ready and able to flower.
The Garden has a fairly good collection of Ariocarpus (although it lacks examples of the newest discoveries), including a few very nice specimens. From time to time we have Ariocarpus seedlings (non-grafted and often twelve or fifteen years old) available for purchase at sales.
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