| Magic Rocks
Of the succulent plants that resemble the rocks and pebbles among which they grow, the best known is undoubtedly Lithops, the living stone of the South African and Namibian deserts. Lithops, however, isn't the only genus of Mesembryanthema (the major subdivision of the family Aizoaceae) that has developed a geologic camouflage as a strategy for survival. In this column I'm going to discuss four other genera, each as remarkable for its ability to flourish in extremely hostile environments as for its peculiar evolution into something that looks more like stone than plant.
The members of Argyroderma, the first of these genera, resemble little eggs made out of quartz or jade. Think of argyrodermas--the name means silver skin-- as silvery-blue to blue-green Easter eggs cut almost in two along their width, with a v-shaped fissure extending along the cut. Each half of the cut open egg is actually a greatly thickened leaf, and each year a new set of leaves appears in the fissure, arranged perpendicularly to the preceding pair of leaves. The width or narrowness of the fissure can be useful in determining the species; some species are rounder in shape, some more ovoid. Some species (Argyroderma delaetii) remain solitary, some (Argyroderma congregatum) slowly clump and form clusters of little blue eggs at the ends of short, prostrate stems. Some argyrodermas (Argyroderma crateriforme, with leaves the color of the bluest sky imaginable) live half-buried in the soil, while others (Argyroderma pearsonii, for example) stay above ground level at all times. The flowers, proportionately large and yellow, red-purple, or rarely white, also emerge out of the central fissure. All eleven species live in one of the most bizarre ecozones on Earth, the quartz fields of coastal Namaqualand in northwest South Africa . This region, called the Knersvlakte, consists of flat plains or slightly rolling slopes, completely covered with small (inch to fist size) chunks of white quartz atop rocky, saline soil. Hidden among the quartz chunks or clustering on top of them, in the company of other miniaturized, extremely odd succulent plants from several families, argyrodermas make their yearly set of leaves, flower and set seed. And their reproductive abilities are impressive. In a good year, with enough winter rain--that is, more than an inch or two--to supplement the frequent fog that keeps the whole ecosystem surviving, some quartz patches are so dense with Argyroderma that the ground is blue with them rather than white and it's impossible to walk (even on tip-toe) without stepping on them.
Far from the seacoast, around the edges of the arid intermountain region called the Little Karoo, we find another genus of stone imitators, this time copying bits of granite rather than white or bluish quartz. These are the at least somewhat familiar members of the genus Pleiospilos, familiarly called "split rocks." Of the relatively few species, most consist of angular pairs of very thick, soft leaves, gray or somewhat reddish and covered with dots, mimicking the angular chunks of rock in which they grow. One, Pleiospilos nelii, has thick, oval-shaped pairs of leave, often several at a time, up to four inches tall, wide and thick. In habitat it looks less like a rock than a pile of something unpleasant deposited by a large quadruped. Other than that, Pleiospilos nelii is an adorable little plant, the entry point for many a child's fascination with succulents, and when it, or any of the angular-leaved pleiospilos (P. bolusii, P. simulans, P. compactum) bloom, their shimmering yellow and orange-rimmed flowers belie any resemblance to rock.
The third genus is the most surreal, in looks, habitat and culture. Dinteranthus consists of about a half-dozen species from the region east of Namaqualand called Bushmanland, with the town of Pofadder at its center. Dinteranthus also grow in small quartz fields, often ones in which the quartz has a crystalline, transparent quality, and some of the species themselves have a glowing appearance as if they were lit from within. As with all these plants, Dinteranthus consist of pairs of fantastically swollen leaves, often with the remnants of last year's leaves around their base. One of them (Dinteranthusvanzyjlii) looks very much like a lithops; the others form little spheres, basically gray, with textures like pearl or marble or velvet. Dinteranthusmicrospermus is covered with dots, some forms of Dinteranthuswilmotianus have faintly glowing orange ridges along the rims of their leaves, and Dinteranthus pole-evansii simply looks likes it is made of mother-of-pearl.
The last of our four genera is monotypic; it consists of only one species. Lapidariamargaretae ranges over a fairly broad area that includes most Dinteranthus territory. It lives in quartz, but in places that actually have a little soil as well, and it is not quite as specialized as its neighbor. Lapidaria--the name means rock--look like little prismatic chips of quartz, pearly colored with blue and rose tips to their chiseled leaves. A large Lapidaria, with several sets of leaves. really does look something like a sharp edged chunk of quartz, at least until it puts up its yellow flowers.
Dinteranthus look impossibly fragile, but winter nights where they live can drop below 20 degrees Fahrenheit. The little rain that falls in their habitat mostly comes in Spring and Fall, but even when actively growing Dinteranthus should receive considerably less water than, for example, Lithops. Watering them once a month from April through September and not at all the rest of the year might suffice to keep them alive, but humidity is their enemy and it's hard to keep them going for many years. Pleiospilos, on the other hand, are easy to grow, liking regular water in the warm months--every week to ten days--and a dry rest in winter. Lapidaria also don't present much of a problem, needing a little less frequent water (every ten days to two weeks) in the warm months and needing to be dry in winter. Argyroderma too, although they come from a strict winter rainfall region, are not that difficult to grow, and can be switched to warm season growth habits, just as with most Lithops, with water every ten days to two weeks in the warmer months and, once again, dry in winter. After seeing them in habitat I'm going to try switching them to a winter watering regime, but it's definitely not needed to keep them growing happily for decades (and not outgrowing a 2 1/2 inch pot in that time). All of these plants want a very fast draining soil mix, preferably low on organic matter, and all of them thrive in very bright sun. Pleiospilos will be quite happy on a sunny windowsill, the others (Dinteranthus in particular) are more difficult, but all grow well from seed and are available from the better succulent nurseries for those interested in adventures in growing.
The Garden has some Pleiospilos and Argyroderma, but its Dinteranthus (the first I ever saw) departed years ago. Conversely, one of the oldest plants in the collection, dating from 1933, is a healthy specimen of Lapidariamargaretae, still fitting in its four-inch pot and blooming regularly every year. We usually have some seed-grown lapidarias to sell at sales and sometimes argyrodermas and pleiospilos as well. |
