Dr. T. Ombrello - UCC Biology Department

LIVING STONES

Common names:

Living Stones, Stoneface, Pebble Plants, Flowering Stones, Mimicry Plant

Scientific name: Lithops species

Explanation of scientific name:

Lithops - from the Greek “lithos” meaning stone and “ops” meaning face.

A pot of Living Stone plants growing in one of Union County College's greenhouses.

The 37 species of the plant genus Lithops are the most well known members of the group of succulent plants commonly referred to as Living Stones. Native only to the deserts of South Africa, these small curious plants escaped detection by botanists until 1811 when Thomas Burchell, a European traveler, discovered one by accident. In his 1822 book entitled Travels in the Interior of Southern Africa, Burchell described his discovery as follows:

“On picking up from the stony ground what I supposed was a curiously shaped pebble, it proved to be a plant . . . . but in color and appearance bore the closest resemblance to the stones between which it was growing.”

Living Stones are perennials, often found growing in clumps although individual plants usually consist of just 2 very succulent leaves. The paired leaves are united and together are shaped like an inverted cone. The stem is practically non-existent, and the roots seem to arise from the base of the leaves. From the fissure across the top of the united leaves arises either flowers or a new pair of leaves to replace the older ones. The delicate growing point is safely hidden beneath the soil between the leaves near their bases.

In nature, only the tops of the leaves are exposed above the soil surface. When cultivated under other than desert conditions, Lithops do best when the leaves are left well above the soil surface.

Having most of the plant’s leaf surface below the ground has certain advantages. It is cooler; there is less exposure to drying winds, and less chance of being spotted by a grazing animal (although no animals are known to use Living Stones as food). Even gas exchange occurs underground. The pores (stomates) open without any exposure to wind or sun, so water loss is minimized during this process.

The disadvantage, however, is the limited amount of leaf surface area exposed to the sun for photosynthesis to supply food for the plant. The Living Stones demonstrate an interesting evolutionary adaptation to overcome this light problem; their leaves have transparent “windows”. In fact, they are the most advanced “windows” of all succulent plants. The clear tips of the leaves have a crude optical system that permits light striking the windows to be diffused by crystals of calcium oxalate onto the green photosynthetic area below. So, with a minimum of exposure to the outside environment, a maximum area of photosynthetic tissue can be illuminated.

Another survival feature built into the Living Stones involves seed dispersal and longevity. Since water is essential for germination, but quite rare in their habitat, seed capsules will not open to disperse their long-lived seeds until they experience rainfall or heavy dew. Sometimes the seeds are retained for many months. This ensures that the dispersed seeds have sufficient moisture to begin the next generation.

All these adaptations contribute to the Living Stones’ ability to thrive in habitats with 120^oF temperatures, full sun, and minimal rainfall.

Living Stones are also masters of mimicry. The circular or oval tips of the leaves resemble weathered stones on the soil surface. Each species harmonizes to some extent with its native background, and can be found only in habitats that provide the right colors and textures. For example, where several rock formations occur side by side, Lithops species are confined to places where they blend best with the background. The colors of leaf tips range from plain gray to brown, with some species showing intricately mottled patterns. Just why they have evolved to mimic their habitat is quite puzzling, in light of the aforementioned statement on their undesirability as food for animals. Mimicry is abandoned, however, when the Living Stones flower. Their large, showy, white or yellow colored blossoms are quite conspicuous. This is not surprising, since they are insect pollinated and, therefore, must be attractive to potential pollinators.

Living Stones certainly rank among the marvels of the Plant Kingdom, and they never fail to generate interest and comments from botanists and the general public alike. Unfortunately, this has led to exploitation by overzealous plant collectors that has driven several species to the verge of extinction and has made most species of Living Stones endangered. In their native South Africa they are now protected by law. Plants are still available at garden centers, but these have been raised from seeds or divisions of cultivated plants.