Lithops are probably the most well known and popular member of the large succulent plant family, the Mesembryanthemaceae. They are commonly called living stones or stone plants because they so closely resemble the surrounding pebbles of their natural habitat, either in coloration or shape or both. This camouflage allows them to escape detection and is a very effective strategy for escaping predation. Their resemblance to stones has also given the genus its scientific name: from the Greek lithos meaning stone and opsis meaning appearance or resemblance. They are known in Afrikaans as beeskloutjies (bees means ox/cattle and kloutjie feet), because the plants also look like miniature hoofprints of cattle.

A lithops plant consists of a two-lobed obconical (upside down cone-shaped) body that is in fact a fused and thickened pair of opposite leaves. The stem is very short and not visible. In their native habitat the leaf pair grows sunken in the gravely soil with only the upper portion visible. This part is called the window, and it allows light into the inner portion of the leaf where it is diffused before reaching the green chlorophyll, which is scattered along the inner leaf margins, where the process of photosynthesis is carried out. The leaf pair is replaced by a new one every year and the leaf sap is recycled from the older to the younger leaf pair, thus maximising on moisture and nutrient conservation. Lithops plants will produce more than one leaf pair so that a single body gradually becomes a smallish cluster.

Each leaf pair will produce one solitary bright daisy-like flower during autumn. This flower is usually white or yellow in colour, about 25 mm in diameter, and appears in the fissure of the leaf pair. They open late in the day and close up again at dusk and could be pollinated by any number of bees, flies, wasps, gnats, bugs etc that are common in the area. Lithops are self-sterile, so the effective pollinator is more likely to be a flying insect that will visit more than one group within a population. The seed is held inside a 4-8 chambered fruiting capsule. This capsule only opens when moistened, exposing the tiny seeds. In nature, falling rain drops splash out the seeds to a distance of a few centimetres to up to a metre or more from the parent plant. After the capsule dries up it closes again, protecting any seeds left behind. All the different species look quite similar to each other, differing mainly in the shape, markings, colour and texture of the body.

The genus Lithops consists of about 33 species widely distributed in the western, southern and central regions of southern Africa, i.e. Namibia, Botswana and all provinces of South Africa except for Mpumalanga and KwaZulu-Natal. They occur mainly in the Nama Karoo and succulent Karoo and are especially common along the Orange River valley in the Northern Cape. The first Lithops was discovered by William John Burchell in 1811 to the south east of the present-day Northern Cape town of Prieska during his nearly five year journey of exploration in southern Africa.

Rainfall in lithops habitats ranges between 25 and 400 mm per annum and falls mainly during spring and autumn with less frequent showers during the rest of the year. Temperatures vary, the average daily maximum ranges between 18 and 28 degrees centigrade and the daily minimum between 8 and 12 degrees centigrade.

Our species, Lithops optica, was first collected by Dr Rudolph Marloth in 1909 near Lüderitz in Namibia. It is endemic (i.e. it occurs naturally nowhere else on Earth) to the Lüderitz district of the southern Namib Desert where it grows on the coastal plains. Its habitat is very sandy and it is often found growing among rocks and gravel where it is very difficult to spot. Its climate is cool due to the cold Atlantic Ocean and frequent fog, and rainfall is mainly during the winter, ranging between 20-50 mm per annum.

Lithops optica consists of an oblong obconical almost club-shaped leaf pair up to 20 mm long. The leaf pairs are often unequal, rounded-truncate at their tops and up to 15 x 12 mm in diameter with a smooth texture, deeply fissured and whitish grey to grey green in colour. There is a reddish colour form, Lithops optica cv. Rubra, with ruby milky pink sides and light to dark ruby-red windows. The bodies become wrinkled during the dry summers. They divide with age forming small clusters of up to 10 bodies. It is possible to estimate the age of a Lithops optica colony by counting the dried-up shells of old bodies, and specimens have been found with ages estimated at 50 to 95 years or more. The flowers are white, sometimes with pink tips, 12-20 mm in diameter and appear during autumn. The fruiting capsule is mostly 5-chambered.

Growing Lithops optica

Lithops has been very well spread by cultivation and is an example of ex-situ conservation, i.e. where plants have been effectively conserved out of habitat. Seed and plants of all species and many cultivars are today available from specialist succulent nurseries not only in South Africa but also in the USA, Europe and Japan. Plants are relatively easily grown but great care needs to be taken with soil, temperature, time and amount of watering. If it is mastered, growing them becomes a pleasure. Lithops out of habitat are best grown in a container in a greenhouse where watering and temperature can be controlled. Although lithops originate from a very sunny terrain care should be taken in cultivation not to expose a plant that has been grown in partial shade too suddenly to full sun as the sun will cause leaf burn resulting in death or disfigurement. Plants should gradually be hardened off by exposing them to brighter light conditions over a period of time to allow the plant to adjust.

Grow Lithops optica in soil that consists of a sandy gravel mixture (e.g. 2 parts sand : 1 part clay loam : 1 part gravel). Add ample bone meal and dolomitic lime. Lithops optica is best planted in a sunny and airy part of the greenhouse, and not too close to the glass roof or sides of the house as the plants can overheat during hot spells. They do not necessarily need direct sun all day and are best protected from the withering midday and afternoon sun. They do best in a situation under a translucent roof fitted with shade cloth that gives them bright but filtered sunlight all day. Watering should be very sparing and only during the late autumn and early winter months, e.g. no more frequently than once in two weeks, even once every three to four weeks, but when watered, they should be drenched so that the water reaches the bottom of the container. A diluted fertiliser can be added to the water, one with trace elements and a low nitrogen high potassium content is best. It is also best to water in the mornings so that the excess water evaporates and the upper layers of soil dry out fairly quickly. Lithops like nothing less than wet, soggy soil. Although the plant will survive mild frost it should be protected from severe cold and prolonged frost conditions. Plants can be hand pollinated, using a small paint brush. Remember always to cross different clones as the plants are self-sterile. The seed will remain viable for many years provided it is stored in a cool dry place.

Lithops optica seeds germinate readily. It is best sown during the warmer summer months when germination is rapid. Sow the seed evenly in sandy free-draining soil with little or no organic matter in it, and cover with a very thin layer of fine sand. This layer acts to support the young seedling after germination. Because the seed is so fine, it is best to water gently with a very fine rose, or to plunge the seed tray into a tray of water and allow the water to seep upwards to avoid displacing the seed. Place the seedling trays in a warm sunny position, keep them moist but not soggy and add a fungicide with each watering. Humidity can be increased by placing a sheet of fibre-glass or glass over the tray, but raised slightly at one end to allow for some air movement. Germination occurs within 3 weeks and the seedlings develop rapidly. After germination, if you have been watering from below, don't do it anymore. Reduce watering so that the top 6mm layer may dry out but not the soil below. Excessive watering during this period may cause damping off of the seedlings. After they are two or three months old, they can be allowed to dry out completely for a few days between waterings, and the period between waterings gradually increased so as to harden them for adulthood. The young plants can be transplanted after a year, and flowering can be achieved within 3-4 years. Take note that not all the seed will germinate immediately and you may find seedlings appearing months, even a year later.

It is also possible to divide a multiheaded plant, but lithops prefer to grow clustered together and many growers prefer to leave them undisturbed to see how large a cluster can get. To separate a cluster, lift the plant, carefully cut through the rootstock and replant them immediately.

References and suggested reading:

• Hammer, Steven & Barnhill, Chris. Lithops, Treasures of the Veld. British Cactus and Succulent Society, 1999.
• Cole, D. T. 1988. Flowering stones, Acorn Books, Randburg.
• Smith et al. 1998. Mesembs of the World. Briza.Pretoria
• Leistner, O.A. (ed.), 2000, Seed plants of southern Africa: families and genera, Strelitzia 10., National Botanical Institute, Pretoria

Ernst van Jaarsveld
Kirstenbosch National Botanical Garden
March 2002