Living Stones (Lithops spp.)

Lithops -  Living Stones - Lithops spp.

Lithops is a genus of succulent plants in the ice plant family, Aizoaceae. Members of the genus are native to southern Africa. The name is derived from the Ancient Greek words λίθος (lithos), meaning "stone," and οψ (ops), meaning "face," referring to the stone-like appearance of the plants. They avoid being eaten by blending in with surrounding rocks and are often known as pebble plants or living stones.

Although it is hard to imagine, the body of a lithops is in fact a pair of leaves that have evolved to efficiently retain whatever moisture becomes available to the plant. In some areas of their habitat rainfall may not occur for many months so conservation of water is of prime importance.

To minimise any evapouration the leaves have become so truncated that they have lost the appearance of a normal leaf and have become rounded like a pebble.

This has helped to make them less obvious to foraging animals so the plants that best mimic the colour and appearance of the soil and stones of their surroundings stand the best chance of survival. This in turn has led to the evolution of different species and varieties.

Individual Lithops plants consist of one or more pairs of bulbous, almost fused leaves opposite to each other and hardly any stem. The slit between the leaves contains the meristem and produces flowers and new leaves. The leaves of Lithops are mostly buried below the surface of the soil, with a partially or completely translucent top surface or window allowing light to enter the interior of the leaves for photosynthesis.

During winter a new leaf pair, or occasionally more than one, grows inside the existing fused leaf pair. In spring the old leaf pair parts to reveal the new leaves and the old leaves will then dry up. Lithops leaves may shrink and disappear below ground level during drought. Lithops in habitat almost never have more than one leaf pair per head, the environment is just too arid to support this. Yellow or white flowers emerge from the fissure between the leaves after the new leaf pair has fully matured, one per leaf pair. This is usually in autumn, but can be before the summer equinox in L. pseudotruncatella and after the winter equinox in L. optica. The flowers are often sweetly scented.

The most startling adaptation of Lithops is the colouring of the leaves. The leaves are not green as in almost all higher plants, but various shades of cream, grey, and brown, patterned with darker windowed areas, dots, and red lines. The markings on the top surface disguise the plant in its surroundings.

Lithops are obligate outcrossers and require pollination from a separate plant. Like most mesembs, Lithops fruit is a dry capsule that opens when it becomes wet; some seeds may be ejected by falling raindrops, and the capsule re-closes when it dries out. Capsules may also sometimes detach and be distributed intact, or may disintegrate after several years.

Lithops are popular novelty house plants and many specialist succulent growers maintain collections. Seeds and plants are widely available in shops and over the Internet. They are relatively easy to grow if given sufficient sun and a suitable well drained-soil.

Normal treatment in mild temperate climates is to keep them completely dry during winter, watering only when the old leaves have dried up and been replaced by a new leaf pair. Watering continues through autumn when the plants flower and then stopped for winter. The best results are obtained with additional heat such as a greenhouse. In hotter climates Lithops will have a summer dormancy when they should be kept mostly dry, and they may require some water in winter. In tropical climates, Lithops can be grown primarily in winter with a long summer dormancy. In all conditions, Lithops will be most active and need most water during autumn and each species will flower at approximately the same time.

Lithops thrive best in a coarse, well-drained substrate. Any soil that retains too much water will cause the plants to burst their skins as they over-expand. Plants grown in strong light will develop hard strongly coloured skins which are resistant to damage and rot, although persistent overwatering will still be fatal. Excessive heat will kill potted plants as they cannot cool themselves by transpiration and rely on staying buried in cool soil below the surface.

Propagation of Lithops is by seed or cuttings. Cuttings can only be used to produce new plants after a plant has naturally divided to form multiple heads, so most propagation is by seed. Lithops can readily be pollinated by hand if two separate clones of a species flower at the same time, and seed will be ripe about 9 months later. Seed is easy to germinate, but the seedlings are small and vulnerable for the first year or two, and will not flower until at least two or three years old.

The first scientific description of a Lithops was made by William John Burchell, explorer of South Africa, botanist and artist, although he called it Mesembryanthemum turbiniforme. In 1811 he accidentally found a specimen when picking up from the ground a "curiously shaped pebble". Unfortunately his description isn't detailed enough to be sure which Lithops he had discovered and the name Lithops turbiniformis is no longer used, although for many years it was applied to what is now known as Lithops hookeri.

Several more Lithops were published as Mesembryanthemum species until in 1922 N E Brown (Nicholas Edward Brown) started to split up the overly large genus on the basis of the capsules. The genus Lithops was created and dozens more species were published in the following decades. Brown, Gustav Schwantes, Kurt Dinter, Gert Nel, and Louisa Bolus continued to document Lithops from across southern Africa, but there was little consensus on the relationships between them, or even which populations should be grouped as species. As recently as the 1950s, the genus was little known in cultivation and not well understood taxonomically.

In the 1950s, Desmond and Naureen Cole began to study Lithops. They eventually visited nearly all habitat populations and collected samples from approximately 400, identifying them with the Cole numbers which have been used ever since and distributing Cole numbered seed around the world. They studied and revised the genus, in 1988 publishing a definitive book (Lithops: Flowering Stones) describing the species, subspecies, and varieties which have been accepted ever since.

New species continue to be discovered steadily in remote regions of Namibia and South Africa, most recently L. coleorum in 1994, L. hermetica in 2000, and L. amicorum in 2006.

After flowering in the fall and extending through winter, when the new 'bodies' are forming within the old leaves, the latter become soft and flaccid and begin to shrivel. Some may split on the sides from the pressure of the new body inside, and often there will be dry or 'dead' spots on the old leaves at this stage. This is perfectly normal. Eventually the old leaves dry up, leaving the plant with a perfect set of new ones.

Source:
http://en.wikipedia.org/wiki/Lithops 
http://www.lithop.supanet.com/