| Lithops | |
|---|---|
 | |
| Lithops sp. byRudolf Marloth, 1929 | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Order: | Caryophyllales |
| Family: | Aizoaceae |
| Subfamily: | Ruschioideae |
| Tribe: | Ruschieae |
| Genus: | Lithops N.E.Br.[1] |
| Species | |
See text | |
Lithops is agenus ofsucculent plants in the ice plant family,Aizoaceae. Members of the genus are native tosouthern Africa. They avoid being eaten by herbivores withtheir camouflage as small stones, and are often known aspebble plants orliving stones.
"Lithops" is both the genus name and the common name, and is singular as well as plural. The name is derived from theAncient Greek wordsλίθος (líthos) 'stone' andὄψ (óps) 'face', referring to the stone-like appearance of the plants.
IndividualLithops consist of one or more pairs of bulbous, almost fusedleaves opposite each other and hardly anystem. The slit between the leaves contains themeristem and producesflowers and new leaves. The leaves ofLithops are mostly buried below the surface of thesoil, with a partially or completely translucent top surface known as aleaf window which allows light to enter the interior of the leaves forphotosynthesis.[2]
During winter a new leaf pair, or occasionally more than one, grows inside the existing fused leaf pair. Inspring 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 duringdrought. Yellow or white flowers emerge from the fissure between the leaves after the new leaf pair fully matures, one per leaf pair. This is usually in autumn, but can be before the summersolstice inL. pseudotruncatella and after the wintersolstice inL. optica. The flowers are often sweetly scented.
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The most startlingadaptation ofLithops is the colouring of the leaves. The leaves arefenestrated, and theepidermal windows are patterned in various shades of cream, grey, and brown, with darker windowed areas, dots, and red lines, according to species and local conditions. The markings function as remarkablecamouflage for the plant in its typically stony environment. As is typical of a window plant, the green tissue lines the inside of the leaves and is covered withtranslucent tissue beneath the epidermal windows.
Lithops are obligateoutcrossers and requirepollination from a separate plant. Like mostmesembs, Lithops fruit is a drycapsule that opens when it becomes wet; someseeds 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 occur naturally across wide areas ofNamibia andSouth Africa, as well as small bordering areas inBotswana and possiblyAngola, from sea level to high mountains. Nearly a thousand individual populations are documented, each covering just a small area of dry grassland,veld, or bare rocky ground. DifferentLithops species are preferentially found in particular environments, usually restricted to a particular type of rock.Lithops have notnaturalised outside this region.
Rainfall inLithops habitats ranges from approximately 700 mm/year to near zero. Rainfall patterns range from exclusively summer rain to exclusively winter rain, with a few species relying almost entirely on dew formation for moisture. Temperatures are usually hot in summer and cool to cold in winter, but one species is found right at the coast with very moderate temperatures year round.
Lithops are popularhouse plants and many specialistsucculent growers maintain collections.Seeds and plants are widely available in shops and over theInternet. They are relatively easy to grow and care for if given sufficient sun and kept in well-draining soil.
Normal treatment in mildtemperateclimates is to keep them completely dry during winter, watering only when the old leaves have dried up and are replaced by a new leaf pair. Watering continues through autumn, when the plants flower, and then stops for winter. The best results are obtained in an environment with additional heat such as a greenhouse. In hotterclimates,Lithops will have a summer dormancy when they should be kept mostly dry, and they may require some water in winter. Intropical 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 most 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 bytranspiration and rely on staying buried in cool soil below the surface. Commercial growers mix a mild fungicide or weak strength horticultural sulfur into the plant's water to prevent rotting. Lithops are sensitive to watering during hot weather, which can cause the plants to rot; in habitat the plants are often dormant when the temperatures are high, doing most of their growing during the cool months of the year. Low light levels will make the plants highly susceptible to rotting and fungal infection.[3]
In theUnited Kingdom the following species have gained theRoyal Horticultural Society'sAward of Garden Merit:[4]
Mr Keith Green was appointedInternational Cultivar Registration Authority forLithops in 2013, and recognises over 100 registeredcultivars.[10]
SinceLithops are mostly propagated by seed, cultivars require to be stabilised as seed strains. Most cultivars are either abnormally green or abnormally red plants, lacking most of the normal leaf pigments. Some were initially found as isolated unusual plants in habitat, but increasingly have arisen from cultivation, sometimes by deliberately selecting mildly-coloured plants to achieve intense colours for a cultivar. The term "aberrant colour form" (acf.) has been used for these unusually-colouredLithops.[11]
There are also so-called "pattern cultivars" ofLithops, seed strains which have been selectively-bred or stabilised from isolated unusual plants to have intensified or unusual leaf patterns, and sometimes unusual flowers. In some cases, these arehybrids.[10]
Propagation ofLithops is byseed orcuttings. 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 separateclones of a species flower at the same time, and seed will beripe about 9 months later. Seed is easy togerminate, 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.
At least half of the species listed in theRed List of South African Plants are classified as endangered or threatened for various reasons, including poaching for the succulent horticultural trade, habitat degradation, and decreased or restricted range due to urban and agriculture expansion.[12]
| Species | Conservation status |
|---|---|
| Lithops aucampiae | VU |
| Lithops coleorum | VU |
| Lithops dinteri | VU |
| Lithops divergens | NT |
| Lithops dorotheae | EN |
| Lithops geyeri | Rare |
| Lithops helmutii | VU |
| Lithops herrei | VU |
| Lithops lesliei subsp. burchellii | NT |
| Lithops lesliei subsp. lesliei | VU |
| Lithops meyeri | VU |
| Lithops naureeniae | VU |
| Lithops olivacea | VU |
| Lithops otzeniana | VU |
| Lithops salicola | NT |
| Lithops viridis | VU |
The first scientific description ofLithops was made bybotanist and artistWilliam John Burchell, explorer ofSouth Africa, although he called itMesembryanthemum turbiniforme. In 1811, Burchell discovered a specimen when picking up a "curiously shaped pebble" from the ground.[11] Unfortunately the documented physical description was not detailed enough to be sure whichLithops he had discovered and the nameLithops turbiniformis is no longer used, although for many years it was applied to what is now known asLithops hookeri.
Several moreLithops were published asMesembryanthemumspecies until in 1922N E Brown started to split up the overly large genus on the basis of thecapsules. ThegenusLithops was created and dozens more species were published in the following decades. Brown,Gustav Schwantes,Kurt Dinter,Gert Nel, and Louisa Bolus continued to documentLithops 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 remained rather unknown in cultivation and was not well understoodtaxonomically.
In the 1950s, Desmond and Naureen Cole began to studyLithops. Together, the couple visited nearly all natural habitats of the different lithops populations and collected samples from approximately 400. They document and identify them, assigning a number, which is now known as the Cole number still used today all around the world. They studied and revised the genus, in 1988 publishing a definitive book(Lithops: Flowering Stones) describing thespecies,subspecies, andvarieties which have been accepted ever since.
Because their camouflage is so effective, new species continue to be discovered. Recent discoveries includeL. coleorum in 1994,L. hermetica in 2000, andL. amicorum in 2006.[13]
Many of the species listed have namedsubspecies orvarieties and some have many regionalforms identified by old names or habitat locations. Identification of species is primarily by flower colour and leaf patterns. The species list here follows Cole & Cole (2006).[11]
| Specific epithet | Meaning |
|---|---|
| amicorum[15] | of the friends |
| aucampiae | named after Juanita Aucamp |
| bromfieldii | named after H. Bromfield |
| coleorum | named after Desmond & Naureen Cole |
| comptonii | named after Prof.Robert Harold Compton |
| dinteri | named afterMoritz Kurt Dinter |
| divergens | divergent lobes |
| dorotheae | named after Dorothea Huyssteen |
| francisci | named after Frantz de Laet |
| fulviceps | tawny head |
| gesineae | named after Gesine de Boer |
| geyeri | named after Albertus Geyer |
| gracilidelineata | thin lined |
| hallii | named afterHarry Hall |
| helmutii | named after Helmut Meyer |
| hermetica | named after the 'hermetically sealed' location,Sperrgebiet |
| herrei | named after Adolar 'Hans' Herre |
| hookeri | named afterSir Joseph Hooker |
| julii | named after Julius Derenberg |
| karasmontana | named after theGreat Karas Mountains |
| lesliei | named after T. N. Leslie |
| localis | of a place |
| marmorata | marbled |
| meyeri | named after Rev. Gottlieb Meyer |
| naureeniae | named after Naureen Cole |
| olivacea | olive-green |
| optica | eye-like |
| otzeniana | named after M. Otzen |
| pseudotruncatella | had been confused withConophytum truncatum |
| ruschiorum | named after Rusch family |
| salicola | salt-dweller |
| schwantesii | named afterGustav Schwantes |
| vallis-mariae | named after the locationMariental (Latinised) |
| verruculosa | warty |
| villetii | named after C. T. Villet |
| viridis | green |
| werneri | named after Werner Triebner |
Although the species, subspecies, and varieties published by Cole & Cole[11] largely remain accepted and in widespread use,[14] some variations have been published by other authors. Some published changes since 2006 include:
One study ofnon-codingchloroplast DNA (trnS-trnG intergenic spacer), nuclear ribosomalinternal transcribed spacer (nrITS) sequences andAFLP data found that Lithops was notmonophyletic sinceDinteranthus,Schwantesia, andLapidaria were involved. It identified 9 clades which did not closely frame the accepted 37 species.[19]
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