Source:Burgess and Speirs
San Francisco State University
Geography 316: Biogeography
The Biogeography of Living Stones(Mesembryanthemaceae: Lithops spp.)
Source:Burgess and Speirs
by Shannon Raider, student in Geography 316, Fall 1999
Kingdom: Plantae
Phylum: Angiospermae
Class: Dicotyledonae
Order: Caryophyllidae
Suborder: Caryophylles
Family: Mesembryanthemaceae
Subfamily: Ruschioidene Schwant
Subtribe: Lithopinae Schwant
Genus: Lithops
Description
The Greek lithos means stones and ops means like, translating to the
common name of Living stones. First noticed by William John Burchell in 1811,on a
botanical expedition to S. Africa he went to pick up an interesting stone and found it was
a plant (Thonner 1984). Since their discovery many different names have been given
including “belly plants,” (because you need to crawl on your belly to see them)
(Sprechman 1970) to beeskloutjies ‘little cow hoofs’. In Namibia they have been
called ombuma yombwa or dog testicles (Cole 1988). However, it was in 1922 that Botanist
Dr. N.E. Brown gave them the name Lithops (Cole 1988).
They have a very short stem with two thick, fleshy leaves separated by
a small slit from which a small flower will emerge about once a year. They blend in with
the surrounding soil in nearly perfect stone mimicry regarding size, texture, color and
shape (Turner and Picker 1993). Lithops are unique in that they grow almost completely
imbedded in the soil exposing only a small oval window to the sun. Botanists have
studied whether imbedding is an adaptation to the extreme temperatures and
Lithops julii var. fulleri source:
Burgess and Spiers 1999
dryness of their habitat (Eller and Grobbelaar 1986 and Nobel 1989). However, detailed
research has shown that imbedding did not develop as a means of thermal tolerance (Turner
and Picker 1993), but has more likely developed as a means of mimicry to evade grazing
animals. Imbedding can also be a method to protect from excessive water loss by
transpiration (Eller and Nipkow 1983).
Lithops are commonly polycephalous with some species growing in clumps
of six heads, as in L. lesilei (Hardy 1992) to groups of sixteen as with L. fulleri
(Thonner 1984). They can grow alone or in clusters. The morphology of different
species can be reflected in the soils where they grow in and the dryness of the season.
There is a considerable correlation between individual species and the nature of their
habitat (Cole 1988). For example L. olivacea is more red-brown, green-brown in appearance
in its westerly locations becoming more yellowed in the dry season and more
purplish-red-grey further north. Its hard translucent gloss closely resembles the white
quartz crystalline rubble of its habitat (Thonner 1984). The more widespread a taxa is the
more variation within that taxa (Cole 1988).
Once a year during the rainy season, each head extends a daisy-like
flower lasting for four to five days. These flowers reflect different hues of white and
yellow, smelling quite sweet to attract bees and flies (Hardy 1992). After flowering and
seeding the body withers and breaks open bearing a new set of fleshy leaves (Chidamian
1958). During long stretches without water Lithops enter a dormant stage when no growth
occurs (Pott 1999). Some species of Lithops growing along the Orange River may get their
water from dew or fog. Seeds can remain viable for along time and are dispersed waiting
for the right time to germinate (Ihlenfeldt and Straka 1971).
Habitat
Different species of Lithops are not often found inhabiting the same
area. A more common distribution is to see one species with all its respective
subspecies and varieties colonating a contiguous area (Cole 1988). Some species such as
Lithops weneri have a very limited distribution, only found in one colony while Lithops
lesliei are very widespread, found at more then fifty sites (Cole 1988). Water and soil
most likely limit distribution. Lithops are unlikely to be found on sandy or shifting
dunes or in densely wooded areas. Most species occur in areas with less then 500 mm of
precipitation per year and many receive less then 100mm per year (Cole 1988).
Lithops can be found in a wide variety of habitats from well draining
rocky soil to hard pans, in crevasses, low ridges, mountain summits and slopes (Cole
1988). Different soil habitats include, limestone, sandstone, ironstone, granite,
gneiss, quartz grit, shale, and schist in various mixtures (Sprechman 1970). They are
found at sea level up to altitudes of almost 8,000 feet (Sprechman 1970). They often grow
in open areas away from other plants (Eller and Nipkow, 1983) where the general vegetation
is scrubby and sparse. Lithops can stand temperatures up to 120 degrees Fahrenheit, to
just above freezing in more mountainous area (Eller and Nipkow, 1983).
Different species have developed to survive different extreme
temperature conditions. In experiments L. turbiniformis did not survive about 45 degrees
C. (Eller and Nipkow, 1983) while L. leslei tolerated 56 degrees C. (Nobel, 1989). Both
species inhabit the same areas of the Cape Province, various sites in Orange Free State
and the in Transvaal. Lithops near Windhoek, Namibia can experience –10 Degree
C.
Natural History
Two distinct adaptations to desert life have been the development of
succulence and CAM4 photosynthesis (Turner and Picker 1993). It has been interesting for
botanists to study the anatomical structure of Lithops as an adaptation to its harsh
environment. Within its cone-like structure the cloroplast cells are located in the
inner layers of the epidermis so that sunlight becomes diffused and not so intense
(Sprechman 1970). Light must first pass through many water storage cells before it
can reach the chloroplast cells and because a Lithops is cone shaped, the more directly
overhead the sun is the more the light is diffused and the weaker slanting rays have less
area to penetrate (Sprechman 1970).
All species of Lithops contain tannins in the epidermal layers of the
leaves. It is not fully understood what these tanniniferous idioplasts do. It is
speculated that they inhibit passage of too much light or may absorb ultra-violet light
Lithops dinteri var. brevis in situ source:Burgess
and Spiers 1999
(Sprechman 1970). Taniniferous idioplasts are visible on the face of the plant through
a ‘window’ where light enters (Turner and Picker 1993). This pigmentation
produces Lithops with one of the most attractive aspects of their character; ranging from
soft green with brown marbling, red marbling on gray background, reddish and purple matte,
blue and purple dots and in combination. It is apparent that the presence of these
tannins aids the plants in camouflaging themselves.
It is interesting that these succulents have adapted to survive under
such extreme temperatures and are not able to self-shade or orient away from the sun
(Turner and Picker 1993). Their habitat is not only dry and hot but can experience very
cold nights in winter (Eller and Nipkow 1983). Lithops’ desert environment
experiences sporadic rainfall and periods of drought. Water is the main limiting factor.
Their roots, while enabling the plant to endure long periods of drought will easily rot
with too much water (Pott, 1998).
The taxonomic investigation of this genus is complicated. Since its
primary classification by N.E. Brown, different traits have been included in deciphering
its taxa. Even the most meticulous expert often mixes up, the numerous species of Lithops.
On my windowsill in the bathroom, where I get the most direct light, I am growing
Lithops. They are Lithops lesliei, Lithops dorotheae and I can not quite figure out
the third. Literature often does not have an analytical key to help with identification.
Also characteristics are always being re-evaluated and existing species are re-grouped
while new ones are found (Sprechman 1970).
Currently, and not without continuing debate, Lithops are classified
under the family Mesembryanthemaceae, otherwise known as subterranean dwarf succulents
(Turner and Picker 1993). At least 99 per cent of all Mesembryanthemums are indigenous to
South Africa (Ihlenfeldt and Straka 1971). Some botanists do not consider Mesembryanthemum
to be its own family and include it as a subgroup of the family Aizoaceae (Klak 1997). Dr.
N.E. Brown, Dr. L. Bolus in South Africa and Dr. Schwantes in Germany were the pioneer
botanist who studied Lithops in particular. In the 1920’s and 1930’s several
different species were discovered and documented.
Morphology has played a leading role in identifying different species
of this genus. Flower color has been important in organizing the taxonomy of Lithops. It
is strongly correlated with other reproductive characteristics and breeding barriers
(Thonner 1984). The morphology of the fruit or seed has also been a determining
factor in classifying these succulents. Color, shape of each head, window shapes and size
and density of colonies has been used to differentiate different varieties of Lithops
(Cole 1988). It is difficult for the layman to see exactly what distinguishes one species
from another. Dr. Lance Speirs at Rhodes University in Grahamstown South Africa told me in
an email, “Variability within a species is so great, that us in the field often use
locality info to determine variations, subspecies etc., rather than even looking at
the plant closely.” Lithops currently maintain 37 species in 93 varieties and
forms (Thonner 1984 ).
Evolution
The geologic history of Southern Africa can explain the high number of
endemic species found there. Flora in this southern area include some of the oldest on the
continent (Riley 1963). South Africa experienced its last ice age and glaciation at
the end of the Paleozoic era. This means there has been relatively little disturbance to
the vegetation since this time (Riley 1963). Many of the floras of this area may
well have descended from the Mesozoic era, the era when flowering plants first appeared
(Riley 1963). This area has been isolated by mountainous terrain elevated upon a
continental plateau (Riley 1963). This isolation may have contributed to Lithops
developing distinct morphological characteristics in response to particular microclimates
and soils.
Lithops are a highly developed succulent and belong to a natural group.
This meaning that they are probably of a phylogenically uniform origin (Ihlenfeldt and
Straka 1971). “Generally one can state that transitions between closely related
taxa must have existed at a time; the finding of fossilized remains or even living
descendants of these connecting links must be fortuitous” (Ihlenfeldt and Straka
1971). It remains difficult to tell if certain characteristics of Lithops are
adaptations to the arid environment or indications of close relationships to other genera
in the family (Klak1997).
Lapidaria margaretae source: Pott
1999.
In subdividing Lithops from other members of Mesembryanthemaceae new
taxonomic items have been identified: morphology of fruits, morphology of pollen, nectary
glands, number of chromosomes and distribution area (Ihlenfeldt and Straka 1971). Lithops
may be the culmination of G. schwantesia and being related to G. lapidaria (Thonner 1984).
All are members of the mesembryanthemum family, all compact and succulent. The
former has toothed leaves while the latter has no teeth. The leaves become progressively
more shortened culminating with Lithops, which have no stem and thick fleshy leaves
(Thonner 1984). Also related is G. dinteranthus with leaves that are more keeled and
round (Thonner 1984). All these plants have evolved toward reduction of size to retain
water and limit exposure to the sun (Chidamian 1958).
The speciation of distinctive varieties and subspecies of this genus
appears to be due to divergent evolution due to natural selection. The broader the
distribution of a species, L. lesliei for example, the younger it is on the evolutionary
line because it has not specialized as much as others (Cole 1988). However, there also
appears to be evidence of convergent evolution within the genera. Several species have
developed remarkable similar traits in shape and color yet are not closely related (Cole
1988).
Most research into evolution remains at the level of family and
order. Mesembryanthemums were once part (and are sometimes still included) in the
family Aizoaceae. Mesembryanthemum was one of five subfamilies of Aizoaceae that split
away based upon differences in the number of chromosomes and the structure of petals (Klak
1997). They are closest in relationship to subfamily Ruschioideae. The cellular anatomy
and structure of Lithops has been analyzed without drawing comprehensive correlation
between the different types of species. The anatomical details of L. tubiniformis, L.
helmutii, L. lesliei, and L. aucampiae were presented without discussing any significant
differences among species (Dugdale 1966). Unfortunately I was unable to find a
family tree or cladogram.
Distribution
Growing in the driest desert regions, Lithops are endemic to the arid
and semi-arid environments of Southern Africa in Namibia, Botswana and the Republics of
South Africa (Eller and Nipkow 1983). More specifically in three provinces of South
Africa: Cape, Transvaal and Orange Free State, in the most southern edges of Botswana and
throughout Namibia (Cole 1988). The first species of Lithops were discovered in the
Republic of South Africa where several species have been associated with the Orange River,
a possible source of water. The fog that is typical of the Namibia Desert is also believed
to be a source of water (Spiers and Burgess, 1999). More colonies are believed to grow
further into Botswana.
Different species of Lithops are not often found inhabiting the same
area. A more common distribution is to see one species with all its respective
subspecies and varieties colonating a contiguous area (Cole 1988). Some species such as
Lithops weneri have a very limited distribution, only found in one colony while Lithops
lesliei are very widespread, found at more then fifty sites (Cole 1988). Water and soil
most likely limit distribution. Lithops are unlikely to be found on sandy or shifting
dunes or in densely wooded areas. Most species occur in areas with less then 500 mm of
precipitation per year and many receive less then 100mm per year (Cole 1988).
While the natural distribution remains in southern Africa, they are
raised in Germany, Japan, England and the United States in highly controlled gardens.
Lithops are grown in outdoor gardens in the American Southwest and England (Thonner,
1984). They could become naturalized there if the microclimate is dry enough. Also,
cultivators often only select the most beautiful specimens to grow therefore gardens
cannot represent the total variation that might exist in the wild (Sprechman 1970).
Maps of Distribution:
Other interesting issues
Due to their interesting quality, these Living Stones have become
quite the collector’s item. Cultivated seeds are traded all over the world.
Avid collectors are growing little armies of Lithops in the most controlled environment.
They are relatively easy to cultivate and grow. One quick search on the World Wide Web
will bring you into contact with cultivators from all over the world offering to sell
seeds and growing tips. There is concern among Lithops enthusiasts that their fragile
ecosystem is being destroyed. Intensified grazing, mining, pollution and encroaching
settlements threaten the areas where they grow. By establishing reserves and
educating farmers on the ecology of Lithops botanists aim to prevent further loss of
habitat.
Bibliography
Chidamian, Claude. 1958. The Book of Cacti and Other Succulents. New York City, Doubleday and Co, Inc.
Cole, D.T. 1988. Lithops; Flowering Stones. Johanessburg, South Africa. Acorn Press.
Dugdale, Chester B. 1966. “Some Notes on the
Internal Anatomy of Lithops”. Journal of South African
Botany. 32(4) 305-312.
Eller, Benno M. and Nipkow, Andre. 1983. “(Mesembryanthemaceae Fenzl) and its Surrounding Soil.” Institute of Plant Biology, University of Zurich.
Hardy, David.1992. Succulents of the Transvaal. Cape Town, South Africa. Southern Book Publication.
Heywood, V.H. 1978. Flowering Plants of the World. Boston, Mayflower Books.
Ihlenfeldt, Dr. H.D. and Straka, Dr. H. 1971. “On the Delimination and the Taxonomic Rank of the Mesembryanthemaceae” and “Some Aspects of the Biology of the Mesembryanthemaceae” The Genera of the Mesembryanthemaceae Capetown, S.Africa, Taflberg-Uitgewers Beperk.
Klak, Cornelia. 1997. “The Mesembryanthema: A Brief Synopsis of Their Systematics.” Mesemb Study Group. 12, 46
Lamb, Edgar. 1955. The Illustrated Reference On Cacti and Other Succulents. London, Blandford Press.
Nobel, P.S. 1989. “Shoot Temperatures and Thermal Tolerances for Succulent Species of Hawthoria and Lithops” Plant, Cell, and Environment. 12, 643-651
Pott, K. (12/01/98) Mesembryanthemaceae Home Page and Classification. http://members.tripod.com/~mesembry/html/mesembs.htm
Riley, Herbert Parkes. 1963. Families of Flowering Plants of Southern Africa. Louisville, University of Kentucky Press.
Spiers, Lance and Burgess, Kerri. Lance and Kerri's World of Lithops Home Page 10/5/99 http://www.geocities.com/RainForest/Canopy/6378/index2.html
Sprechman, David L. 1970. Lithops. Cranbury, New Jersey, Associated University Presses, Inc.
Thonner, Fronts. 1984. “Lithops Fulleri var. Fulleri and Lithops Olivacea”. Flowering Plants of Africa 48:1-2, June
Turner, J.S. and Picker, M.D. 1993 “Thermal
Ecology of an Embedded Dwarf Succulent from Southern Africa (Lithops spp:
Mesembryanthemaceae)” Journal of Arid Environments. 24: 361-385.
send comments to bholzman@sfsu.edu
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