The Lithops Genus

Many species of plants and animals have evolved striking visual resemblances to inanimate objects found in the same locality. Examples can be found in a diverse array of taxa (Skelhorn, Rowland, and Ruxton, 2010). For example, plants from the genus Lithops, also called living stone, flowering stone, or stoneface,  are named so because they visually resemble stones. These flowering stones can be any of a group of about 40 species of succulent plants of the carpetweed family and are native to southern Africa. The plants are virtually stemless, with two leaves growing during each rainy season forming a fleshy, roundish structure that is slit across the top (Encyclopaedia Britannica, 2015).

Structure of Lithops plant from "The Morphology of Lithops"

The succulent leaves are hypothesized to have three evolutionary purposes. First, because the leaves are sunken into the ground and lack the ability for stomatal transpiration, they are thought to serve as protection against evaporation (Eller and Ruess, 1982). The succulent leaves also effectively serve as a light transmitter to photosynthetic tissue below ground (Bennett et al., 1988). These adaptations have allowed the Lithops plants to thrive in very dry, arid environments and radiated and found small niches throughout southern Africa, promoting rapid speciation (Kellner et al., 2011). Lastly, but certainly not the least of all evolutionary adaptions, is the remarkable specialisation of vascular tissues to mimic local soil types. This adaptation has been found to create a camouflage for protection against herbivory (Kellner et al., 2011). It is quite amazing that Lithops can be found on every soil formation, ranging from granite to sandstone or limestone.

Collection of Lithops courtesy of Index of Aizoaceae

Little is known about the genetic relationships within the Lithops genus at present. As of now, its taxonomy is completely based on morphology. Kellner and his colleagues created one of the first phylogenetic trees in 2011, showing that the many species within Lithops are possibly a result of parapatric and allopatric speciation. More research is needed in order to determine the evolutionary relationships between the species in the Lithops genus, but it is clear that because of the large morphological variability due to geographic distribution, morphological trees are inaccurate. 

References

Bennett, B., Brito, C., Calvert, B., Cooper, J., Dennis, N., Holman, W., Patmore, J., and Stiver, J., 1988, British Cactus & Succulent Journal, 6 (2), pp. 44-45. Retrieved 07, April, 2015, from http://www.jstor.org/stable/42794129

Eller, B. M., and Ruess, B., 1982, Physiologia Plantarum, 55, pp. 329-334. Retrieved 07, April, 2015, from http://onlinelibrary.wiley.com/doi/10.1111/

Encyclopaedia Britannica, 2015, Encyclopaedia Britannica Online, lithops. Retrieved 07, April, 2105, from http://www.britannica.com/EBchecked/topic/343776/lithops

Kellner, A., Ritz, C. M., Schlittenhardt, P., and Hellwig, F. H., 1982, Plant Biology, 13, pp. 368-380. Retrieved 07, April, 2015, from http://onlinelibrary.wiley.com/doi/10.1111/

Skelhorn, J., Rowland, H. M., and Ruxton, G. D., 2010, Biological Journal of the Linnean Society, 99, pp. 1-8. Retrieved 07, April, 2015, from http://onlinelibrary.wiley.com/doi/10.1111/

Photos

Structure of Lithops retrieved 07, April, 2015 from http://www.floweringstones.co.za/morphology/morphology.html

Collection of Lithops retrieved 07, April, 2015 from http://www.flowershots.net/web-content/Aizoaceae/lithops2_.jpg