What phytolith are

Biomineralization is a common process in nature (in both the plant and animal kingdoms) and four major minerals can be precipitated: calcite, aragonite, apatite and opal. In addition, there is a wide variety of other less common biominerals, including calcium, strontium and iron oxides as well as hydroxides, fluorides, sulphates, and oxalates. In plants, biomineralization mainly serves as a structural aid and/or defensive resource (e.g. silicified or calcified tissues act as deterrent for grazers). Calcification takes place in marine and freshwater macroalgae, coccolithophorids, and perhaps bacteria and fungi, while silification occurs in diatoms and some flowering plants.

Silicon (Si) is ubiquitous in nature and the soil content in this element can vary between less than 1% to 45% of dry weight (Somner et al 2006). Biogenic opal or amorphous opaline silica is essentially hydrated silica (SiO2•nH2O). Diatoms deposit opal silica in the cell walls to produce the so-called frustule. Higher plants absorbe biogenic silica as silicic acid Si(OH)4 and this is moved all over the plant by the transport system but generally deposition happen in the organs with highest evapo-transpiration (e.g. leaves) due to loss of water and concentration of the solutes (Chave 1984; Currie & Perry 2007).

Bibliography

• Chave, K. E. 1984. Physics and chemistry of biomineralization. Annals Review of Earth Planet, 12:293-305
• Currie, H. A. and Perry, C. C. 2007. Silica in Plants: Biological, Biochemical and Chemical Studies. Annals of Botany 100:1383-1389.

Calcium biomineralizations

The most common form of biomineralization in higher plants, however, is calcium oxalate as calcium is acquired from the soil solution by the root system (White and Broadley 2003). However, due to its high solubility once deposited and the low taxonomical specificity, it is rarely used in archaeobotanical studies. Calcium oxalate (CaC2O4.nH2O) is precipitated in crystalline forms within the cell lumen (White and Broadley 2003). Calcium oxalate crystals may form in any organ or tissue within the plant and it is known from roots, stems, leaves, flowers, fruits, and seeds and within epidermal, ground, and vascular tissues (Prychid and Rudall 1999). Calcium oxalate often forms in idioblasts, cells that develop in isolation with structure or content distinct from surrounding cells. It can take different crystalline forms...

Calcium oxalates will not be further discussed in this tutorial and for a general overview on this biomineralization see Mulholland & Rapp (1992) work.

Bibliography

• Mulholland, S. C. and Rapp, G. Jr. 1992. Phytolith systematics: an introduction. In: Mulholland, S. C. and Rapp, G. Jr. Phytolith systematics: emerging issues, Advances in Archaeological and Museum Science vol. 1, Plenum Press, New York, pp. 1-13.
• Prychid, C. J. and Rudall, P. J. 1999. Calcium Oxalate Crystals in Monocotyledons: A Review of their Structure and Systematics. Annals of Botany 84:725-739.
• White, P. J. and Broadley, M. R. 2003. Calcium in plants. Annals of Botany 92:487-511.

Production of phytoliths in higher plants

Different levels of silicification in the leaf epidermal cells of a grass (Poaceae). 1) highly silicified bilobate short cells and 2) less silicified long cells (Photo M. Madella).

Monosilicic acid Si(OH)4, which is present in soils as a result of the weathering of silica minerals (e.g. feldspar or quartz), is absorbed through the roots and taken in the xylem to the aerial organs of the plant (Raven 1983) where it is deposited as porous opal-A SiO2·nH2O (Mann et al. 1983). Silicon accumulation is greater (but not exclusive) in monocotyledonous plants and Equisetaceae (horsetails), Poaceae (grasses), Cyperaceae (sedges) show high deposition of silicon (more than 4% Si in weight). Plants such as Cucurbitales (pumpkins and squashes), Urticales (nettle) and Commelinaceae (spiderwort family) have an intermediate deposition of silicon (between 2% and 4%). Hodson et al (2005) have observed that Si concentration in plant shoots (which can be taken as a proxy of Si deposition) decline in the following order:

liverworts > horsetail > clubmosses > mosses > angiosperms > gymnosperms > ferns

These variables are all important in the process of phytolith production.

The silica polymerisation can occur through one or more of the following processes (Curry & Perry 2007; Kaufman et al. 1985; Perry 2003):

Most of the mechanisms of opal deposition are genetically determined (Hodson et al., 2005). Some cells are devoted to active accumulation of silica, an example of these are the silica-cells coupled with the cork cells in grass leaf epidermis or the so-called short cells (dumbbell, saddle, cross phytolith types). The hairs silicification is another example in which deposition of silica happens regardless of external conditions, as this is connected with the specific function of the cells for improving pest resistance (Richmond and Sussman 2003). Recently, the expression of some genes and phytolith production have been directly related. For instance, the deposition of opal silica in the fruit rinds of Cucurbita has been linked to the hard rind (Hr) genetic locus (Piperno et al. 2002) while locus tga1 has been shown to control deposition of silica in teosinte glumes (Dorweiler and Doebley 1997).

Environmentally controlled silicification happens in those cells that do not have a genetic control on silica deposition in their lumen. These cells are not primarily associated with opal silica deposition (atypical deposition, according to Blackman and Parry 1968) and the mechanism of deposition is related to water flow and excess of monosilicic acid in the plant (Richmond and Sussman 2003). An example of these type of cells are the grass epidermal long cells (Piperno 2007 but also see Madella et al. 2009 for experimental data on short vs. long cell production in controlled environments). A recent article by Tsartsidou et al. (2007) comparing two, albeit small, phytolith reference collections from Israel and Greece clearly shows that there are significant differences in phytolith production for most of the studied grasses (see Tsartsidou et al. 2007). Differences in phytolith production have been also noted by the same authors when comparing the tree leaves from Greece with those from the north central region of the USA or the same plants from East Africa and Israel (Tsartsidou et al. 2007).

Function of phytoltihs within plant organism

There are at least the following known reasons for why phytoliths are deposited in the plant cells. However, more studies in silicon physiology are needed to clearly understand absorption and deposition of silicon in the plant tissues.

• Maintaining turgidity to avoid cell wall collapse (Currie & Perry 2007)
• Protection against toxic elements (e.g. aluminum; Carnelli et al 2002)
• Protection against herbivores or fungi attacks (Hunt et al 2008; Piperno 1988)

Bibliography

• Blackman, E. and Parry, D.W. 1968. Opaline silica deposition in rye (Secale cereale L.). Annals of Botany 32:199–206
• Carnelli, A. L., Madella, M., Theurillat, J-P. and Amman, B. 2002. Aluminum in the opal silica reticule of phytoliths: a new tool in palaeoecological studies. American Journal of Botany 89:346–351.
• Currie, H. A. and Perry, C. C. 2007. Silica in Plants: Biological, Biochemical and Chemical Studies. Annals of Botany 100:1383-1389.
• Dorweiler, J.E. and J. Doebley. 1997. Developmental analysis of teosinte glume architecture1: A key locus in the evolution of maize (Poaceae). American Journal Botany 84:1313-1322.
• Hodson, M. J., White, P. J., Mead, A. and Broadley, M. R. 2005. Phylogenetic Variation in the Silicon Composition of Plants. Annals of Botany 96:1027-1046.
• Hunt, J. W. Dean, A. P., Webster, R. E., Johnson, G. N. and Ennos, A. R. 2008. A Novel Mechanism by which Silica Defends Grasses Against Herbivory. Annals of Botany 102:653-656.
• Kaufman, P.B., Dayanandan, P., Franklin, C.I. and Takeoka, Y. 1985. Structure and function of silica bodies in the epidermis system of grass shoots, Annals of Botany, 55:487-507.
• Madella, M., Jones, M. K., Echlin, P., Powers-Jones, A. and Moore, M. 2009. Plant water availability and analytical microscopy of phytoliths: Implications for ancient irrigation in arid zones. Quaternary International 193:32-40.
• Perry, C. C. 2003. Silicification: The Processes by Which Organisms Capture and Mineralize Silica. Reviews in Mineralogy and Geochemistry 54:291-327.
• Piperno, D. 1988. Phytolith analysis: An archaeological and geological perspective. 280 pp. New York: Academic Press.
• Piperno, D. 2006. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. 238 pp. Lanham, New York, Toronto, Oxford: AltaMira Press (Rowman & Littlefield).
• Piperno, D. R., Holst, I., Wessel-Beaver, L. and Andre, T. A. 2002. Evidence for the control of phytolith formation in Cucurbita fruits by the hard rind (Hr) genetic locus: Archaeological and ecological implications. PNAS 6, vol. 99, no. 16:10923-10928.
• Richmond, K.E. and Sussman, M.R. 2003. Got Silicon? The non-essential, beneficial plant nutrient. Current Opinions in Plant Biology 6:268-272.
• Tsartsidou, G. Lev-Yadun, S., Albert, R.-M., Miller-Rosen, A., Efstratiou, N. and Weiner S. 2007. The phytolith archaeological record: strengths and weaknesses evaluated based on a quantitative modern reference collection from Greece. Journal of Archaeological Science 34:1262–1275.

Patterns of phytolith production in higher plants

Patterns of phytolith production within the plant kingdom are very different. In general, Bryophyta, Lycopsida, Equisetopsida (Pteridophyta) show high levels of silica accumulation, while others like Filicopsida (Pteridophyta), Gymnospermae, and Angiospermae show lower silica accumulation (see Piperno 2006 for an extensive review). Poaceae (grasses) produce higher amounts of phytoliths than arboreal species (Albert and Weiner, 2001), although there are some groups, such as Magnoliaceae, Ulmaceae, Moraceae and others that show amounts of silica deposition comparable to grasses (Kondo, 1977). This knowledge on patterns of silicification in the Plant Kingdom is somehow biased on the basis of research interest. For instance, the interest in cereals domestication processes and early agriculture made the Poaceae one of the best studied families for phytoliths.

On the basis of phytolith morphology, it is possible to identify plant taxonomic groups at different levels (e.g. species, genera, family, etc.). It is also possible to detect vegetation and plant physiological adaptations:

• Arboreal vs. grass vegetation (steppe, prairies, grasslands, etc.)
• C3 (Festucoideae) vs. C4 (Chloridoideae and Panicoideae) grasses (Twiss 1992)

Bibliography

• Albert, R. M. and Weiner, S. 2001. Study of phytoliths in prehistoric ash layers using a quantitative approach. In: J.D. Meunier and F. Coline (Eds.) Phytoliths: Applications in Earth Sciences and Human History, A.A. Balkema Publishers, Netherlands, pp. 251–266.
• Kondo, R. 1977. Opal phytoliths, inorganic biogenic particles in plants and soils. Japan Agricultural Research Quarterly 11:198-203.
• Piperno, D 2007. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. 238 pp. Lanham, New York, Toronto, Oxford: AltaMira Press (Rowman & Littlefield).
• Twiss, P. C. 1992. Predicted world distribution of C3 and C4 grass phytoliths. In: Mulholland, S. C. and Rapp, G. Jr. (Eds.) Phytolith systematics: emerging issues, Advances in Archaeological ann Museum Science vol. 1, Plenum Press, New York, pp. 113-128.

A very short history of phytolith studies

The term phytolith for defining microscopic opaline bodies deposited in plants initially appeared in a paper by Ruprecht (cited in Baker 1959a, b) but the first observation of mineral particles from plants goes back to 1675 when Loeuwenhoek reported calcium phytoliths (Arnott 1976, cited in Mulholland and Rapp 1992).

Piperno (1998) divides the history of phytolith studies into four periods:

• Discovery and exploration period: 1835 – 1900

Silica phytoliths were first examined by a German scholar, Struve, who produced in 1835 a dissertation at the University of Berlin on silica in plants (cited in Powers 1992), therefore phytoliths were officially discovered one year before pollen was. Ehrenberg, a decade later, described 34 phytolitharia, as part of infusoria (microscopic plant and animal life), from the dust collected by Charles Darwin on the deck of the Beagle (Darwin 1846). He considered this silicified plant material as a vegetal appendix to other microscopic life such as Foraminifera, Protozoa and Coelenterata (Powers 1992).

• Botanical research period 1895 – 1936

In the second half of the 19th century and first half of the 20th century a group of botanists from Germany produced a series of paper that investigated silica bodies in cereals and other species of grass. The works of Hohnel (1875, quoted in Grob 1896), Guntz (1886, cited in Grob 1896) and Grob (1896), in phytolith anatomy and morphology are fundamental for many later studies.

• Phytolith research in ecology

The Welsh investigation of phytoliths started in Bangor after the World War II and continued for some thirty years providing, together with the works of the “German School”, a solid base of information for the modern research in phytolith analysis.

• Modern period:

In the 50’s and 60’s phytoltih analysis is again used in archaeology with Helbaek (1961, 1969) working on ashes and and ceramics from the Near East and Watanabe (1955, 1968, 1970) identifying rice phytoliths in prehistoric deposits from Japan. A turning point was the publication by Rovner (1971) and Piperno (1998) of two seminal works that greatly helped to advertise the study of phytoliths in archaeology and palaeoecology.

Today phytoliths are routinely investigated in Archaeology, Palaeoenvironment and Palaeoclimate studies, Palaeontology and Botany (both in physiology and taxonomy).

Bibliography

For a comprehensive review of the history of phytolith studies:

• Madella, M. 2008. The “stones from plants”: A review of phytolith studies and classification in Europe, Asia and North America. In A. Zucol, M. Osterrieth and M. Brea (Eds.) Fitolitos: Estado Actual del Conocimiento en America del Sur. Editorial Universidad Nacional de Mar del Plata (updated to year 2000).
• Piperno, D 2007. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. 238 pp. Lanham, New York, Toronto, Oxford: AltaMira Press (Rowman & Littlefield).
• Powers, A.H. 1992. Great expectations: a short historical review of European phytolith systematics, In G. Rapp, Jr. And S.C. Mulholland (eds.) Phytolith Systematics Emerging Issues, Advances in Archaeology and Museum Science, vol. 1, Plenum Press, New York, 15-35.

Papers cited above:

• Baker, G. 1959a. Opal phytoliths in some Victorian soils and “Red Rain” residues, Australian Journal of Botany, 7:64:87.
• Baker, G. 1959b. Fossil opal phytoliths and phytolith nomenclature, Australian Journal of Science, 21:305-306.
• Darwin, C. 1846. An account of the fine dust which often falls on vessels in the Atlantic Ocean, Quarterly Journal of the Geological Society of London, 2:26-30.
• Grob, A. 1896. Beitrage zur Anatomie der Epidermis der Gramineenblatter, Bibliotheca Botanica, 36:1-63.
• Helbaek, H. 1961 Studying the diet of ancient man. Archaeology 14: 95–101.
• Helbaek, H. 1969. Plant-collecting, dry-farming and irrigation agriculture in prehistoric Deh Luran. In: Prehistory and human ecology of the Deh Luran plain. An early village sequence from Khuzistan, Iran.—Hole F, Flannery KV, Neely JA, (eds.) University of Michigan: Memoirs of the Museum of Anthropology. 383–426.
• Mulholland, A.C. and Rapp, G. Jr 1992. Phytolith systematics: an introduction, In G. Rapp, Jr. And S.C. Mulholland (eds.) Phytolith Systematics Emerging Issues, Advances in Archaeology and Museum Science, vol. 1, Plenum Press, New York, 15-35.
• Piperno, D.R. 1988. Phytolith analysis, an archaeological and geological perspective, Academic Press, New York.
• Powers, A.H. 1992. Great expectations: a short historical review of European phytolith systematics, In G. Rapp, Jr. And S.C. Mulholland (eds.) Phytolith Systematics Emerging Issues, Advances in Archaeology and Museum Science, vol. 1, Plenum Press, New York, 15-35.
• Rovner, I. 1971. Potential of opal phytoliths for use in paleoecological reconstruction. Quaternary Research 1: 345-359.
• Watanabe, N. 1955. Ash in archaeological sites, Rengo-Taikai Kiji, Proceedings of the Joint Meeting of the Anthropological Society of Nippon and Japan Society of Ethnology, 9:169-171 (in Japanese).
• Watanabe, N. 1968. Spodographic evidence of rice from prehistoric Japan, Journal of the Faculty of Science of the University of Tokyo, 3(3):217-234.
• Watanabe, N. 1970. A spodographic analysis of millet from prehistoric Japan, Journal of the Faculty of Science of the University of Tokyo, 3(5):357-359.

International venues in phytolith studies

Since 1996 researchers from around the world have met at two major international gatherings: the International Meetings on Phytolith Research (IMPR) and the Meetings on Phytolith Research in the Conosur - South America (Encuentros de Investigaciones Fitolíticas). These meetings also produce some of the most up-to-date phytolith research in special proceedings volumes. The following is a list of the past meetings.

International Meetings on Phytolith Research (IMPR) - Under the ages of the Society for Phytolith Research

• 1st IMPR - 1996 Center for Environmental Sciences, CSIC - Spanish National Research Council, Madrid (Spain).

Proceedings: Pinilla, J. Juan-Tresserras & M.J. Machado eds. (1997), The State-of-the-art of Phytoliths in Soils and Plants. Monografia 4 del Centro de Ciencias Medioambientales, CISC. Madrid.

• 2nd IMPR - 1998 CEREGE - Centre Européen de Recheerches et d’Etudes de Géosciences de l’Environnement, Aix en Provence (France).

Proceedings: Meunier, Jean Dominique / Fabrice Colin (eds). (2001), Phytoliths - Applications in earth science and human history. Balkema Press.

• 3rd IMPR - 2000 Museum of Central Africa, Bruxelles (Belgium)

Proceedings: not yet published.

• 4th IMPR - 2002 Department of Archaeology, University of Cambridge (UK).

Proceedings: M. Madella & D. Zurro (2007). Plants, People and Places: Recent Studies in Phytolith Analysis. Oxbow Books, Oxford.

• 5th IMPR - 2004 Institute of Geography, RAN - Russian Academy of Sciences, Moscow (Russia).

Proceedings: not yet published.

• 6th IMPR - 2006 ICREA (Institució Catalana de Recerca i Estudis Avançats), Universidad de Barcelona, CSIC (Spanish National Research Council), Barcelona (Spain).

Proceedings: Special volume of Quaternary International edited by R.M. Albert and M. Madella (2009).

• 7th IMPR - 2008 held in conjunction with the 4th South American Meeting on Phytolith Research (EIF) at Centro de Geologia de Costas y del Cuaternario, Universidad de Mar del Plata, Argentina.

Proceedings: to be published in 2010.

Meetings on Phytolith Research of the GEFACS - Grupo de Estudios Fitolíticos Aplicados del Cono Sur

• 1st South American Meeting on Phytolith Research - 1999 "J. Frenguelli" Diamante, Argentina.
• 2nd South American Meeting on Phytolith Research - 2001 Mar del Plata, Argentina.
• 3rd South American Meeting on Phytolith Research - 2005 Tucumán, Argentina.
• 4th South American Meeting on Phytolith Research - 2008 Mar del Plata, Argentina.