Phytoliths (from Greek, "plant stone") are rigid, microscopic mineral deposits found in some plant tissues, often persisting after the decay of the plant. Although some use "phytolith" to refer to all mineral secretions by plants, it more commonly refers to siliceous plant remains. Phytoliths come in varying shapes and sizes. The plants which exhibit them take up dissolved silica from the groundwater, whereupon it is deposited within different intracellular and extracellular structures of the plant.Piperno, Dolores R. (2006). Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. AltaMira Press .
The silica is absorbed in the form of monosilicic acid (Si(OH)4), and is carried by the plant's vascular system to the cell walls, cell lumen, and intercellular spaces. Depending on the plant taxa and soil condition, absorbed silica can range from 0.1% to 10% of the plant's total dry weight. When deposited, the silica replicates the structure of the cells, providing structural support to the plant. Phytoliths strengthen the plant against abiotic stressors such as salt runoff, metal toxicity, and extreme temperatures. Phytoliths can also protect the plant against biotic threats such as insects and fungal diseases.Rajendiran et al., (2012).Role of phytolith occluded carbon of crop plants for enhancing soil carbon sequestration in agro-ecosystems. Current Science, 103(8), 911-920.
Functions There is still debate in the scientific community as to why plants form phytoliths, and whether silica should be considered an essential nutrient for plants. Studies that have grown plants in silica-free environments have typically found that plants lacking silica in the environment do not grow well. For example, the stems of certain plants will collapse when grown in soil lacking silica. In many cases, phytoliths appear to lend structure and support to the plant, much like the spicules in sponges and leather corals. Phytoliths may also provide plants with protection. These rigid silica structures help to make plants more difficult to consume and digest, lending the plant's tissues a grainy or prickly texture. Phytoliths also appear to provide physiologic benefits. Experimental studies have shown that the silicon dioxide in phytoliths may help to alleviate the damaging effects of toxic heavy metals, such as aluminum. Finally, calcium oxalates serve as a reserve of carbon dioxide in Alarm photosynthesis. Cacti use these as a reserve for photosynthesis during the day when they close their pores to avoid water loss; baobabs use this property to make their trunks more flame-resistant.
History of phytolith research According to Dolores Piperno, an expert in the field of phytolith analysis, there have been four important stages of phytolith research throughout history.Phytoliths Gallery . Smithsonian National Museum of Natural History. Discovery and exploratory stage (1835–1895): The first report on phytoliths was published by a German botanist named in 1835. During this time another German scientist named Christian Gottfried Ehrenberg was one of the leaders in the field of phytolith analysis. He developed the first classification system for phytoliths, and analyzed soil samples that were sent to him from all around the world. Most notably, Ehrenberg recorded phytoliths in samples he received from the famous naturalist, Charles Darwin, who had collected the dust from the sails of his ship, HMS Beagle, off the coast of the Cape Verde Islands. Botanical phase of research (1895–1936): Phytolith structures in plants gained wide recognition and attention throughout Europe. Research on production, taxonomy and morphology exploded. Detailed notes and drawings on plant families that produce silica structures and morphology within families were published. Period of ecological research (1955–1975): First applications of phytolith analysis to paleoecological work, mostly in Australia, the United States, the United Kingdom, and Russia. Classification systems for differentiation within plant families became popular. Modern period of archaeological and paleoenvironmental research (1978–present): Archaeobotanists working in the Americas first consider and analyze phytolith assemblages in order to track prehistoric plant use and domestication. Also for the first time, phytolith data from pottery are used to track history of clay procurement and pottery manufacture. Around the same time, phytolith data are also used as a means of vegetation reconstruction among paleoecologists. A much larger reference collection on phytolith morphology within varying plant families is assembled.
Development in plants
Soluble silica, also called monosilicic or orthosilicic acid with a chemical formula of (Si(OH)4), is taken up from the soil when plant roots absorb groundwater. From there, it is carried to other plant organs by the xylem. By an unknown mechanism, which appears to be linked to genetics and metabolism, some of the silica is then laid down in the plant as silicon dioxide. This biological mechanism does not appear to be limited to specific plant structures, as some plants have been found with silica in their reproductive and sub-surface organs.
Chemical and physical characteristics Phytoliths are composed mainly of noncrystalline silicon dioxide, and about 4% to 9% of their mass is water. Carbon, nitrogen, and other major nutrient elements comprise less than 5%, and commonly less than 1%, of phytolith material by mass. These elements are present in the living cells in which the silica concretions form, so traces are retained in the phytoliths. Such immobilised elements, in particular carbon, are valuable in that they permit radiometric dating in reconstructing past vegetation patterns. The silica in phytoliths has a refractive index ranging from 1.41 to 1.47, and a specific gravity from 1.5 to 2.3. Phytoliths may be colorless, light brown, or opaque; most are transparent. Phytoliths exist in various three-dimensional shapes, some of which are specific to plant families, genera or species.
Single cell and conjoined phytoliths Phytoliths may form within single cells, or multiple cells within a plant to form 'conjoined' or multi-cell phytoliths, which are three-dimensional replicas of sections of plant tissue. Conjoined phytoliths occur when conditions are particularly favourable for phytolith formation, such as on a silica rich substrate with high water availability
Pathogenic stress on phytolith formation Silica is not considered an essential nutrient for plants such as nitrogen or phosphorus. However, silica-aided phytoliths can help a plant be more resilient against biotic and abiotic stressors. Silica is bioactive, meaning it is able to change the expression of certain plant genes to jumpstart a defensive response against these stressors. In terms of fungal infections, the deposition of silica has been shown to create a physical barrier between invading fungi and the plant. Some factors however can have very damaging effects on the plant and limit or alter phytolith production.Kistler et al., (2013). Experimental investigation of pathogenic stress on phytolith formation in Cucurbita pepo var. texana (wild gourd). Vegetation History and Archaeobotany, 22(3), 165-170.
In 2009, researchers at the Rock Springs Agricultural Experiment Station at The Pennsylvania State University investigated the effects of pathogenic viruses on phytolith production in Cucurbita pepo var. Texana. The plants that were affected by either mosaic virus (carried by aphids) or bacterial wilt disease (carried by cucumber beetles) were infected on their own to replicate natural conditions and all plants were grouped into three categories: healthy plants sprayed to prevent insect herbivory, plants infected with mosaic disease, and plants infected with bacterial wilt disease.
Analysis after harvest yielded 1,072 phytoliths from forty-five plants. Plants affected by mosaic disease experienced a decrease in phytolith size. This is because the virus constricts overall plant growth and therefore phytolith growth as well. Contrastingly, plants affected with bacterial wilt disease resulted in much larger phytoliths but they were abnormally shaped. This could be due to the bacteria causing constriction of the hypodermal cells, causing an influx of silica deposits.
Patterns of phytolith production Because identification of phytoliths is based on morphology, taxonomical differences in phytolith production are important.
Families with high phytolith production; family and genus-specific phytolith morphology is common: Acanthaceae, Aceraceae, Annonaceae, Arecaceae, Asteraceae, Boraginaceae, Bromeliaceae, Burseraceae, Chrysobalanaceae, Commelinaceae, Costaceae, Cucurbitaceae, Cyatheaceae, Cyperaceae, Dilleniaceae, Equisetaceae, Heliconiaceae, Hymenophyllaceae, Magnoliaceae, Marantaceae, Moraceae, Musaceae, Orchidaceae, Poaceae, Podostemaceae, Selaginellaceae, Ulmaceae, Urticaceae, Zingiberaceae
Families where phytolith production may not be high; family and genus-specific phytolith morphology is common: Capparaceae, Cupressaceae, Dipterocarpaceae, Euphorbiaceae, Fagaceae, Flacourtiaceae, Flagellariaceae, Joinvilleaceae, Pinaceae, Polypodiaceae, Restionaceae, Taxaceae, Taxodiaceae
Families where phytolith production is common; family and genus-specific phytolith morphology is uncommon: Aristolochiaceae, Chloranthaceae, Combretaceae, Hernandiaceae, Loranthaceae, Menispermaceae, Piperaceae, Sapotaceae, Verbenaceae
Families where phytolith productions varies; family and genus-specific phytolith morphology is uncommon: Clusiaceae, Fabaceae, Malvaceae, Sterculiaceae
Families where phytolith production is rare or not observed: Agavaceae, Alismataceae, Amaranthaceae, Amaryllidaceae, Apiaceae, Apocynaceae, Araceae, Araliaceae, Araucariaceae, Asclepiadaceae, Bignoniaceae, Bixaceae, Bombacaceae, Burmanniaceae, Cactaceae, Campanulaceae, Caricaceae, Cartonemataceae, Chenopodiaceae, Convolvulaceae, Cycadaceae, Cyclanthaceae, Dioscoreaceae, Ericaceae, Eriocaulaceae, Gnetaceae, Guttiferae, Hydrocharitaceae, Iridaceae, Juglandaceae, Juncaceae, Labiatae, Lacistemnaceae, Lauraceae, Lecythidaceae, Lentibulariaceae, Liliaceae, Loganiaceae, Malpighiaceae, Mayacaceae, Melastomataceae, Meliaceae, Myristicaceae, Myrtaceae, Myrsinaceae, Nymphaeaceae, Olacaceae, Oxalidaceae, Pedaliaceae, Podocarpaceae, Polygonaceae, Pontederiaceae, Potamogetonaceae, Primulaceae, Proteaceae, Ranunculaceae, Rhamnaceae, Rosaceae, Rubiaceae, Rutaceae, Salicaceae, Sapindaceae, Saxifragaceae, Smilacaceae, Solanaceae, Theaceae, Tiliaceae, Trioridaceae, Typhaceae, Vitaceae, Violaceae, Winteraceae, Xyridaceae, Zygophyllaceae
Archaeology Phytoliths are very robust, and are useful in archaeology because they can help to reconstruct the plants present at a site when the rest of the plant parts have been burned up or dissolved. Because they are made of the inorganic substances silica or calcium oxalate, phytoliths don't decay with the rest of the plant and can survive in conditions that would destroy organic residues. Phytoliths can provide evidence of both economically important plants and those that are indicative of the environment at a particular time period. Cabanes, D. (2020). Phytolith Analysis in Paleoecology and Archaeology. In Interdisciplinary Contributions to Archaeology (pp. 255-288) doi: 10.1007/978-3-030-42622-4_11
Phytoliths may be extracted from residue on many sources: dental calculus (buildup on teeth); food preparation tools like rocks, grinders, and scrapers; cooking or storage containers; ritual offerings; and garden areas.
Sampling strategies
There are several things to be taken into consideration when designing a sampling strategy: