Approaches to phytoliths classification

There have been many authorities who have asserted that the basis of science lies in counting or measuring, i.e. in the use of mathematics. Neither counting nor measuring can however be most fundamental processes in our study of the material universe - before you can do either to any purpose you must first select what you propose to count or measure, which presupposes a classification. (Crowson 1970:2)

Phytolith classifications have followed three general approaches (Piperno 2006; Powers 1992):

• taxonomic
• typological
• taxonomic-typological

All are based on the morphological appearance of the silica particles. The taxonomic approach emphasises morphology in relation to the structure of the original plant tissues, and considers phytoliths as one of the anatomical characteristics of the plant. Phytoliths with a taxonomic relevance are, for instance, trichomes, stomata or bulliforms. Each one of these categories has a defined anatomical origin, and a shape that is repetitive, well determined and easily identifiable. The taxonomic approach is designed to work with modern plant tissues or articulated material from archaeological and geological sites but it may be unsuitable for disarticulated material. Most of the phytoliths recovered (with some exceptions) are discrete particles dispersed in the soil. Distinctive groupings of phytoliths and cells are not available for identification purposes and the typological approach may be more appropriate. Typology can be defined as “the study and interpretation of types” (Oxford Dictionary of the English Language) and, in this case, types are morphological forms of silica bodies. Twiss et al. (1969) first proposed a classification system based on the morphology of the silica bodies from 17 species of grasses from the North American Mid-Plains. The phytolith groups were based on Metcalfe’s (1960) system but, because of the use of discrete bodies, emphasis was placed on the morphology rather than on the position and orientation of the phytoliths in the plant tissues. Powers (1992) developed a simple system based on the visual appearance of the phytoliths without any effort to link the form to a particular anatomical structure or plant taxon. The system was further develop by Madella (2008) to include some forms believed to be secondary forms, originating through pre and post-depositional taphonomic processes. The action of these processes changes the original type so much that it is sometime difficult to recall its earlier form. An attempt to produce an easy and comprehensive single phytolith classification system is the work by Bowdery et al. (2001), which presents a preliminary discussion of the classification system and a key for surface ornamentation.

A further step in phytolith classification was the establishment, by the Society for Phytolith Research, of the International Committee on Phytolith Nomenclature. The committee proposed in 2005 a set of rules on how to describe and name phytolith morphologies, taking into consideration both geometrical and anatomical characteristics. However, other nomenclature approaches have been proposed and/or are still in use (see Piperno 2006 for a general review). In South America, and especially Argentina, the classification set out by Bertoldi de Pomar (1971) is still very much in use and recently it has been proposed by Zucol and Brea (2005) a classification that consider phytoliths as plant fossils and therefore are described and named according to the International Code of Botanical Nomenclatural.

Bibliography

• Bertoldi de Pomar H. 1971. Ensayo de clasificación morfológica de los silicofitolitos. Ameghiniana 7: 317-328.
• Bowdery D., D. M. Hart, C. Lentfer, and Lynley A. Wallis. 2001. A universal phytolith key. Pages 267–278 in Jean Dominique Meunier and Fabrice Colin, eds. Phytoliths: Applications in earth sciences and human history. A. A. Balkema, Lisse, The Netherlands.
• Crowson R. A. 1970. Classification and biology. Heinemann, London.
• 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
• Metcalfe C. R. 1960. Anatomy of the Monocotyledons I Gramineae. Clarendon Press, Oxford.
• Piperno, D. R. 2006. Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. Altamira Press.
• 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
• Twiss P. C., Suess E. and Smith R. M. 1969. Morphological classification of grass phytoliths. Soil Science Society of America Proceedings 33:109-115.
• Zucol, A. F. and Brea, M. 2005. Sistemática de fitolitos, pautas para un sistema clasificatorio. Un caso en estudio en la Formación Alvear (Pleistoceno inferior), Entre Ríos, Argentina. Ameghiniana, vol.42, no.4, p.685-704.

Description and nomenclature

Probably the major problems encountered by researchers working with phytoliths is the great variability of forms (morphological variability), the variability within the forms as well as multiplicity and redundancy (sensu Rovner 1971, see also Piperno 2006). Multiplicity is the production of different phytolith morphologies in a single taxon. This is because opal silica can be deposited in different tissues of a same plant and therefore the phytolith morphologies reproduce the variability of cell morphologies of the different tissues. Redundancy is the presence of similar phytolith morphologies in different taxa, sometime not even taxonomically related.

Here there is an example of variability within a single phytolith type, the so-called dendritic morphology. All the photographs show a dendritic but, as you can see, they are not identical. The key factor in defining all of them as dendritics is the presence of ornamentations in the form of needle-like protrusions from the main body that are then further subdivided (see arrows).

Bibliography

• Madella M., Alexandre A. and Ball T. 2005. International Code for Phytolith Nomenclature 1.0. Annals of Botany 96:253–260.
• Piperno, D. R. 2006. Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. Altamira Press.
• Rovner I. 1971. Potentials of opal phytoliths for use in palaeoecological reconstruction. Quaternary Research I:343-359.

Preparing and analysing reference collections

Tissue reference collections

In a tissue reference collection phytoliths are still in situ, within the cells of the tissue in which they have formed. It is a useful collection for understanding the origin of the different phytolith morphologies as well as their anatomical relationship. To make a reference collection of this type, small pieces of tissue from the sampled plant are treated with sodium hypochlorite (thin bleach) for several hours (the piece is ready when it is almost completely transparent) to eliminate some of the organic matter and colour. The tissue fragments are then washed in distilled water to eliminate any residual sodium hypochlorite, dried on a microscopy slide and fixed with a permanent mounting medium (e.g. Entellen™ or Styrolite™). Care should be taken to use fragments of tissue that are not too thick because otherwise it will be very difficult to observe the phytoliths.

Each sample should carry the following information: genus and species (including the author of the binomial attribution as this is part of the plant name), family, locality of collection, name or acronym of collector, date of collection. For slide storage see below in "Spodograms reference collections".

Here following are some examples of photographs from a tissue reference collection.

Spodograms reference collections

Spodograms result from the incineration of plant tissues at high temperatures (e.g. 500°C) until all organic matter has been eliminated, leaving behind ashes made up of opal silica and/or calcium carbonates/oxalates. To preserve only the silica fraction, the ashes can be treated with weak hydrochloric acid (e.g. 5% in volume) and then washed several times with distilled water. Phytolith for reference can be extracted from both herbaceous and woody plants. Normally, different plant parts are treated separately: leaves, stem, flowers, wood, etc., so to have the possibility to understand the provenance of the different phytolith morphologies. Spodograms are useful reference because the different phytolith types can be view as single elements and, if mounted in liquid mounting, the 3D shape and characteristics can be easily observed.

Specimens for phytolith extraction can be specifically collected for the purpose or, if necessary, herbarium specimens can also be used (especially for grasses where only a small amount of tissues is needed). The amount of dry material used for phytolith extraction can vary but the following can be considered minimum weights for obtaining reasonable amounts of silica fraction for handling and for mounting several duplicate slides:

For other protocols and more in-depth discussion you can also consult Piperno (2008) or Parr et al (2001).

In certain cases when material for reference is collected during field work in substantial amounts and there is no access to a processing laboratory, it might be useful to reduce the volume/weight before transporting the samples. This can be done by simply burning the dry plant material. However, a lot of care has to be taken to avoid cross contamination (e.g. the container in which the material is burnt must be thoroughly washed and cleaned between samples). This simple step will allow to collect substantial amounts of green plant material. For wood is better to do the entire process in the laboratory.

When processing woody parts it is important to separate the bark from the wood itself. This is because bark incorporates dust and impurities during its long period of growth (if the samples come from a trunk this period could be several tens of years) and it is impossible to separate alien morphologies from the ones that might have been produced in the bark. It is therefore safer to consider the wood morphologies (the ones effectively encountered in the wood) and the bark morphologies (which can be also impurities) as two different entities.

Bibliography

• Parr, J., Lentfer C. J. and Boyd W. E. 2001. A Comparative Analysis of Wet and Dry Ashing Techniques for the Extraction of Phytoliths from Plant Material. Journal of Archaeological Science 28:875-886
• Piperno, D. R. 2006. Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. Altamira Press.

Storage of phytolith reference collections

Storage of ash

Ash from modern plants can be stored in glass vials with pressure or screw cap. It is important not to use plastic vials as these can have a static charge that attracts phytoliths to the walls, especially the smaller morphologies.